Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

22. Functional Systems

22.1 Sensory System, Overview

A Simplified diagram of the sensory pathways of the spinal cord

Stimuli generate impulses in various receptors in the periphery of the body (see C, p. 179) which a re transmitted to the cerebrum and cerebellum along the sensory (afferent) pathways or tracts shown here (see В for details). While most of the sensory qualities listed in В are intuitively clear (e.g. pain and temperature sensation), the concept of proprioception is more difficult to convey and will be explained in more detail. Proprioception is concerned with the position of the limbs in space (= position sense). The types of information involved in proprioception are complex: position sense (the position of the limbs in relation to one another) is distinguished from motion sense (speed and direction of joint movements) and force sense (the muscular force associated with joint movements). Accordingly, the receptors for proprioception (proprioceptors) consist mainly of muscle and tendon spindles and joint receptors (see p.328). We also distinguish between conscious and unconscious proprioception. Information on conscious proprioception travels in the posterior funiculus of the spinal cord (fasciculus gracilis and fasciculus cuneatus) and is relayed through its nuclei (nucleus gracilis and nucleus cuneatus) to the thalamus. From there it is conveyed to the sensory cortex (postcentral gyrus), where the information presumably rises to consciousness (“I know that my left hand is making a fist, even though my eyes are closed”). Unconscious proprioception, which enables us to ride a bicycle and climb stairs without thinking about it, is conveyed by the spinocerebellar tracts to the cerebellum, where it remains at the unconscious level. Sensory information from the head is mediated by the trigeminal nerve and is not depicted here (see p. 330).

В Synopsis of sensory pathways

The various stimuli generate impulses in different receptors which are transmitted in peripheral nerves to the spinal cord. The perikarya of the first afferent neuron (to which the receptors are connected) for all pathways are located in the spinal ganglion. The axons from the ganglion pass along various tracts in the spinal cord to the second neuron. Its axons either pass directly to the cerebellum or are relayed by a third neuron to the cerebrum.

Name of Sensory quality

pathway

Receptor

Course in the spinal cord

Central course (above the spinal cord)

Spinothalamic tracts

Anterior spinothalamic tract

• Crude touch

• Hair follicles

• Various skin receptors

The perikaryon of the second neuron is located in the posterior horn and may be up to 15 segments above or 2 segments below the entry of the first neuron. Its axons cross in the anterior commissure (see p. 274)

The axons of the second neuron (spinal lemniscus) terminate in the ventral posterolateral nucleus of the thalamus (see D, p.219). There they synapse onto the third neuron, whose axons project to the postcentral gyrus

Lateral spinothalamic tract

• Pain and temperature

• Mostly free nerve endings

The perikaryon of the second neuron is in the substantia gelatinosa. Its axon crosses at the same level in the anterior commissure (see p. 274)

The axons of the second neuron (spinal lemniscus) terminate in the ventral posterolateral nucleus of the thalamus. There they synapse onto the third neuron, whose axons project to the postcentral gyrus

Tracts of the posterior funiculus

Fasciculus gracilis

• Fine touch

• Conscious proprioception of lower limb

• Vater-Pacini corpuscles

• Muscle and tendon receptors

The axons of the first neuron pass to the nucleus gracilis in the lower medulla oblongata (second neuron) (see p. 276 and B, p. 233)

The axons of the second neuron cross in the brainstem and traverse the medial lemniscus (see B, p. 233) to the ventral posterolateral nucleus of the thalamus. There they synapse onto the third neuron, whose axons project to the postcentral gyrus

Fasciculus cuneatus

• Fine touch

• Conscious proprioception of upper limb

• Vater-Pacini corpuscles

• Muscle and tendon receptors

The axons of the first neuron pass to the nucleus cuneatus in the lower medulla oblongata (second neuron) (see p. 276 and B, p. 233)

The axons of the second neuron cross in the brainstem and traverse the medial lemniscus (see B, p. 233) to the ventral posterolateral nucleus of the thalamus. There they synapse onto the third neuron, whose axons project to the postcentral gyrus

Spinocerebellar tracts

Anterior spinocerebellar tract(of Gowers)

• Unconscious crossed and uncrossed extero- and proprioception to the cerebellum

• Muscle spindles

• Tendon receptors

• Joint receptors

• Skin receptors

The second neuron is located in the dorsal column in the central part of the gray matter. The axons of the second neuron run directly to the cerebellum, both crossed and uncrossed, without synapsing with a third neuron (see p. 278)

The axons of the second neuron pass through the superior cerebellar peduncle to the vermian part of the spino- cerebellum (no third neuron) (see also p. 243)

Posterior spinocerebellar tract(of Flechsig)

• Unconscious uncrossed extero- and proprioception to the cerebellum

• Muscle spindles

• Tendon receptors

• Joint receptors

• Skin receptors

The second neuron is located in the thoracic nucleus (Clarke column, Stilling nucleus) in the gray matter at the base of the posterior horn. The axons of the second neuron run directly to the cerebellum without crossing (see p. 278)

The axons of the second neuron pass through the inferior cerebellar peduncle to the vermian part of the spinocerebellum (no third neuron) (see also p. 243)

22.2 Sensory System:

Stimulus Processing

A Receptors of the somatosensory system

a Skin receptors: Various types of stimuli generate impulses in different receptors in the periphery of the body (illustrated here in sections through hair-bearing and hairless skin). These impulses are transmitted through peripheral nerves to the spinal cord, from which they are relayed and carried by specific tracts to the sensory cortex (see previous unit). Sensory qualities cannot always be uniquely assigned to specific receptors. The figure does not indicate the prevalence of the different receptor types. Nociceptors (= pain receptors), like heat and cold receptors, consist of free nerve endings. Nociceptors make up approximately 50% of aII receptors.

b Joint receptors: Proprioception encompasses position sense, motion sense, and force sense. Proprioceptors include muscle spindles, tendon sensors, and joint sensors (not shown).

В Receptive field sizes of cortical modules in the upper limb of a primate

Sensory information is processed in cortical “modules" (seeC, p.201). This drawing shows the size of the receptive fields supplied by modules. In areas where high resolution of sensory information is not required (e.g., the forearm), one module supplies a large receptive field. In areas that require finer tactile sensation (e.g., the fingers), one module supplies a much smaller receptive field. The size of these fields determines the overall proportions of the sensory homunculus (seeC). Because one skin area may be innervated by several neurons, many of the receptive fields overlap. Information is transmited from the receptive field to the cortex by a chain of neurons and their axons. These neurons and axons are located at specific sites in the CNS (topographical principle).

C Arrangement of sensory pathways in the cerebrum

Anterior view of the right postcentral gyrus. The perikarya of the third neurons of the sensory pathways are located in the thalamus. Their axons projecttothe postcentral gyrus, wheretheprimarysomatosensory cortex is located. The postcentral gyrus has a somatotopic organization, meaning that each body region is represented in a particular cortical area. The body regions in the cortex are not represented in proportion to their actual size, but in proportion to the density of their sensory innervation. The fingers and head have abundant sensory receptors, and so their cortical representation is correspondingly large (seeB). Conversely, the less dense sensory innervation of the buttocks and legs results in smaller areas of representation. Based on these varying numbers of peripheral receptors, we can construct a “sensory homunculus” whose parts correspond to the cortical areas concerned with their perception.

Note: The head of the homunculus is upright while the trunk is upside down.

The axons of the sensory neurons ascending from the thalamus travel side by side with the axons forming the pyramidal tract (red) in the dorsal part of the internal capsule. Because of this arrangement, a large cerebral hemorrhage involving the internal capsule produces sensory as well as motor deficits (see Kell et al.).

D Primary somatosensory cortex and parietal association cortex

a Left lateral view. The Brodmann areas are numbered in the sectional view (b). The contralateral body half is represented in the primary somatosensory cortex (except the perioral region, which is represented bilaterally: speech). This area of the cortex is concerned with somatosensory perception. The parietal association cortex receives information from both sides of the body. Thus, the processing of stimuli becomes increasingly complex in these cortical areas.

E Activity of cortical cell columns In the primary somatosensory cortex

a Amplitude of the neuronal response in the primary somatosensory cortex to a peripheral pressure stimulus. The intensity of the stimulus is shown in b. The diagrams illustrate the principle of sensory information processing in the cortex. When approximately 100 intensity detectors in the fingertip are stimulated by pressure, approximately 10,000 neurons in the corresponding cell column in the primary somatosensory cortex (see columnar organization of the cortex, p. 201) respond to the stimulus. Because the intensity of the peripheral pressure stimulus is maximal at the center and fades toward the edges, it is processed in the cortex accordingly. Cortical processing amplifies the contrast between the greater and lesser stimulus intensities, resulting in a sharper peak (a). While the stimulated area on the fingertip measures approximately 100 mm2, the information is processed in only a 1-mm2 area of the primary somatosensory cortex.

22.3 Sensory System: Lesions

A Sites of occurrence of lesions in the sensory pathways

(after Bahr and Frotscher)

The central portions of the sensory pathways may be damaged at various sites from the spinal root to the somatosensory cortex as a result of trauma, tumor mass effect, hemorrhage, or infarction. The signs and symptoms are helpful in determining the location of the lesion. This unit deals strictly with lesions in conscious pathways. The innervation of the trunk and limbs is mediated by the spinal nerves. The innervation of the head is mediated by the trigeminal nerve, which has its own nuclei (see below).

Cortical or subcortical lesion (1,2): A lesion at this level is manifested by paresthesia (tingling) and numbness in the corresponding regions of the trunk and limbs on the opposite side of the body. The symptoms may be most pronounced distally because of the large receptive fields on the fingers and the relatively small receptive fields on the trunk (see previous unit). The motor and sensory cortex are closely interlinked because fibers in the sensory tracts from the thalamus also terminate in the motor cortex, and because the cortical areas are adjacent (pre- and postcentral gyrus).

Subthalamic lesion (3): All sensation is abolished in the contralateral half of the body (thalamus = “gateway to consciousness”). A partial lesion that spares the pain and temperature pathways (4) is characterized by hypesthesia (decreased tactile sensation) on the contralateral face and body. Pain and temperature sensation are unaffected.

Lesion of the trigeminal lemniscus and lateral spinothalamic tract (5): Damage to these pathways in the brainstem causes a loss of pain and temperature sensation in the contralateral half of the face and body. Other sensory qualities are unaffected.

Lesion of the medial lemniscus and anterior spinothalamic tract (6):

All sensory qualities on the opposite side of the body are abolished except for pain and temperature.

The medial lemniscus transmits the axons of the second neurons of the anterior spinothalamic tract and both tracts of the posterior funiculus.

Lesion of the trigeminal nucleus, spinal tract of the trigeminal nerve, and lateral spinothalamic tract (7): Pain and temperature sensation are abolished on the ipsilateral side of the face (uncrossed axons of the first neuron in the trigeminal ganglion) and on the contralateral side of the body (axons of the crossed second neuron in the lateral spinothalamic tract).

Lesion of the posterior funiculi (8): This lesion causes an ipsilateral loss of position sense, vibration sense, and two-point discrimination. Because coordinated motor function relies on sensory input that operates in a feedback loop, the lack of sensory input leads to ipsilateral sensory ataxia.

Posterior horn lesion (9): A circumscribed lesion involving one or a few segments causes an ipsilateral loss of pain and temperature sensation in the affected segment(s), because pain and temperature sensation are relayed to the second neuron within the posterior horn. Other sensory qualities including crude touch are transmitted in the posterior funiculus and relayed in the dorsal column nuclei; hence they are unaffected. The effects of a posterior horn lesion are called a “dissociated sensory deficit."

Dorsal root lesion (10): This lesion causes ipsilateral, radicular sensory disturbances that may range from pain in the corresponding dermatome to a complete loss of sensation. Concomitant involvement of the ventral root leads to segmental weakness. This clinical situation may be caused by a herniated intervertebral disk (see p. 345).

Lesions of unconscious cerebellar tracts that lead to sensorimotor deficits are not considered here. The volume on General Anatomy and Musculoskeletal System may be consulted for information on peripheral sensory nerve lesions.

22.4 Sensory System: Pain Conduction

A Synopsis of pain modalities

The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." Pain is classified by its site of origin as somatic or visceral. Somatic pain generally originates in the trunk, limbs, or head, while visceral pain originates in the internal organs. Neuropathic pain is caused by damage to the nerves themselves. It may involve nerves of the somatic and/or autonomic nervous system. The somatic pain fibers described below travel with the spinal or cranial nerves, while the visceral pain fibers travel with the autonomic nerves (see p. 322).

В Peripheral somatic pain conduction (after Lorke)

Somatic pain impulses from the trunk and limbs are conducted by myelinated A8 fibers (temperature, pain, position) and unmyelinated C fibers (temperature, pain). The perikarya (cell bodies) for these afferent nerve fibers are located in the spinal ganglion (pseudounipolar neurons). Their axons terminate in the posterior horn of the spinal cord, chiefly in the Rexed laminae I, II, and IV—VI. The nociceptors, afferent fibers ascend after synapsing in the posterior horn (see C).

Note: Most somatosensory pain fibers are myelinated, while the viscerosensory fibers are unmyelinated.

C Ascending pain pathways from the trunk and limbs

The axons of the primary afferent neurons for pain sensation in the trunk and limbs terminate on the projection neurons (shown above) located in the posterior horn of the spinal-cord gray matter. The lateral spinothalamic tract is subdivided into a neo- and paleospinothalamic part. The second neuron of the neospinotholamic port of the pain pathway (red) terminates in the ventral posterolateral nucleus of the thalamus. The third neuron projects from there to the primary somatosensory cortex (postcentral gyrus) of the brain. The second neuron of the paleospinothalamic tract (blue) terminates in the intralaminar and medial nuclei of the thalamus, whose third neurons then project to a variety of brain regions. This pain pathway is mainly responsible for the emotional component of pain. In addition to these pain pathways that end in the cortex, there are also pain pathways that end in subcortical regions—the spinomesencephalic tract and spinoreticular tract. The second neuron of the spinomesencephalic tract (green) terminates mainly in the central gray matter, which surrounds the aqueduct. Other axons terminate in the cuneiform nucleus or anterior pretectal nucleus. The second neuron of the spinoreticular tract (orange) ends in the reticular formation, represented here by the nucleus raphes magnus and the gigantocellular nucleus. Reticulothalamic fibers transmit the pain impulses onward to the medial thalamus, hypothalamus, and limbic system.

22.5 Sensory System: Pain Pathways in the Head and the Central Analgesic System

A Pain pathways in the head (after Lorke)

The pain fibers in the head accompany the principal divisions of the trigeminal nerve (CN Vt-V-j). The perikarya of these primary afferent neurons of the pain pathway are located in the trigeminal ganglion. Their axons terminate in the spinal nucleus of the trigeminal nerve.

Note the somatotopic organization of this nuclear region: The perioral region (a) is cranial and the occipital regions (c) are caudal. Because of this arrangement, central lesions lead to deficits that are distributed along the Solder lines (see D, p.75).

The axons of the second neurons cross the midline and travel in the trigeminothalamic tractto the ventral posteromedial nucleus and to the intralaminar thalamic nuclei on the opposite side, where they terminate. The third (thalamic) neuron of the pain pathway ends in the primary somatosensory cortex. Only the pain fibers of the trigeminal nerve are pictured in the diagram. In the trigeminal nerve itself, the other sensory fibers run parallel to the pain fibers but terminate in various trigeminal nuclei (see p.74).

В Pathways of the central descending analgesic system

(after Lorke)

Besides the ascending pathways that carry pain sensation to the primary somatosensory cortex, there are also descending pathways that have the ability to suppress pain impulses. The central relay station for the descending analgesic (pain-relieving) system is the central gray matter of the mesencephalon. It is activated by afferent input from the hypothalamus, the prefrontal cortex, and the amygdaloid bodies (part of the limbic system, not shown). It also receives afferent input from the spinal cord (see p.333). The axons from the excitatory glutaminergic neurons (red) of the central gray matter terminate on serotoninergic neurons in the raphe nuclei and on noradrenergic neurons in the locus ceruleus (both shown in blue). The axons from both types of neuron descend in the posterolateral funiculus. They terminate directly or indirectly (via inhibitory neurons) on the analgesic projection neurons (second afferent neuron of the pain pathway), thereby inhibiting the further conduction of pain impulses.

22.6 Motor System, Overview

A Simplified representation of the anatomical structures involved in a voluntary movement (pyramidal motor system)

(after Klinke and Silbernagl)

The first step in performing a voluntary movement is to plan the movement in the association cortex of the cerebrum (e.g., goal: “I want to pick up my coffee cup”). The cerebellar hemispheres and basal ganglia work in parallel to program the movement and inform the premotor cortex of the result of this planning. The premotor cortex passes the information to the primary motor cortex (M1), which relays the information through the pyramidal tract to the alpha motor neuron (pyramidal motor system). The alpha motor neuron then initiates the process whereby the skeletal muscle transforms the program into a specific voluntary movement. Sensorimotor functions supply important feedback during this process (How fa г has the movement progressed? How strong is my grip on the cup handle?—different from gripping an eggshell, for example). Although some of the later figures portray the primary motor cortex as the starting point for a voluntary movement, this diagram shows that many motor centers are involved in the execution of a voluntary movement (including the extrapyramidal motor system, see C and D; cerebellum). For practical reasons, however, the discussion commonly begins at the primary motor cortex (Ml).

В Cortical areas with motor function: initiating a movement

Lateral view of the left hemisphere. The initiation of a voluntary movement (reaching for a coffee cup) results from the interaction of various cortical areas. The primary motor cortex (Ml, Brodmann area 4) is located in the precentral gyrus (execution of a movement). The rostrally adjacent area 6 consists of the lateral premotor cortex and medial supplementary motor cortex (initiation of a movement). Association fibers (see p.376) establish close functional connections with sensory areas 1, 2, and 3 (postcentral gyrus with primary somatosensory cortex, S1) and with areas 5 and 7 (= posterior parietal cortex), which have an associative motor function. These areas provide the cortical representation of space, which is important in precision grasping movements and eye movements.

C Connections of the cortex with the basal ganglia and cerebellum: programming complex movements

The pyramidal motor system (the primary motor cortex and the pyramidal tract arising from it) is assisted by the basal ganglia and cerebellum in the planning and programming of complex movements. While afferent fibers of the motor nuclei (green) project directly to the basal ganglia (left) without synapsing, the cerebellum is indirectly controlled via pontine nuclei (right; seeC, p.233). The motor thalamus provides a feedback loop for both structures (seep.341). The efferent fibers of the basal nuclei and cerebellum are distributed to lower structures including the spinal cord. The importance of the basal ganglia and cerebellum in voluntary movements can be appreciated by noting the effects of lesions in these structures. While diseases of the basal ganglia impair the initiation and execution of movements (e.g., in Parkinson’s disease), cerebellar lesions are characterized by uncoordinated writhing movements (e.g., the reeling movements of inebriation, caused by a temporary toxic insult to the cerebellum).

D Simplified block diagram of the sensorimotor system in movement control

Voluntary movements require constant feedback from the periphery (muscle spindles, tendon organs) in order to remain within the desired limits. Because the motor and sensory systems are so closely interrelated functionally, they are often described jointly as the sensorimotor system. The spinal cord, brainstem, cerebellum, and cerebral cortex are the three control levels of the sensorimotor system. All information from periphery, cerebellum, and the basal ganglia passes through the thalamus on its way to the cerebral cortex. The clinical importance of the sensory system in movement is illustrated by the sensory ataxia that may occur when sensory function is lost (see D, p.353). The oculomotor component of the sensorimotor system is not shown.

22.7 Motor System:

Pyramidal (Corticospinal) Tract

A Course of the pyramidal (corticospinal) tract

The pyramidal tract consists of three fiber systems: corticospinal fibers, corticonuclear fibers, and corticoreticular fibers (the latter are not shown here; they pass to the gigantocellular nucleus of the reticular formation in the brainstem and will not be discussed further). These groups of fibers constitute the descending motor pathways from the primary motor cortex. The corticospinal fibers pass to the motor anterior horn cells in the spinal cord, while the corticonuclear fibers pass to the motor nuclei of the cranial nerves.

Corticospinal fibers: Only a small percentage of the axons of the corticospinal fibers originate from the large pyramidal neurons in lamina V of the precentral gyrus (the laminar structure of the motor cortex is shown inD). Most of the axons arise from small pyramidal cells and other neurons in laminae V and VI. Other axons originate from adjacent brain regions. All of them descend through the internal capsule. Eighty percent of the fibers cross the midline at the level of the medulla oblongata (decussation of the pyramids) and descend in the spinal cord as the lateral corticospinal (pyramidal) tract The uncrossed fibers descend in the cord as the anterior corticospinal (pyramidal) tract and cross later at the segmental level. Most of the axons terminate on intercalated cells whose synapses end on motor neurons.

Note: the basic pattern of somatotopic organization described earlier at the spinal cord level is found at all levels of the pyramidal tract. This facilitates localization of the lesion in the pyramidal tract. Corticonuclear fibers: The motor nuclei and motor segments of the cranial nerves receive their axons from pyramidal cells in the facial region of the premotor cortex. These corticonuclear fibers terminate in the contralateral motor nuclei of cranial nerves III—VII and IX—XII in the brainstem (the fibers to other brainstem nuclei are shown in C). Besides this contralateral supply, axons also pass to several cranial nerve nuclei on the same (ipsilateral) side, resulting in a bilateral innervation pattern (not shown here). This dual supply is clinically important in lesions of the frontal branch of the facial nerve, for example (see D, p. 79).

Notes on the “pyramidal tract”: Some authors interpret this term as applying strictly to the portion of the tract below the decussation of the pyramids, while other authors apply the term to the entire tract. Most publications, including this atlas, use “pyramidal tract” as a collective term for all of the fiber tracts described here. Some authors derive the term not from the decussation of the pyramids but from the giant pyramidal cells (Betz cells) in the cerebral cortex (see D and p. 281).

В Somatotopic representation of the skeletal muscle in the precentral gyrus (motor homunculus)

Anterior view. Regions in which the muscles are very densely innervated (e.g., the hand) must be supplied by many neurons In the precentral gyrus. As a result, they require a larger representation area in the cortex than regions supplied by fewer neurons (e.g., the trunk). This cortical representation is analogous to that in sensory innervation, where areas of varying size are also represented in the cortex (postcentral gyrus; compare with the sensory homunculus in C, p. 329). One cortical area is devoted to the trunk and limbs and another to the head. The axons for the head area are the corticonuclear fibers, and the axons for the trunk and limbs are the corticospinal fibers. The latter fibers split into two groups below the telencephalon, forming the lateral and anterior corticospinal tracts.

C Variety of cortical efferent fibers

Anterior view. Besides the corticospinal and corticonuclear fibers described above, a variety of axons descends from the cortex to various subcortical regions and into the spinal cord. The following subcortical regions also receive cortical efferent fibers: the corpus striatum, thalamus, red nucleus, pontine nuclei, reticular formation, inferior olive, dorsal column nuclei (these nuclear regions are described on p.342), and spinal cord. The supraspinal efferent fibers listed above consist partially of axon collaterals from pyramidal tract neurons and partially of separate axons.

D Laminar structure of the motor cortex (= area 4 In the precentral gyrus)

The axons from giant pyramidal cells (Betz cells) in lamina V account for only a small percentage (< 4%) of the axons that make up the corticospinal tract. Small pyramidal cells and other neurons from laminae V and VI contribute the rest. In all, however, only about 40% of the axons of the pyramidal tract originate in area 4. The remaining 60 % come from neurons in the supplementary motor fields (see p. 336).

22.8 Motor System: Motor Nuclei

A Motor nuclei

Coronal section. The basal ganglia are subcortical nuclei of the telencephalon that have a role in the planning and execution of movements. They are the central relay station of the extra pyramidal motor system and make up almost all the central gray matter of the cerebrum. The only other central gray-matter structure is the thalamus, which is primarily sensory (“gateway to consciousness") and is involved only secondarily, through feedback mechanisms, in motor sequences. The three largest motor nuclei are:

 Caudate nucleus,

 Putamen, and

 Globus pallidus (develop mentally, part of the diencephalon).

These three nuclei are sometimes known by varying collective designations:

• The lentiform nucleus is formed by the putamen, globus pallidus, and intervening fiber tracts.

 The corpus striatum consists of the putamen, caudate nucleus, and intervening streaks of gray matter. In addition to these three nuclei, there are other nuclei that are considered functional components of the motor system (also shown here).

In a strictly anatomical sense, only the telencephalic structures listed above are constituents of the basal ganglia. Some textbooks mistakenly include the subthalamic nucleus of the diencephalon (see p.224) and the substantia nigra of the mesencephalon (see p. 228) among the basal ganglia because of their close functional relationship to nuclei. Functional disturbances of the basal nuclei are characterized by movement disorders (e.g., Parkinson’s disease).

В Flow of information between motor cortical areas and basal ganglia: motor loop

The basal ganglia are concerned with the controlled, purposeful execution of fine voluntary movements (e.g., picking up an egg without breaking it). They integrate information from the cortex and subcortical regions, which they process in parallel and then return to motor cortical areas via the thalamus (feedback). Neurons from the premotor, primary motor, supplementary motor, and somatosensory cortex and from the parietal lobe send their axons to theputamen (see p.209). Initially there is a direct (yellow) and indirect (green) pathway for relaying the information out of the putamen. Both pathways ultimately lead to the motor cortex by way of the thalamus. In the direct pathway (yellow), the neurons of the putamen project to the medial globus pallidus and to the reticular part of the substantia nigra. Both nuclei then return feedback signals to the motor thalamus, which projects back to motor areas of the cortex. The indirect pathway (green) leads from the putamen through the lateral globus pallidus and subthalamic nucleus back to the medial globus pallidus, which then projects to the thalamus. An alternate indirect route leads from the subthalamic nucleus to the reticular part of the substantia nigra, which in turn projects to the thalamus. When inhibitory dopaminergic neurons in the compact part of the substantia nigra cease to function, the indirect pathway is suppressed and the direct pathway is no longer facilitated. Both effects lead to the increased inhibition of thalamocortical neurons, resulting in decreased movements (= hypokinetic disorder, e.g., in Parkinson’s disease). Conversely, reduced activation of the internal part of the globus pallidus and the reticular part of the substantia nigra leads to increased activation of the thalamocortical neurons, resulting in abnormal spontaneous movements (= hyperkinetic disorder, e.g., Huntington’s disease).

The diagram at lower left shows a close-up view of the boxed area (thalamus).

22.9 Motor System: Extra pyramidal Motor System and Lesions

A Descending tracts of the extrapyramidal motor system

The neurons of origin of the descending tracts of the extra pyramidal motor system* arise from a heterogeneous group of nuclei that includes the basal ganglia (putamen, globus pallidus, and caudate nucleus), the red nucleus, the substantia nigra, and even motor cortical areas (e.g., area 6). The following descending tracts are part of the extra pyramidal motor system:

 Rubrospinal tract

 Olivospinal tract

 Vestibulospinal tract

 Reticulospinal tract

 Tectospinal tract

These long descending tracts terminate on interneurons which then form synapses onto alpha and gamma motor neurons, which they control. Besides these long descending motor tracts, the motor neurons additionally receive sensory input (blue). All impulses in these pathways are integrated by the alpha motor neuron and modulate its activity, thereby affecting muscular contractions. The functional integrity of the alpha motor neuron is tested clinically by reflex testing.

* The term “extrapyramidal motor system” has been criticized because its functional and anatomical components are so closely linked to the pyramidal motor system that the distinction seems arbitrary in an anatomical sense—particularly since the system does not include cerebellar tracts that are also involved in the control of motor function.

В Lesions of the central motor pathways and their effects Lesion near the cortex (1): paralysis of the muscles innervated by the damaged cortical area. Because the face and hand are represented by particularly large areas in the motor cortex (see B, p. 339), paralysis often affects primarily the arm and face (“brachiofacial” paralysis). The paralysis invariably affects the side opposite the lesion (decussation of the pyramids) and is flaccid and partial (paresis) rather than complete because the extrapyramidal fibers are not damaged. If the extrapyramidal fibers were also damaged, the result would be complete spastic paralysis (see below).

Lesion at the level of the internal capsule (2): This leads to chronic, contralateral, spastic hemiplegia (= complete paralysis) because the lesion affects both the pyramidal tract and the extra pyramidal motor pathways, which mix with pyramidal tract fibers in front of the internal capsule. Stroke is a frequent cause of lesions at this level.

Lesion at the level of the cerebral peduncles (crura cerebri) (3): contralateral spastic hemiparesis.

Lesion at the level of the pons (4): contralateral hemiparesis or bilateral paresis, depending on the size of the lesion. Because the fibers of the pyramidal tract occupy a larger cross-sectional area in the pons than in the internal capsule, not all of the fibers are damaged in many cases. For example, the fibers for the facial nerve and hypoglossal nerve are usually unaffected because of their dorsal location. Damage to the abducent nucleus may cause ipsilateral damage to the trigeminal nucleus (not shown).

Lesion at the level of the pyramid (5): Flaccid contralateral paresis occurs because the fibers of the extrapyramidal motor pathways (e.g., the rubrospinal and tectospinal tract) are more dorsal than the pyramidal tract fibers and are therefore unaffected by an isolated lesion of the pyramid.

Lesion at the level of the spinal cord (6,7): A lesion at the level of the cervical cord (6) leads to ipsilateral spastic hemiplegia because the fibers of the pyramidal and extra pyramidal system are closely interwoven at this level and have already crossed to the opposite side. A lesion at the level of the thoracic cord (7) leads to spastic paralysis of the ipsilateral leg.

Lesion at the level of the peripheral nerve (8): This lesion damages the axon of the alpha motor neuron, resulting in flaccid paralysis.

* Thus, spastic paralysis is actually a sign of extra pyramidal motor damage. This fact was unknown when pyramidal tract lesions were first described, however, and it was assumed that a pyramidal tract lesion led to spastic paralysis. Because this fact has few practical implications, spasticity is still described in some textbooks as the classic sign of a pyramidal tract lesion. It would be better simply to regard spastic paralysis as a form of central paralysis.

22.10 Radicular Lesions: Sensory Deficits

A Caudal end of the spinal cord and cauda equina in the vertebral canal

Midsagittal section viewed from the left side. The spinal cord ends approximately at the LI level, and the neural tissue in the vertebral canal below that level consists only of ventral and dorsal roots (see also p. 269). The ventral motor root and dorsal sensory root unite in the intervertebral foramen to form the spinal nerve. The roots enter and emerge from the spinal dural sac through two separate openings (b). This is the anatomical basis for the fact that sensory deficits (pain, loss of sensation) and motor deficits (muscular weakness ranging to paralysis) may develop separately in patients with nerve root compression (see E).

В Projection of radicular innervation to the skin: dermatomes

After the dorsal and ventral roots unite to form the spinal nerve (see A), their nerve fibers are distributed to their respective territories. The area of skin that is innervated by the fibers of a single dorsal root is called a dermatome. If the dorsal root is damaged (e.g., by pressure from a herniated intervertebral disk), sensation may be altered in the area supplied by the root. As a result, the level of the damaged nerve root can be identified by noting the dermatome affected by the sensory loss. Because the Cl segment contains only motor fibers, there is no Cl dermatome.

C Location of a radicular lesion

A radicular lesion is located on the ventral motor root or dorsal sensory root between its site of emergence from the spinal cord and the union of both roots to form a peripheral nerve. Accordingly, a lesion of the ventral root leads to motor deficits (see p.346) while a dorsal root lesion leads to sensory disturbances in the corresponding dermatome. The dermatomes on the limbs are shifted because of migratory processes during embryonic development, but the dermatomes on the trunk retain their segmental pattern of innervation (see В and D). Due to the overlap between adjacent dermatomes, the sensory loss that results from damage to a dermatome may be smaller than the size of the dermatome as it appears in the diagram. The brain does not “know” the location of the lesion; it processes information as if the lesion were located in the area supplied by the nerve, i.e., in the dermatone.

D Radicular innervation of the trunk

The segmental arrangement of the musculature is preserved in the trunk, and so the trunk retains a segmental (radicular) innervation pattern. Because the nerves in the trunk do not form plexuses, the radicular innervation pattern continues into the peripheral territory of a cutaneous nerve (T2-T12; seeB). It can be seen that afferent fibers from the sympathetic trunk reach the peripheral nerves distal to the roots. This explains why radicular lesions are usually not associated with autonomic deficits in the affected dermatomes.

E Pressure on spinal nerve roots from a herniated lumbar disk of L4/5

A herniated intervertebral disk may exert pressure on the spinal nerve root or cauda equina. The disk consists of a central gelatinous core (nucleus pulposus) and a peripheral ring of fibrocartilage (anulus fibrosus). When the anulus fibrosus is damaged, material from the gelatinous core may be extruded through the ring defect and impinge upon the root at its entry into the intervertebral foramen. This is a frequent cause of radicular symptoms, which have two grades of severity: *

* Irritation of the nerve root in the region of the intervertebral foramen. This leads to pain in the low back (lumbago), potentially accompanied by pain radiating into the lower limb in the dermatone of the affected root (sciatica).

* A large disk herniation may compress the dorsal and/or ventral spinal nerve root, causing severe pain in addition to sensory deficits and (if the ventral root is affected) motor deficits.

a Posterolateral disk herniation at the L4/5 level. This damages the L5 root passing behind the herniated disk but not the descending L4 root, which has already entered the intervertebral foramen at that level. As a result, the sensory deficits are manifested in the L 5 dermatome (see B). Only a far lateral disk herniation will damage the root that exits at the same level as the affected disk, b Posteromedial disk herniation at the L4/5 level. The material herniates through the posterior longitudinal ligament and impinges on the cauda equina. Cauda equina syndrome may develop if a lesion in this region compresses multiple roots. The locations of the deficits associated with specific root lesions are described in the next unit.

22.11 Radicular Lesions: Motor Deficits

A Indicator muscles of radicular lesions—limb muscles and diaphragm (after Kunze)

While a lesion of the sensory dorsal roots leads to sensory disturbances in specific dermatomes (see p. 344 and C, p. 345), a lesion of the motor ventral roots will cause weakness to develop in specific muscles. Just as the affected dermatome indicates the site of the sensory root lesion, the affected muscle indicates the level of the damaged spinal cord segment or its root. The muscles that are predominantly supplied by a particular spinal cord segment are called its indicator muscles (analogous to the dermatomes for the dorsal roots). Because indicator muscles are supplied predominantly but, as a rule, not exclusively by a single segment, a lesion in one segment or spinal nerve root usually causes weakness (paresis) of the affected muscle rather than complete paralysis (plegia). Slight weakness may also be noted in muscles that receive some innervation from the affected segment but are not principally supplied by it. The indicator muscles in the upper and lower limbs are listed in the tables below. Whereas sensory (dorsal) root lesions may occur in isolation, motor (ventral) root lesions usually occur in association with dorsal root lesions, and therefore the dermatomes are also listed in the tables.

Note: Because these nerves of the trunk are derived directly from the spinal nerve roots without any intervening plexuses, the pattern of segmental innervation in the trunk is identical to the pattern of peripheral innervation.

Location of pain or sensory disturbance

Shoulder

Indicator muscle

Diaphragm

Reflexes abolished by a segmental lesion

None

         

Location of pain or sensory disturbance

Anterior side of thigh, passing obliquely downward from the approximate level of the trochanter to the medial side of the knee

Posterolateral thigh, extensor side of knee to anteromedial side of lower leg

Posterior side of thig h, lateral side of knee, anterolateral lower leg, dorsum of foot to big toe

Lateral surface of thigh and calf, heel to lateral edge of foot

Indicator muscle (and other affected muscles)

© Quadriceps femoris (Adductors)

© Quadriceps femoris, especially the vastus medialis

® (Tibialis anterior, adductors)

© Extensor hallucis longus © Tibialis anterior © Gluteus médius

© Triceps surae, peronei, thigh flexors ® Gluteus maxi mus

Reflexes abolished by a segmental lesion

Quadriceps reflex (= patellar tendon reflex = knee-jerk reflex)

Quadriceps reflex (Adductor reflex)

Tibialis posterior reflex

Triceps surae reflex (= Achilles tendon reflex = ankle-jerk reflex)

В Principal indicator muscles of the spinal cord segments

The table lists the typical indicator muscles for each cord segment.

Cord

segment

Indicator muscle

C4

Diaphragm

C5

Deltoid

C6

Biceps brachii

C7

Triceps brachii

C8

Hypothenar muscles, long digital flexors on ulnar side

L3

Quadriceps femoris

L4

Quadriceps femoris, vastus medialis

L5

Extensor hallucis longus, tibialis anterior

SI

Triceps surae, peronei, gluteus maximus

C Clinical manifestations of nerve root irritation

 Pain in the affected dermatome

 Sensory losses in the affected dermatome

 Increased pain during coughing, sneezing, or straining

 Pain fibers more severely affected than other sensory fibers

 Motor deficits in the indicator muscles of the segment

 Reflexes associated with the affected segment are absent or diminished.

22.12 Lesions of the Brachial Plexus

A Brachial plexus paralysis

Anterior view of the right side. Lesions are circled. By definition, two forms of brachial plexus paralysis are distinguished: upper brachial plexus paralysis, which is caused by a lesion of the C5 and C6 ventral rami (seeC), and lower brachial plexus paralysis, which is caused by a lesion of the C8 and T1 ventral rami (seeD). C7 forms a “watershed” between the two forms of paralysis and is typically unaffected by either form. A complete lesion of the brachial plexus may also occur in severe trauma.

В Site of lesion in brachial plexus paralysis

A brachial plexus lesion affects the ventral rami of several spinal nerves, which transmit afferent signals to the plexus. Because the ventral rami carry both motor and sensory fibers, a brachial plexus lesion always causes a combination of motor and sensory deficits. The resulting paralysis (seeC) is always of the flaccid type because of its peripheral nature (= lesion of the second motor neuron).

C Example: upper brachial plexus paralysis (Erb's palsy)

This condition results from a lesion of the ventral rami of the C5 and C6 spinal nerves, causing paralysis of the abductors and external rotators of the shoulder joint and of the upper arm flexors and supinator. The arm hangs limply at the side (loss of the upper arm flexors), and the palm faces backward (loss of the supinator with dominance of the pronators). There may also be partial paralysis of the extensor muscles of the elbow joint and hand. Typical cases present with sensory disturbances on the lateral surface of the upper arm and forearm, but these signs may be absent. A frequent cause of upper brachial plexus paralysis is obstetric trauma.

D Example: lower brachial plexus paralysis

(Dejerine-Klumpke palsy)

This paralysis results from a lesion of the ventral rami of the C8 and T1 spinal nerves (see A). It affects the hand muscles, the long digital flexors, and the flexor muscles in the wrist (claw hand with atrophy of the small hand muscles, a). Sensory disturbances affect the ulnar surfaces of the forearm and hand. Because the sympathetic fibers for the head leave the spinal cord at T1 (b), the sympathetic innervation of the head is also lost. This is manifested by a unilateral Horner syndrome, characterized by miosis (contracted pupil due to paralysis of the dilator pupillae) and narrowing of the palpebral fissure (not ptosis) due to a loss of sympathetic innervation to the superior and inferior tarsal muscles. The narrowed palpebral fissure mimics enophthalmos (sinking of the eyeball into the orbit).

22.13 Lesions of the Lumbosacral Plexus

A Lumbosacral plexus

Anterior view. The lumbosacral plexus is divided into a lumbar plexus (T12-L4) and sacral plexus (L5-S 4).

Note: The nerves of the lumbar part (yellow) pass anteriorly while those of the sacral part (green) pass posteriorly. The connection between the two parts of the plexus is the lumbosacral trunk.

Because the lumbosacral plexus is in a protected location deep within the pelvis, it is less commonly affected by lesions than the brachial plexus, which is much more superficial. The lumbosacral plexus may be injured by pelvic ring fractures, a sacral bone fracture, or hip fractures, or as a complication of hip replacement.

В Lesion of the left lumbar plexus (T12-L4)

The dominant feature of this condition is femoral nerve paralysis affecting the hip flexors, knee extensors, and the external rotators and adductors of the thigh (a). A sensory deficit is found on the anteromedial aspect of the thigh and calf. The lesion also disrupts the sympathetic fibers for the leg, which arise from the lumbar cord and pass through the lumbar plexus. The clinical manifestations (b) include: increased warmth of the foot (loss of sympathetic vasoconstriction) and anhidrosis on the sole of the foot (sweating is absent because of loss of sympathetic innervation to the sweat glands). When sweating is intact, the ninhydrin test is positive (footprint on a sheet of paper stains purple with 1 % ninhydrin solution).

Note: Manifestations in the limbs are recognized by comparison with the unaffected side.

C Muscular and cutaneous distribution of the femoral nerve (L1-L4)

Anterior view.

D Lesion of the right sacral plexus (L5-S4)

This lesion presents clinically with paralysis of the sciatic nerve and its two main branches, the tibial and common fibular nerves, which are jointly affected. The results are loss of plantar flexion (tibial nerve paralysis, inability to walk on the toes) and paralysis of the foot and toe extensors (common fibular nerve, steppage gait: the patient must raise the knee abnormally high while walking to avoid dragging the toes on the ground). Sensory disturbances are noted on the posterior surfaces of the thigh, lower leg, and foot. Because the superior gluteal nerve is involved, the gluteus médius and minimus are also paralyzed. These two muscles stabilize the pelvis of the stationary side during gait. When they are paralyzed, the pelvis tilts toward the swinging leg, producing a “waddling” gait (known also as a positive Trendelenburg sign). The superior gluteal nerve also innervates the tensor fasciae latae, which normally acts in the same manners as the two gluteal muscles. Specific categories of peripheral nerve lesions are described in the volume on General Anatomy and Musculoskeletal System.

22.14 Lesions of the Spinal Cord and Peripheral Nerves: Sensory Deficits

Overview of the next three units (after Bahr and Frotscher)

Two questions should be addressed in the diagnostic evaluation of spinal cord lesions:

1. What structure(s) within the cross-sect/on of the spinal cord is (are) affected? This is determined systematically by proceeding from the periphery of the cord toward the center.

2. At what level of the spinal cord (in longitudinal section) is the lesion located?

In these units we will first correlate various deficit patterns (syndromes) with the structures in the cross-section of the spinal cord. We will then discuss the level of the lesion in the longitudinal or craniocaudal dimension. Since these syndromes present with deficits that result from damage to specific anatomical structures, they can be explained in anatomical terms. Based on the lesions and syndromes described here, the reader can test his or her ability to relate what has already been learned to the locations and effects of spinal cord lesions.

A Spinal ganglion syndrome illustrated for an isolated lesion of T6

As part of the dorsal roots, the spinal ganglia are concerned with the transmission of sensory information. (Recall that the ganglia contain the perikarya of the first sensory neuron.) When only a single spinal ganglion is affected (e.g., by a viral infection such as herpes zoster), the resulting pain and paresthesia are limited to the sensory distribution (dermatome) of the ganglion. Because the dermatomes show considerable overlap, adjacent dermatomes can assume the function of the affected dermatome. As a result, the area that shows absolute sensory loss, called the “autonomous area” of the dermatome, may be quite small.

В Dorsal root syndrome Illustrated for a lesion at the C4-T6 level

When a lesion (trauma, degenerative spinal changes, tumor) affects multiple successive dorsal roots as in this example, complete sensory loss occurs in the affected dermatomes. When this sensory loss affects the afferent limb of a reflex, that reflex will be absent or diminished. If the sensory dorsal roots are irritated but not disrupted, as in the case of a herniated intervertebral disk, severe pain may sometimes be perceived in the affected dermatome. Because pain fibers do not overlap as much as other sensory fibers, the examiner should have no difficulty in identifying the affected dermatome, and thus the corresponding spinal cord segment, from the location of the pain.

C Posterior horn syndrome Illustrated for a lesion attheC5~C8 level

This lesion, like a dorsal root lesion of the spinal nerves, is characterized by a segmental pattern of sensory disturbance. But with a posterior horn lesion of the spinal cord, unlike a dorsal root lesion, the resulting sensory deficit is incomplete. Pain and temperature sensation are abolished in the dermatomes on the ipsilateral side because the first peripheral/ afferent neuron of the lateral spinothalamic tract is relayed in the posterior horn, which is within the damaged area. Position sense and vibration sense are unaffected because the fibers for these sensory qualities are both conveyed in the posterior funiculus. Bypassing the posterior horn, these fibers pass directly via the posterior funiculi to their synapses in the nucleus gracilis or nucleus cuneatus (see p.276). A lesion of the anterior spinothalamic tract does not produce striking clinical signs. The deficit (loss of pain and temperature sensation with preservation of position and vibration sense) is called a dissociated sensory loss. Pain and temperature sensation are preserved below the lesion because the tracts in the white matter (lateral spinothalamic tract) are undamaged. This type of dissociated sensory loss occurs in syringomyelia, a congenital or acquired condition in which threre is an expanded cavity in or near the central canal of the spinal cord. (According to the strictest terminology, expansion of the central canal itself = hydromyelia).

D Lesion of the posterior funiculi at the T8 level

A lesion of the posterior funiculi (see also p.276) is characterized by a loss of:

 Position sense,

 Vibration sense, and

 Two-point discrimination.

These deficits occur distal to the lesion, hence they involve the legs and lower trunk when the lesion is at the T8 level. When the legs are affected, as in the present example, the loss of position sense (mediated by proprioception, see p. 179) leads to an unsteady gait (ataxia).

When the arm is affected (not shown here), the only clinical finding is sensory impairment. The lack of feedbackto the motor system also prevents the precise interaction of different muscle groups during fine movements (asynergy). Ataxia results from the fact that information on body position is essential for carrying out movements. Vision can (partly) compensate for this loss of information when the eyes are open, and so the ataxia worsens when the eyes are closed (Romberg’s sign). This sensory ataxia differs from cerebellar ataxia in that the latter cannot be compensated by visual control.

E Gray matter syndrome illustrated for a lesion attheC4-T4level

This syndrome results from a pathological process (e.g., a tumor) in and around the central canal. All tracts that cross through the gray matter are damaged, i.e., the anterior and lateral spinothalamic tracts. The result is a dissociated sensory loss (loss of pain and temperature sensation with preservation of position, vibration, and touch), in this case involving the arms and upper chest (compare with C). A relatively large lesion may additionally affect the anterior horns, which contain the alpha motor neuron, causing a flaccid paralysis in the distal portions of the upper limb. An even larger lesion may concomitantly affect the pyramidal tract, causing spastic paralysis of the distal muscles (here in the legs). This syndrome may result from syringomyelia (seeC) or tumors located near the central canal.

F Combined disease of the posterior funiculi and pyramidal tract illustrated for a lesion at the T6 level

A lesion of the posterior funiculi leads to loss of position and vibration sense. A concomitant pyramidal tract lesion additionally leads to spastic paralysis of the legs and abdominal muscles below the affected dermatome, i.e., below T6 in the example. This predominantly cervico-thoracic lesion typically occurs in funicular myelosis (vitamin B12 deficiency), in which the posterior funiculi are affected initially, followed by the pyramidal tract. This disease is characterized by degeneration of the myelin sheaths.

22.15 Lesions of the Spinal Cord and Peripheral Nerves: Motor Deficits

A Anterior horn syndrome illustrated for a lesion attheC7~C8 level

Damage to the motor anterior horn cells leads to ipsilateral paralysis, in this case involving the hands and forearm muscles because the lesion is at C7-C8 and these segments innervate the muscles in this region. The paralysis is flaccid because the alpha motor neuron that supplies the muscles (lower motor neuron = second motor neuron, seep. 181) has ceased to function. Because larger muscles are supplied by motor neurons from more than one segment (see A, p.270), damage to a single segment may lead only to muscular weakness (paresis) rather than complete paralysis of the affected muscle group. When the lateral horns are additionally involved, decreased sweating and vasomotor function will also be noted because the lateral horns contain the cell bodies of the sympathetic neurons that subserve these functions. This type of lesion may occur in poliomyelitis or in spinal muscular atrophy, for example. These relatively rare diseases are relentlessly progressive.

В Combined lesions of the anterior horn and lateral corticospinal tract

These lesions produce a combination of flaccid and spastic paralysis. Damage to the motor anterior horns or “lower” motor neuron (= second motor neuron) causes flaccid paralysis, while a lesion of the lateral corticospinal tract or “upper” motor neuron (=first motor neuron) causes spastic paralysis. The degree of injury to both types of neuron may be highly variable. In the example shown, an anterior horn lesion at the C7-C8 level has caused flaccid paralysis of the forearm and hand. By contrast, a lesion of the lateral corticospinal tract at the T 5 level would cause spastic paralysis of the abdominal and leg muscles.

Note: When the second motor neuron in the anterior horn is already damaged (flaccid paralysis), an additional lesion of the lateral corticospinal tract at the level of the same segment will not produce any noticeable effects.

This lesion pattern occurs in amyotrophic lateral sclerosis, in which the first cortical motor neuron (pyramidal tract lesion) and second spinal motor neuron (anterior horn lesion) both undergo progressive degeneration (etiology unclear). The end stage is marked by additional involvement of the motor cranial nerve nuclei, with swallowing and speaking difficulties (bulbar paralysis).

C Corticospinal tract syndrome

Progressive spastic spinal paralysis (Erb-Charcot disease) is characterized by a progressive degeneration of the cortical neurons in the motor cortex with increasing failure of the corticospinal pathways (axonal degeneration of the first motor neuron). The course of the disease is marked by a progressive spastic paralysis of the limbs that begins in the legs and eventually reaches the arms.

D Combined lesions of the posterior funiculus, spinocerebellar tracts, and pyramidal tract

This syndrome begins with destruction of the neurons in the spinal ganglia, which transmit information on conscious position sense (loss: ataxia, asynergy), vibration sense, and two-point discrimination. This neuronal destruction leads to atrophy of the posterior funiculi. There is little or no impairment of pain and temperature sensation, which are still transmitted to higher centers in the unaffected lateral spinothalamic tract. The loss of conscious proprioception alone is sufficient to cause sensory ataxia (lack of feedback to the motor system, see D, p. 353). But the lesions additionally affect the spinocerebellar tracts (unconscious proprioception), injury to which suffices to cause ataxia, and so this dual injury causes a particularly severe loss of conscious and unconscious proprioception. This is the main clinical feature of the disease. Spastic paralysis also develops as a result of pyramidal tract dysfunction. The prototype of this disease is hereditary Friedreich ataxia, which has several variants. The gene has been localized to chromosome 19.

E Spinal hemiplegia syndrome (Brown-Séquard syndrome) illustrated for a lesion at the T10 level on the left side

Hemisection of the spinal cord, though uncommon (e.g., in stab injuries), is an excellent model for testing our understanding of the function and course of the nerve tracts in the spinal cord. Spastic paralysis due to interruption of the pyramidal tract (see footnote on p.343) occurs on the side of the lesion (and below the level of the lesion). The interruption of the posterior funiculi (pathways for conscious proprioception) causes a loss of position and vibration sense and two-point discrimination on the side of the lesion. After spinal shock has subsided, spastic paralysis develops belowthe level of the lesion (here affecting the left leg). Of course, this paralysis does not produce an ataxia like that described following interruption of the posterior funiculi. Destruction of the alpha motor neurons in the locally damaged segment (in this case T10) leads to ipsilateral flaccid paralysis associated with this segment. Because the axons of the lateral spinothalamic tract have already crossed to the unaffected side belowthe lesion, pain and temperature sensation are preserved on the ipsilateral side belowthe lesion. These two types of sensation are lost on the contralateral side, however, because the crossed axons on the opposite side have been interrupted at the level of the lesion. If spinal root irritation occurs at the level of the lesion, radicular pain may occur because of the descending course of the sensory (and motor) roots in the segment above the lesion (see E, p. 345).

22.16 Lesions of the Spinal Cord, Assessment

A Deficits caused by complete cord lesions at various levels

Having explored the manifestations of lesions at different sites in the cross-section of the spinal cord, we will now consider the effects of lesions at various levels of the cord. An example is the paralysis caused by a complete spinal cord lesion, which occurs acutely after a severe injury and is considerably more common than the incomplete lesions described earlier (seeE, p.355). A complete cord lesion following acute trauma is initially manifested by spinal shock, the pathophysiology of which is not yet fully understood. This condition is marked by complete flaccid paralysis below the site of the lesion, with a loss of all sensory qualities from the level of the lesion downward. Loss of bladder and rectal function and impotence are also present. Because the lesion also interrupts the sympathetic fibers, sweating and thermoregulation are impaired. The gray matter of the spinal cord recovers over a period ranging from a few days to eight weeks. The spinal reflexes return, and the flaccid paralysis changes to a spastic paralysis. There is a recovery of bladder and rectal function, but only at a reflex level since voluntary control has been permanently lost. Impotence is permanent. Lesions of the cervical cord above C3 are swiftly fatal because they disrupt the efferent supply of the phrenic nerve (main root at C4), which innervates the diaphragm and maintains abdominal respiration, while innervation to the intercostal muscles is also lost, causing a failure of thoracic respiration. A complete lesion of the lower cervical cord causes paralysis of all four limbs (quadriplegia), and respiration is precarious because of paralysis of the intercostal muscles. Lesions of the upper thoracic cord (T2 downward) spare the arms but respiration is compromised because of paralysis of the abdominal muscles. A lesion of the lower thoracic cord (the exact site is unimportant) has little or no effect on the abdominal muscles, and respiration is not impaired. If the sympathetic splanchnic nerves are also damaged, there may be compromise of visceral motor function ranging to paralytic ileus (seep.324).

With lesions of the lumbar cord, a distinction is drawn between epiconus syndrome (L4-S2) and conus syndrome (S3 downward). Epiconus syndrome is characterized by a flaccid paralysis of the legs (only the roots are affected, causing peripheral paralysis), and reflex but not conscious emptying of the bladder and rectum is preserved. Sexual potency is lost. In conus syndrome, the legs are not paralyzed and only the foregoing autonomic disturbances are present. The motordeficits described here are also associated with sensory deficits (see B).

В Deficits associated with complete spinal cord lesions at various levels (after Rohkamm)

Level of lesion

Motor deficits

Sensory deficits

Autonomic deficits

C1-C3 (high cervical cord lesion)

• Quadriplegia

• Paralysis of nuchal muscles

• Spasticity

• Respiratory paralysis (immediate death if not artificially ventilated)

• Sensory loss from occiput or mandibular border downward

• Pain in occipital region, back of neck, and shoulder region

• Reflex viscera I functions (bladder, bowel) with no voluntary control

• Horner syndrome

C4-C5

• Quadriplegia

• Diaphragmatic respiration only

• Sensory loss from clavicle or shoulder downward

• See above

C6-C8 (lower cervical cord lesion)

• Quadriplegia

• Diaphragmatic respiration

• Spasticity

• Sensory loss from upper chest wall and backdownward, and on the arms (sparing the shoulders)

• See above

T1 -T5

• Paraplegia

• Decreased respiratory volume

• Sensory loss from inside of forearm, upper chest wall and back

• Reflex function of bladder and rectum

T5-T10

• Paraplegia, spasticity

• Sensory loss from affected level in chest wall and back

• See above

Til-L3

• Paraplegia

• Sensory loss from groin region or front of thigh, depending on site of lesion

• See above

L4-S2

(epiconus, spinal nerve roots paralyzed)

• Distal paraplegia

• Sensory loss from front of thigh, dorsum of foot, sole of foot, or back of thigh, depending on site of lesion

• Flaccid paralysis of bladder and rectum

• Impotence

S3-S5

(conus)

• No deficit

• Sensory loss in perianal region and inside of thigh

• See above

C Determining the level of spinal cord lesions a Muscles and the spinal cord segments that innervate them. Most muscles are multisegmental, i.e., they receive innervation from several spinal cord segments. Thus, for example, a lesion at the C7 level will not necessarily cause complete paralysis of the latissimus dorsi, because that muscle is also innervated by C6. This is not the case with the “indicator muscles,” which are supplied almost exclusively by a single segment (see B, p.347). A lesion at the L3 level, for example, will cause almost complete paralysis of the quadriceps femoris because that muscle is innervated almost entirely by L3. b The degree of disability varies, depending on the level of the complete cord lesion.

22.17 Visual System, Overview and Geniculate Part

A Overview of the visual pathway

Left lateral view. The visual pathway extends from the eye, an anterior prolongation of the diencephalon, back to the occipital pole. Thus it encompasses almost the entire longitudinal axis of the brain. The principal stations are as follows:

Retina. The retina contains the first three neurons of the visual pathway (b):

 First neuron: photoreceptor rods and cones, located on the deep retinal surface opposite to the direction of the incoming light (“inversion of the retina").

 Second neuron: bipolar cells.

 Third neuron: ganglion cells whose axons are collected to form the optic nerve.

Optic nerve, optic chiasm, and optic tract: This neural portion of the visual pathway is part of the central nervous system (optic nerve = cranial nerve II) and is surrounded by meninges. Thus, the optic nerve is actually a tract rather than a true nerve. The optic nerves join below the base of the diencephalon to form the optic chiasm, which then divides into the two optic tracts. Each of these tracts divides in turn into a lateral and medial root.

Lateral geniculate body: Ninety percent of the axons of the third neuron (=90% of the optic nerve fibers) terminate in the lateral geniculate body on neurons that project to the striate area (visual cortex, see below). This is the geniculate part of the visual pathway (discussed here). It is concerned with conscious visual perception and is conveyed by the lateral root of the optic tract. The remaining 10% of the third-neuron axons in the visual pathway do not terminate in the lateral geniculate body. This is the nongeniculate part of the visual pathway (medial root, see B, p. 361), and its signals are not consciously perceived.

Optic radiation and visual cortex (striate area): The optic radiation begins in the lateral geniculate body, forms a band that winds around the inferior and posterior horns of the lateral ventricles, and terminates in the visual cortex or striate area (=Brodmann area 17). Located in the occipital lobe, the visual cortex can be grossly identified by a prominent stripe of white matter in the otherwise gray cerebral cortex (the stria of Gennari, see c). This white stripe runs parallel to the brain surface and is shown in the inset, where the gray matter of the visual cortex is shaded light red.

В Representation of each visual field in the contralateral visual cortex

Superior view. The light rays in the nasal part of each visual field are projected to the temporal half of the retina, while those from the temporal part are projected to the retinal half. Because of this arrangement, the left half of the visual field projects to the visual cortex of the right occipital pole, and the right half projects to the visual cortex of the left occipital pole. For clarity, each visual field in the diagram is divided into two halves, and the reader should understand this basic division before we explore how the visual fields are divided into four quadrants (C).

Note: The axonal fibers from the nasal half of each retina cross to the opposite side at the optic chiasm and then travel with the uncrossed fibers from the temporal half of each retina.

C Topographic organization of the geniculate part of the visual pathway

The fovea centralis, the point of maximum visual acuity on the retina, has a high receptor density. Accordingly, a great many axons pass centrally from its receptors, and so the fovea centralis is represented by an exceptionally large area in the visual cortex. Other, more peripheral portions of the retina contain fewer receptors and therefore fewer axons, resulting in a smaller representational area in the visual cortex.

Note: Only the left half of the complete visual field is shown. It is subdivided into four quadrants (clockwise from top left in 1): upper temporal, upper nasal, lower nasal, and lower temporal. The representation of this subdivision is continued into the visual cortex.

1 The three zones that make up a particular visual hemifield (left, in this case) are each indicated by color shading of decreasing intensity:

 The smallest and darkest zone is at the center of the fovea centralis; it corresponds to the central visual field.

 The largest zone is the macular visual field, which also contains the “blind spot” (= optic disk, see 2).

• The “temporal crescent” represents the temporal, monocular part of the visual field.

• Note that the lower nasal quadrant of each visual field is indented by the nose (small medial depression).

2 Because all light that reaches the retina must first pass through the narrow pupil (which is like the aperture of a camera), up/down and temporal/nasal are exactly reversed when the image is projected onto the retina.

3,4 In the initial part of the optic nerve, the fibers that represent the macular visual field first occupy a lateral position (3) and then move increasingly toward the center of the nerve (4).

5 In traversing the optic chiasm, the nasal fibers of the optic nerve cross the midline to the opposite side.

6 At the start of the optic tract, the fibers from the corresponding halves of the retinae unite—the right halves of the retinae in the right tract, the left halves in the left tract. The impulses from the right visual field finally terminate in the left striate area. Initially the macular fibers continue to occupy a central position in the optic tract.

7 At the end of the optic tract, just before it enters the lateral geniculate body, the fibers are collected to form a wedge.

8 In the lateral geniculate body, the wedge shape is preserved, the macular fibers occupying almost half the wedge. After the fibers are relayed to the fourth neuron, they project to the posterior end of the occipital pole (= visual cortex).

9 This figure shows that the central part of the visual field is represented by the largest area in the visual cortex compared with other portions of the field. This is due to the large number of axons that run to the optic nerve from the fovea centralis. This large proportion of axons is continued into the visual cortex, establishing a point-to- point (retinotopic) correlation between the fovea centralis and the visual cortex. The other parts of the visual field also show a point-to- point correlation but have fewer axons. The central lower half of the visual field is represented by a large area on the occipital pole above the calcarine sulcus, while the central upper half of the visual field is represented below the sulcus. The region of central vision also occupies the largest area within the lateral geniculate body (see 8).

D Informal visual field examination with the confrontation test

The visual field examination is an essential step in the examination of lesions of the visual pathway (see A,p.360). The confrontation test is an informal test in which the examiner (with an intact visual field) and the patient sit face-to- face, cover one eye, and each fixes their gaze on the other’s open eye, creating identical visual axes. The examiner then moves his or her index finger from the outer edge of the visual field toward the center until the patient signals that he or she can see the finger. With this test the examiner can make a gross assessment as to the presence and approximate location of a possible visual field defect. The precise location and extent of a visual field defect can be determined by perimetry, in which points of light replace the examiner’s finger. The results of the test are entered in charts that resemble the small diagrams in C.

22.18 Visual System, Lesions and Nongeniculate Part

A Visual field defects (scotomata) and their location along the visual pathway

Visual field defects and lesion sites are illustrated here for the left visual pathway. Lesions of the visual pathway may result from a large number of neurological diseases. The patient perceives the lesion as a visual disturbance. Because the nature of the visual field defect often points to the location of the lesion, it is clinically important to know the patterns of defects that maybe encountered. Division of the visual field into four quadrants is helpful in determining the location of a lesion. The quadrants are designated as upper and lower tern рога I, and upper and lower nasal (see also p.359).

1 A unilateral optic nerve lesion produces blindness (amaurosis) in the affected eye only.

2 A lesion of the optic chiasm causes bitemporal hemianopia (as in a horse wearing blinders) because it interrupts the fibers from the nasal portions of the retina (the only ones that cross in the optic chiasm), which represent the temporal visual fields

3 A unilateral lesion of the optic tract causes contralateral homonymous hemianopia because it interrupts fibers from the temporal portions of the retina on the ipsilateral side and the nasal portions on the opposite side. Thus the right or left half of the visual field is affected in each eye.

Note: All homonymous visual field defects are caused by a retro- chiasmal lesion.

4 A unilateral lesion of the optic radiation in the anterior temporal lobe (Meyer’s loop) leads to contralateral upper quadrantanopia (a "pie-in-the sky"deficit). This occurs because the affected fibers wind around the inferior horn of the lateral ventricle in the temporal lobe and are separated from the fibers that come from the lower half of the visual field (see p. 358).

5 A unilateral lesion in the medial part of the optic radiation in the parietal lobe leads to contralateral lower quadrantanopia. This occurs because the fibers course superior to those for the upper quadrant in Meyer’s loop (see p. 358).

6 A lesion of the occipital lobe leads to homonymous hemianopia. Because the optic radiation fans out widely before entering the visual cortex, lesions of the occipital lobe have been described that spare foveal vision. These lesions are most commonly due to intracerebral hemorrhage. The visual field defects may vary considerably because of the variable size of the hemorrhage.

7 A lesion confined to the cortical areas of the occipital pole, which represent the macula, is characterized by a homonymous hemia- nopic central scotoma.

В Nongeniculate part of the visual pathway

Approximately 10% of the axons of the optic nerve do not terminate on neurons in the lateral geniculate body for projection to the visual cortex. They continue along the medial root of the optic tract, forming the nongeniculate part of the visual pathway. The information from these fibers is not processed at a conscious level but plays an important role in the unconscious regulation of various vision-related processes and in visually mediated reflexes (e.g., the afferent limb of the pupillary light reflex). Axons from the nongeniculate part of the visual pathway terminate in the following regions:

 Axons to the superior colliculus: transmit kinetic information that is necessary for tracking moving objects by unconscious eye and head movements (retinotectal system).

 Axons to the pretectal area: afferents for pupillary responses and accommodation reflexes (retinopretectal system). Subdivision into specific nuclei has not yet been accomplished in humans, and so the term “area” is used.

 Axons to the suprachiasmatic nucleus of the hypothalamus: influence circadian rhythms.

 Axons to the thalamic nuclei (optic tract) in the tegmentum of the mesencephalon and to the vestibular nuclei: afferent fibers for optokinetic nystagmus (=jerky, physiological eye movements during the tracking of fast-moving objects). This has also been called the “accessory visual system.”

 Axons to the pulvinar of the thalamus: visual association cortex for oculomotor function (neurons are relayed in the superior colliculus).

 Axons to the parvocellular nucleus of the reticular formation: arousal function.

C Brainstem reflexes: clinical importance of the nongeniculate part of the visual pathway

Brainstem reflexes are important in the examination of comatose patients. Loss of all brainstem reflexes is considered evidence of brain death. Three of these reflexes are described below:

Pupillary reflex: The pupillary reflex relies on the nongeniculate parts of the visual pathway (see p. 363). The afferent fibers for this reflex come from the optic nerve, which is an extension of the diencephalon (since the diencephalon is not part of the brainstem, “brainstem reflex” is a somewhat unfortunate term). The efferents for the pupillary reflex come from the accessory nucleus of the oculomotor nerve (CN III), which is located in the brainstem. Loss of the pupillary reflex may signify a lesion of the diencephalon (interbrain) or mesencephalon (midbrain). Vestibulo-ocular reflex: Irrigating the ear canal with cold water in a normal individual evokes nystagmus that beats toward the opposite side (afferent fibers are conveyed in the vestibulocochlear nerve = CN VIII, efferent fibers in the oculomotor nerve = CN III). When the vestibulo-ocular reflex is absent in a comatose patient, it is considered a poor sign because this reflex is the most reliable clinical test of brainstem function. Corneal reflex: This reflex is not mediated by the visual pathway. The afferent fibers for the reflex (elicited by stimulation of the cornea, as by touching it with a sterile cotton wisp) are conveyed in the trigeminal nerve and the efferent fibers (contraction of the orbicularis oculi in response to corneal irritation) in the facial nerve. The relay center for the corneal reflex is located in the pontine region of the brainstem.

22.19 Visual System: Reflexes

A Pathways for convergence and accommodation

When the head moves closer to an object, the visual axes of the eyes must move closer together (convergence) and simultaneously the lenses must adjust their focal length (accommodation). Both processes are necessary for a sharp, three-dimensional visual impression. Three subprocesses can be identified in convergence and accommodation:

1. In convergence, the two medial rectus muscles move the ocular axis inward to keep the image of the approaching object on the fovea centralis.

2. In accommodation, the curvature of the lens is increased to keep the image of the object sharply focused on the retina. The lens is flattened by contraction of the lenticular fibers, which are attached to the ciliary muscle. When the ciliary muscle contracts during accommodation, it relaxes the tension on the lenticular fibers, and the intrinsic pressure of the lens causes it to assume a more rounded shape.

3. The pupil is constricted by the sphincter pupillae to increase visual acuity.

Convergence and accommodation may be conscious (fixing the gaze on a near object) or unconscious (fixing the gaze on an approaching automobile). Most of the axons of the third neuron in the visual pathway course in the optic nerve to the lateral geniculate body. There they are relayed to the fourth neuron, whose axons project to the primary visual cortex (area 17). Axons from the secondary visual area (19) finally reach the pretectal area by way of synaptic relays and interneurons. Another relay occurs at that level, and the axons from these neurons terminate in Perlia’s nucleus, which is located between the two Edinger-Westphal nuclei (= visceral oculomotor nuclei). Two functionally distinct groups of neurons are located in Perlia’s nucleus:

* For accommodation, one group of neurons relays impulses to the somatomotor oculomotor nucleus, whose axons pass directly to the medial rectus muscle.

 The other group relays the neurons responsible for accommodation and pupillary constriction to the visceromotor (parasympathetic) accessory nuclei of the oculomotor nerve (parasympathetic innervation is illustrated here for one side only).

After synapsing in this nuclear region, the preganglionic parasympathetic axons pass to the ciliary ganglion, where the central neuron synapses with the peripheral parasympathetic neuron. Again, two groups of neurons are distinguished: one passes to the ciliary muscle (accommodation) and the other to the pupillary sphincter (pupillary constriction). The pupillary sphincter light response is abolished in tertiary syphilis, while accommodation (ciliary muscle) and convergence are preserved. This phenomenon, called an Argyll Robertson pupil, suggests that the connections to the ciliary and pupillary sphincter muscles are mediated by different tracts, although the anatomy of these tracts is not yet fully understood.

В Regulation of pupillary size—the light reflex

The pupillary light reflex enables the eye to adapt to varying levels of brightness. When a large amount of light enters the eye, like the beam of a flashlight, the pupil constricts (to protect the photoreceptors in the retina); when the light fades, the pupil dilates. As the term “reflex” implies, this adaptation takes place without conscious input (nongeniculate part of the visual pathway).

Afferent limb of the light reflex: The first three neurons (first neurons: rods and cones; second neurons: bipolar cells; third neurons: ganglion cells) in the afferent limb of the light reflex are located in the retina. The axons from the ganglion cells form the optic nerve. The axons responsible for the light reflex (light blue) pass to the pretectal area (nongeniculate part of the visual pathway) in the medial root of the optic tract. The other axons pass to the lateral geniculate body (dark blue). After synaps- ing in the pretectal nucleus, the axons from the fourth neurons pass to the parasympathetic nuclei (accessory nuclei of the oculomotor nerve = Edinger-Westphal nuclei) of the oculomotor nerve. Because both sides are innervated, a consensual light response can occur (see below). Efferent limb of the light reflex: The fifth neurons located in the Edinger-Westphal nucleus (central parasympathetic neurons) distribute their axons to the ciliary ganglion. There they are relayed to the sixth neurons (peripheral parasympathetic neurons), whose axons then pass to the pupillary sphincter.

The direct pupillary light response is distinguished from the indirect response:

The direct light response is tested by covering both eyes of the conscious, cooperative patient and then uncovering one eye. After a short latency period, the pupil of the light-exposed eye will contract.

To test the indirect light response, the examiner places his hand on the bridge of the patient’s nose, shading one eye from the beam of a flashlight while shining it into the other eye. The object is to test whether shining the light into one eye will cause the pupil of the shaded eye to contract as well (consensual light response).

Loss of the light response due to certain lesions: With a unilateral optic nerve lesion, shining a light into the affected side will induce no direct light response on the affected side. The consensual light response on the opposite side will also be lost because of impairment of the afferent limb of the light response on the affected side. Illumination of the unaffected side will, of course, elicit pupillary contraction on that side (direct light response). A consensual light response is also present because the afferent signals for this reflex are mediated by the unaffected side while the efferent signals are not mediated by the optic nerve. With a lesion of the parasympathetic oculomotor nucleus or ciliary ganglion, the efferent limb of the reflex is lost. In either case the patient has no direct or indirect pupillary light response on the affected side. A lesion of the optic radiation or visual cortex (geniculate part of the visual pathway) does not abolish this reflex, as it will affect only the geniculate part of the visual pathway.

22.20 Visual System:

Coordination of Eye Movement

A Oculomotor nuclei and their higher connections in the brainstem

a Midsagittal section viewed from the left side, b Circuit diagram showing the supranuclear organization of eye movements.

When we shift our gaze to a new object, we swiftly move the axis of vision of our eyes toward the intended target. These rapid, precise, “ballistic" eye movements are called saccades. They are preprogrammed and, once initiated, cannot be altered until the end of the saccadic movement. The nuclei of all the nerves that supply the eye muscles (nuclei of cranial nerves III, IV, and VI, shaded red) are involved in carrying out these movements. They are interconnected for this purpose by the medial longitudinal fasciculus (shaded blue; see В for its location). Because these complex movements essentially involve all of the extraocular muscles and the nerves supplying them, the activity of the nuclei must be coordinated at a higher or supranuclear level. This means, for example, that when we gaze to the right with the right eye, the right lateral rectus muscle (CN VI, abducent nucleus activated) must contract while the right medial rectus muscle (CN III, oculomotor nucleus inhibited) must relax. Forthe/efteye.the left lateral rectus (CNVI) must relax while the left medial rectus (CN III) must contract. Movements of this kind that involve both eyes are called conjugate eye movements. These movements are coordinated by several centers (premotor nuclei, shaded purple). Horizontal gaze movements are programmed in the nuclear region of the paramedian pontine reticular formation (PPRF), while vertical gaze movements are programmed in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). Both gaze centers establish bilateral connections with the nuclei of cranial nerves III, IV, and VI. The tonic signals for maintaining the new eye position originate from the nucleus prepositus hypoglossi (see a).

B Course of the medial longitudinal fasciculus in the brainstem

Midsagittal section viewed from the left side. The medial longitudinal fasciculus runs anterior to the cerebral aqueduct on both sides and continues from the mesencephalon to the cervical spinal cord. It transmits fibers for the coordination of conjugate eye movements. A lesion of the MLF results in internuclear ophthalmoplegia (see C).

C Lesion of the medial longitudinal fasciculus and internuclear ophthalmoplegia

The medial longitudinal fasciculus interconnects the oculomotor nuclei and also connects them with the opposite side (b). When this “information highway” is interrupted, internuclear ophthalmoplegia develops. This type of lesion most commonly occurs between the nucleus of the abducent nerve and the oculomotor nucleus. It may be unilateral or bilateral. Typical causes are multiple sclerosis and diminished blood flow. The lesion is manifested by the loss of conjugate eye movements (a). With a lesion of the left medial longitudinal fasciculus, as shown here, the left medial rectus muscle is no longer activated during gaze to the right. The eye cannot be moved inward on the side of the lesion (loss of the medial rectus), and the opposite eye goes into an abducting nystagmus (lateral rectus is intact and innervated by the abducent nerve). Reflex movements such as convergence are not impaired, as there is no peripheral or nuclear lesion and this reaction is not mediated by the medial longitudinal fasciculus.

22.21 Auditory Pathway

A Afferent auditory pathway of the left ear

The receptors of the auditory pathway are the inner hair cells of the organ of Corti. Because they lack neural processes, they are called secondary sensory cells. They are located in the cochlear duct of the basilar membrane and are studded with stereocilia, which are exposed to shearing forces from the tectorial membrane in response to a traveling wave. This causes bowing of the stereocilia (see p. 151 ). These bowing movements act as a stimulus to evoke cascades of neural signals. Dendritic processes of the bipolar neurons in the spiral ganglion pick up the stimulus. The bipolar neurons then transmit impulses via their axons, which are collected to form the cochlear nerve, to the anterior and posterior cochlear nuclei. In these nuclei the signals are relayed to the second neuron of the auditory pathway. Information from the cochlear nuclei is then transmitted via 4-6 nuclei to the primary auditory cortex, where the auditory information is consciously perceived (analogous to the visual cortex). The primary auditory cortex is located in the transverse temporal gyri (Heschl gyri, Brodmann area 41). The auditory pathway thus contains the following key stations:

 Inner hair cells in the organ of Corti

 Spiral ganglion

 Anterior and posterior cochlear nuclei

 Nucleus of the trapezoid body and superior olivary nucleus

 Nucleus of the lateral lemniscus

 Inferior collicular nucleus

 Nucleus of medial geniculate body

 Primary auditory cortex in the temporal lobe (transverse temporal gyri = Heschl gyri or Brodmann area 41)

The individual parts of the cochlea are correlated with specific areas in the auditory cortex and its relay stations. This is known as the tonotopic organization of the auditory pathway. This organizational principle is simi- larto that in the visual pathway. Binaural processing of the auditory information (= stereo hearing) first occurs at the level of the superior olivary nucleus. At all further stages of the auditory pathway there are also interconnections between the right and left sides of the auditory pathway (for clarity, these are not shown here). A cochlea that has ceased to function can sometimes be replaced with a cochlear implant.

В The stapedius reflex

When the volume of an acoustic signal reaches a certain threshold, the stapedius reflex triggers a contraction of the stapedius muscle. This reflex can be utilized to test hearing without the patient’s cooperation (“objective” auditory testing). The test is done by introducing a sonic probe into the ear canal and presenting a test noise to the tympanic membrane. When the noise volume reaches a certain threshold, it evokes the stapedius reflex and the tympanic membrane stiffens. The change in the resistance of the tympanic membrane is then measured and recorded. The afferent limb of this reflex is in the cochlear nerve. Information is conveyed to the facial nucleus on each side by way of the superior olivary nucleus. The efferent limb of this reflex is formed by special visceromotor fibers of the facial nerve.

C Efferent fibers from the olive to the Corti organ

Besides the afferent fibers from the organ of Corti (see A, shown here in blue), which form the vestibulocochlear nerve, there are also efferent fibers (red) that pass to the organ of Corti in the inner ear and are concerned with the active preprocessing of sound (“cochlear amplifier”) and acoustic protection. The efferent fibers arise from neurons that are located in either the lateral or medial part of the superior olive and project from there to the cochlea (lateral or medial olivocochlear bundle). The fibers of the lateral neurons pass uncrossed to the dendrites of the inner hair cells, while the fibers of the medial neurons cross to the opposite side and terminate at the base of the outer hair cells, whose activity they influence. When stimulated, the outer hair cells can actively amplify the traveling wave. This increases the sensitivity of the inner hair cells (the actual receptor cells). The activity of the efferents from the olive can be recorded as otoacoustic emissions (OAE). This test can be used to screen for hearing abnormalities in newborns.

22.22 Vestibular System

A Central connections of the vestibular nerve

Three systems are involved in the regulation of human balance:

 Vestibular system

 Proprioceptive system

 Visual system

The latter two systems have already been described. The peripheral receptors of the vestibular system are located in the membranous labyrinth (see petrous bone, pp. 140,152), which consists of the utricle and saccule and the ampullae of the three semicircular ducts. The maculae of the utricle and saccule respond to linear acceleration, while the semicircular duct organs in the ampullary crests respond to angular (rotational) acceleration. Like the hair cells of the inner ear, the receptors of the vestibular system are secondary sensory cells. The basal portions of the secondary sensory cells are surrounded by dendritic processes of bipolar neurons. Their perikarya are located in the vestibular ganglion. The axons from these neurons form the vestibular nerve and terminate in the four vestibular nuclei (see C). Besides input from the vestibular apparatus, these nuclei also receive sensory input (seeB). The vestibular nuclei show a topographical organization (seeC) and distribute their efferent fibers to three targets:

* Motor neurons in the spinal cord via the lateral vestibulospinal tract. These motor neurons help to maintain upright stance, mainly by increasing the tone of extensor muscles.

* Flocculonodular lobe of the cerebellum (archicerebellum) via vestibulocerebellar fibers.

* Ipsilateral and contralateral oculomotor nuclei via the ascending part of the medial longitudinal fasciculus.

В Central role of the vestibular nuclei in the maintenance of balance

The afferent fibers that pass to the vestibular nuclei and the efferent fibers that emerge from them demonstrate the central role of these nuclei in maintaining balance. The vestibular nuclei receive afferent input from the vestibular system, proprioceptive system (position sense, muscles, and joints), and visual system. They then distribute efferent fibers to nuclei that control the motor systems important for balance. These nuclei are located in the:

 Spinal cord (motor support),

 Cerebellum (fine control of motor function), and

 Brainstem (oculomotor nuclei for oculomotor function).

Efferents from the vestibular nuclei are also distributed to the following regions:

 Thalamus and cortex (spatial sense)

 Hypothalamus (autonomic regulation: vomiting in response to vertigo) Note: Acute failure of the vestibular system is manifested by rotary vertigo.

C Vestibular nuclei: topographic organization and central connections

Four nuclei are distinguished:

 Superior vestibular nucleus (of Bechterew)

 Lateral vestibular nucleus (of Deiters)

 Medial vestibular nucleus (of Schwalbe)

 Inferior vestibular nucleus (of Roller)

The vestibular system has a topographic organization:

 The afferent fibers of the saccular macula terminate in the inferior vestibular nucleus and lateral vestibular nucleus.

 The afferent fibers of the utricular macula terminate in the medial part of the inferior vestibular nucleus, the lateral part of the medial vestibular nucleus, and the lateral vestibular nucleus.

• The afferent fibers from the ampullary crests of the semicircular canals terminate in the superior vestibular nucleus, the upper part of the inferior vestibular nucleus, and the lateral vestibular nucleus.

The efferent fibers from the lateral vestibular nucleus pass to the lateral vestibulospinal tract. This tract extends to the sacral part of the spinal cord, its axons terminating on motor neurons. Functionally it is concerned with keeping the body upright, chiefly by increasing the tone of the extensor muscles. The vestibulocerebellar fibers from the other three nuclei act through the cerebellum to modulate muscular tone. All four vestibular nuclei distribute ipsilateral and contralateral axons via the medial longitudinal fasciculus to the three motor nuclei of the nerves to the extraocular muscles (i.e., the nuclei of the abducent, trochlear, and oculomotor nerves).

22.23 Gustatory System (Taste)

A Gustatory pathway

The receptors for the sense of taste are the taste buds of the tongue (seeB). Unlike other receptor cells, the receptor cells of the taste buds are specialized epithelial cells (secondary sensory cells, as they do not have an axon). When these epithelial cells are chemically stimulated, the base of the cells releases glutamate, which stimulates the peripheral processes of afferent cranial nerves. These different cranial nerves serve different areas of the tongue. It is rare, therefore, for a complete loss of taste (ageusia) to occur.

 The anterior two-thirds of the tongue are supplied by the facial nerve (CN VII), the afferent fibers first passing in the lingual nerve (branch of the trigeminal nerve) and then in the chorda tympani to the geniculate ganglion of the facial nerve.

 The posterior third of the tongue and the vallate papillae are supplied by the glossopharyngeal nerve (CN IX).

 The epiglottis is supplied by the vagus nerve (CN X).

Peripheral processes from pseudounipolar ganglion cells (which correspond to pseudounipolar spinal ganglion cells) terminate on the taste buds. The central portions of these processes convey taste information to the gustatory part of the nucleus of the solitary tract. Thus, they function as the first afferent neuron of the gustatory pathway. Their perikarya are located in the geniculate ganglion for the facial nerve, in the inferior (petrosal) ganglion for the glossopharyngeal nerve, and in the inferior (nodose) ganglion for the vagus nerve. After synapsing in the gustatory part of the nucleus of the solitary tract, the axons from the second neuron are believed to terminate in the medial parabrachial nucleus, where they are relayed to the third neuron. Most of the axons from the third neuron cross to the opposite side and pass in the dorsal trigeminothalamic tract to the contralateral ventral posteromedial nucleus of the thalamus. Some of the axons travel uncrossed in the same structures. The fourth neurons of the gustatory pathway, located in the thalamus, project to the postcentral gyrus and insular cortex, where the fifth neuron is located. Collaterals from the first and second neurons of the gustatory afferent pathway are distributed to the superior and inferior salivatory nuclei. Afferent impulses in these fibers induce the secretion of saliva during eating (“salivary reflex"). The parasympathetic preganglionic fibers exit the brainstem via cranial nerves VII and IX (see the descriptions of these cranial nerves for details). Besides this purely gustatory pathway, spicy foods may also stimulate trigeminal fibers (not shown), which contribute to the sensation of taste. Finally, olfaction (the sense of smell), too, is a major component of the sense of taste as it is subjectively perceived: patients who cannot smell (anosmosia) report that their food tastes abnormally bland.

В Organization of the taste receptors in the tongue

The human tongue contains approximately 4600 taste buds in which the secondary sensory cells for taste perception are collected. The taste buds (seeC) are embedded in the epithelium of the lingual mucosa and are located on the surface expansions of the lingual mucosa—the vallate papillae (principal site, b), the fungiform papillae (c), and the foliate papillae (d). Additionally, isolated taste buds are located in the mucous membranes of the soft palate and pharynx. The surrounding serous glands of the tongue (Ebner glands), which are most closely associated with the vallate papillae, constantly wash the taste buds clean to allow for new tasting. Humans can perceive five basic taste qualities: sweet, sour, salty, bitter, and a fifth “savory” quality, called umami, which is activated by glutamate (a taste enhancer).

C Microscopic structure of a taste bud

Nerves induce the formation of taste buds in the oral mucosa. Axons of cranial nerves VII, IX, and X grow into the oral mucosa from the basal side and induce the epithelium to differentiate into the light and dark taste cells (= modified epithelial cells). Both types of taste cell have microvilli that extend to the gustatory pore. For sour and salty, the taste cell is stimulated by hydrogen ions and other cations. The other taste qualities are mediated by receptor proteins to which the low-molecular- weight flavored substances bind (details may be found in textbooks of physiology). When the low-molecular-weight flavored substances bind to the receptor proteins, they induce signal transduction that causes the release of glutamate, which excites the peripheral processes of the pseudounipolar neurons of the three cranial nerve ganglia. The taste cells have a life span of approximately 12 days and regenerate from cells at the base of the taste buds, which differentiate into new taste cells.

Note: The old notion that particular areas of the tongue are sensitive to specific taste qualities has been found to be false.

22.24 Olfactory System (Smell)

A Olfactory system: the olfactory mucosa and its central connections

Olfactory tract viewed in midsagittal section (a) and from below (b). The olfactory mucosa is located in the roof of the nasal cavity. The olfactory cells (= primary sensory cells) are bipolar neurons. Their peripheral receptor-bearing processes terminate in the epithelium of the nasal mucosa, while their central processes pass to the olfactory bulb (see В for details). The olfactory bulb, where the second neurons of the olfactory pathway (mitral and tufted cells) are located, is considered an extension of the telencephalon. The axons of these second neurons pass centrally as the olfactory tract In front of the anterior perforated substance, the olfactory tract widens to form the olfactory trigone and splits into the lateral and medial olfactory striae.

 Some of the axons of the olfactory tract run in the lateral olfactory stria to the olfactory centers: the amygdala, semilunar gyrus, and ambient gyrus. The prepiriform area (Brodmann area 28) is considered to be the primary olfactory cortex in the strict sense. It contains the third neurons of the olfactory pathway.

Note: The prepiriform area is shaded in b, lying at the junction of the basal side of the frontal lobe and the medial side of the temporal lobe.

 Other axons of the olfactory tract run in the medial olfactory stria to nuclei in the septal (subcallosal) area, which is part of the limbic system (see p. 374), and to the olfactory tubercle, a small elevation in the anterior perforated substance.

 Yet other axons of the olfactory tract terminate in the anterior olfactory nucleus, where the fibers that cross to the opposite side branch off and are relayed. This nucleus is located in the olfactory trigone, which lies between the two olfactory striae and in front of the anterior perforated substance.

Note: None of these three tracts are routed through the thalamus. Thus, the olfactory system is the only sensory system that is not relayed in the thalamus before reaching the cortex. There is, however, an indirect route from the primary olfactory cortex to the neocortex passing throug the thalamus and terminating in the basal forebrain. The olfactory signals are further analyzed in these basal portions of the forebrain (not shown).

The olfactory system is linked to other brain areas well beyond the primary olfactory cortical areas, with the result that olfactory stimuli can evoke complex emotional and behavioral responses. Noxious smells may induce nausea, while appetizing smells evoke watering of the mouth. Presumably these sensations are processed by the hypothalamus, thalamus, and limbic system (see next unit) via connections established mainly by the medial forebrain bundle and the medullary striae of the thalamus. The medial forebrain bundle distributes axons to the following structures:

 Hypothalamic nuclei

 Reticular formation

 Salivatory nuclei

 Dorsal vagal nucleus

The axons that run in the medullary striae of the thalamus terminate in the habenular nuclei. This tract also continues to the brainstem, where it stimulates salivation in response to smell.

В Olfactory mucosa and vomeronasal organ (VNO)

The olfactory mucosa occupies an area of approximately 2 cm2 on the roof of each nasal cavity, and 107 primary sensory cells are concentrated in each of these areas (a). At the molecular level, the olfactory receptor proteins are located in the cilia of the sensory cells (b). Each sensory cell has only one specialized receptor protein that mediates signal transduction when an odorant molecule binds to it. Although humans are microsmatic, having a sense of smell that is feeble compared with other mammals, the olfactory receptor proteins still make up 2 % of the human genome. This underscores the importance of olfaction in humans. The primary olfactory sensory cells have a lifespan of approximately 60 days and regenerate from the basal cells (life-long division of neurons). The bundled central processes (axons) from hundreds of olfactory cells form olfactory fibers (a) that pass through the cribriform plate of the ethmoid bone and terminate in the olfactory bulb (seeC), which lies above the cribriform plate. The vomeronasal organ (c) is located on both sides of the anterior nasal septum. Its central connections in humans are unknown. It responds to steroids and evokes unconscious reactions in subjects (possibly influences the choice of a mate). Mate selection in many animal species is known to be mediated by olfactory impulses that are perceived in the vomeronasal organ.

C Synaptic patterns in an olfactory bulb

Specialized neurons in the olfactory bulb, called mitral cells, form apical dendrites that receive synaptic contact from the axons of thousands of primary sensory cells. The dendrite plus the synapses make up the olfactory glomeruli. Axons from sensory cells with the same receptor protein form glomeruli with only one or a small number of mitral cells. The basal axons of the mitral cells form the olfactory tract. The axons that run in the olfactory tract project primarily to the olfactory cortex but are also distributed to other nuclei in the CNS. The axon collaterals of the mitral cells pass to granule cells: both granule cells and periglomerular cells inhibit the activity of the mitral cells, causing less sensory information to reach higher centers. These inhibitory processes are believed to heighten olfactory contrast, which aids in the more accurate perception of smells. The tufted cells, which also project to the primary olfactory cortex, are not shown.

22.25 Limbic System

A Limbic system viewed through the partially transparent cortex

Medial view of the right hemisphere. The term “limbic system" (Latin limbus = “border” or “fringe”) was first used by Broca in 1878, who collectively described the gyri surrounding the corpus callosum, diencephalon, and basal ganglia as the grand lobe limbique. The limbic system encompasses neo-, archi- and paleocortical regions as well as subcortical nuclei. The anatomical extent of the limbic system is such that it can exchange and integrate information between the telencephalon (cerebral cortex), diencephalon, and mesencephalon. Viewed from the medial aspect of the cerebral hemispheres, the limbic system is seen to consist of an inner arc and an outer arc. The outer arc is formed by:

• Parahippocampal gyrus,

• Cingulate gyrus (also called the limbic gyrus),

• Subcallosal area (paraolfactory area), and

• Indusium griseum.

The inner arc is formed by:

* Hippocampal formation,

* Fornix,

* Septal area (also known simply as the septum),

* Diagonal band of Broca (not visible in this view), and

* Paraterminal gyrus.

The limbic system also includes the amygdalae and mammillary bodies. The following nuclei are also considered part of the limbic system but are not shown: the anterior thalamic nucleus, habenular nucleus, dorsal tegmental nucleus, and interpeduncular nucleus.

The limbic system is concerned with the regulation of drive and affective behavior and plays a crucial role in memory and learning. The numbers in the diagram indicate the Brodmann areas.

В Neuronal circuit (Papez circuit)

View of the medial surface of the right hemisphere. Several nuclei of the limbic system are interconnected by a neuronal circuit (see below) called the Papez circuit after the anatomist who first described it. The sequence below indicates the nuclei (normal print) and tracts (italic print) that are the successive stations of this neuronal circuit:

Hippocampus → fornix → mammillary body → mammillothalamic tract (Vicq d’Azyr bundle) → anterior thalamic nuclei → thalamocingular tract (radiation) → cingulate gyrus → cingulohippocampal fibers → hippocampus.

This neuronal circuit interconnects ontogenically distinct parts of the limbic system. It establishes a connection between information stored in the unconscious and conscious behavior.

C Cytoarchitecture of the hippocampal formation (after Bahr and Frotscher)

View from anterior left.

Note: The hippocampal formation has a threelayered allocortex instead of a six-layered iso- cortex(lowerleftindiagram).ltisaphylogenetically older structure than the isocortex. At the centeroftheallocortexisabandofneuronsthat forms the neuronal layer of the hippocampus (= hippocampus proper = Ammon’s horn). The neurons in this layer are mainly pyramidal cells. Three regions, designated CA1-CA3, can be distinguished based on differnces in the density of the pyramidal cells. Region CA 1, called also the “Sommer sector,” is important in neuropathology, as the death of neurons in this sector is the first morphologically detectable sign of cerebral hypoxia. Besides the hippocampus proper, we can also identify the cellular sheet of the dentate gyrus (dentate fascia), which consists mainly of granule cells.

D Connections of the hippocampus

Left anterior view. The most important afferent pathway to the hippocampus is the perforant path (blue), which extends from the entorhinal region (triangular pyramidal cells of Brodmann area 28) to the hippocampus (where it ends in a synapse). The neurons that project from area 28 into the hippocampus receive afferent input from many brain regions. Thus, the entorhinal region is considered the gateway to the hippocampus. The pyramidal cells of Ammon’s horn (triangles) send their axons into the fornix, and the axons transmitted via the fornix continue to the mammillary body (Papez neuronal circuit) or to the septal nuclei.

E Important definitions pertaining to the limbic system

Archicortex

Phylogeneticallyold structures of the cerebral cortex; does not have a six-layered architecture

Hippocampus (retrocommissural)

Ammon’s horn (hippocampus proper), dentate gyrus (dentate fascia), subiculum (some authors consider it part of the hippocampal formation rather than the hippocampus itself)

Hippocampal formation

Hippocampus plus the entorhinal area of the parahippocampal gyrus

Limbic system

Important coordinating system for memory and emotions. Includes the following telencephalic structures: cingulate gyrus, parahippocampal gyrus, hippocampal formation, septal nuclei, and amygdala. Its diencephalic components include the anterior thalamic nucleus, mammillary bodies, nucleus accumbens, and habenular nucleus. Its brainstem components are the raphe nuclei. The medial forebrain bundle and the dorsal longitudinal fasciculus contribute to the fiber tracts of the limbic system.

Periarchicortex

A broad transitional zone around the hippocampus, consisting of the cingulate gyrus, the isthmus of the cingulate gyrus, and the parahippocampal gyrus

A Fiber tracts

Fiber tracts are the “information highways” of the white matter of the brain and spinal cord. The most important terms pertaining to CNS fiber tracts are listed in the table.

Projection fibers

Connect the cerebral cortex to subcortical centers, either ascending or descending

• Ascending fibers

• Descending fibers

Association fibers

Commissural fibers

Fornix

Connect subcortical centers to the cerebral cortex

Connect the cerebral cortex to deeper centers

Connect different cortical areas within one hemisphere

Connect like cortical areas in both hemispheres (= interhemispheric association fibers)

Special projection tract of the limbic system

В Brain specimen prepared to show the structure of the projection fibers

Medial view of the right hemisphere. This type of specimen is prepared by fixing the brain in formaldehyde and then freezing it. The gray matter, which has a high water content, is destroyed by ice-crystal formation, while the lipid-containing white matter remains relatively intact. The frozen brain is then thawed, and the tissue is dissected and teased with a spatula to bring out the fiber architecture of the white matter. The fibers represent bundled axons that pass collectively from their site of origination to their destination. Because the brain has a topographic organization, many equidirectional axons passthrough the white matter as fasciculi (for the designations of different fiber types, see A, above). The projection fibers shown here connect the cerebral cortex to subcortical structures (e.g., basal ganglia, spinal cord). A distinction is drawn between ascending and descending fibers and their systems. In descending systems, the cell bodies of the neurons are located in the cerebral cortex and their axons terminate in subcortical structures (e.g., the corticospinal tract). In ascending systems, the neurons from subcortical structures terminate in the cerebral cortex (e.g., sensory tracts from the spinal cord).

22.26 Brain: Fiber Tracts

C Association fibers

a Lateral view of the left hemisphere, b Anterior view of the right hemisphere. c Anterior view of short association fibers.

Long association fibers interconnect different brain areas that are located in different lobes, whereas short association fibers interconnect cortical areas within the same lobe. Adjacent cortical areas are interconnected by short, U-shaped arcuate fibers, which run just below the cortex.

D Commissural fibers

a Medial view of the right hemisphere, b Superior view of the transparent brain.

Commissural fibers interconnect the two hemispheres of the brain. The most important connecting structure between the hemispheres is the corpus callosum. If the corpus callosum is intentionally divided, as in a neurosurgical procedure, the two halves of the brain can no longer communicate with each other (“split-brain” patient, see p. 380). There are other, smaller commissural tracts besides the corpus callosum (anterior commissure, fornical commissure).

E Somatotopic organization of the internal capsule

Transverse section. Both ascending and descending projection fibers pass through the internal capsule. If blood flow to the internal capsule is interrupted, as by a stroke, these ascending and descending tracts undergo irreversible damage. The figure of the child shows how the sites where the pyramidal tract fibers pass through the internal capsule can be assigned to peripheral areas of the human body. Thus, we see that smaller lesions of the internal capsule may cause a loss of central innervation (= spastic paralysis) in certain areas of the body. This accounts forthe great clinical importance of this structure. The internal capsule is bounded medially by the thalamus and the head of the caudate nucleus, and laterally by the globus pallidus and putamen. The internal capsule consists of an anterior limb, a genu, and a posterior limb, which are traversed by specific tracts:

Anterior limb • Frontopontine tracts (red dashes)

 Anterior thalamic peduncle (blue dashes)

Genu of internal capsule • Corticonuclear fibers (red dots)

Posterior limb • Corticospinal fibers (red dots)

 Posterior thalamic peduncle (blue dots)

 Temporopontine tract (orange dots)

 Posterior thalamic peduncle (light blue dots)

22.27 Brain: Functional Organization

A Functional organization of the neocortex

Left lateral view. The primary sensory and motor areas are shown in red, and the areas of the association cortex are shown in different shades of green. Projection tracts begin or end, respectively, in the primary motor or sensory areas. More than 80% of the cortical surface area is association cortex, which is secondarily connected to the primary sensory or primary motor areas. The neuronal processing of differentiated behavior and intellectual performance takes place in the association cortex, which has increased greatly in size over the course of human evolution. The functional organization pattern shown here, such as the localization of the primary motor cortex in the precentral gyrms, can be demonstrated in living subjects with modern imaging techniques. The results of such studies are illustrated in the figures below. Interestingly, the correlations described in these studies correspond reasonably well with the cortical areas defined by Brodmann.

В Analysis of brain function based on studies of regional cerebral blood flow

Left lateral view of the brain. When neurons are activated they consume more glucose and oxygen, which must be delivered to them via the bloodstream. This may produce a detectable increase in regional blood flow. These brain maps illustrate the local patterns of cerebral blood flow at rest (a) and during movement of the right hand (b).

When the right hand is moved, increased blood flow is recorded in the left precental gyrus, which contains the motor representation of the right hand (see motor homunculus in В on p.339). Simultaneous activation is noted in the sensory cortex of the postcentral region, showing that the sensory cortex is also active during motor function (feedback loop).

C Sex differences in neuronal processing

(after Stoppe, Hentschel, and Munz)

Patterns of brain activity can also be demonstrated by functional magnetic resonance imaging (fMRI). This provides a noninvasive method for investigating the metabolic activity of the brain. Because no human brain is identical to any other, a comparison of several brains will show slight variations in the distribution of specific functions. By superimposing the results of examinations in different brains, we can produce a generalized map that shows the approximate distribution of brain functions. Compare the summation map for female brains on the left with a map for male brains on the right. Both groups of subjects were given phonological tasks based on recognizing differences in the meaning of spoken sounds. While the female subjects activated both sides of their brain when solving the tasks, the male subjects activated only the left side (the sectional images are viewed from below).

D Modulating subcortical centers

The cerebral cortex, the seat of our conscious thoughts and actions, is influenced by various subcortical centers. The parts of the limbic system that are crucial for learning and memory are indicated in light red.

22.28 Brain: Hemispheric Dominance

A Demonstration of hemispheric dominance for language in split- brain patients (after Klinke, Pape, and Silbernagl)

The corpus callosum is by far the most important commissural tract, interconnecting areas of I ike function in both hemispheres of the brain. Because lesions of the corpus callosum were once considered to have no clinical effects, surgical division of the corpus callosum was commonly performed at one time in epileptic patients to keep epileptic seizures from spreading across the brain. This operation interrupts the connections in the upper telencephalon while leaving intact the more deeply situated diencephalon, which contains the optic tract. Patients who have undergone this operation are called “split-brain patients.” They have no obvious clinical abnormalities, but special neuropsychological tests reveal deficits, the study of which has improved our understanding of brain function. In one test the patient sits in front of a screen on which words are projected. Meanwhile, the patient can grasp objects behind the screen without being able to see them. When the word “Ball” is flashed briefly on the left side of the screen, the patient perceives it in the visual cortex on the right side (the optic tract has not been cut). Because language production resides in the left hemisphere in 97% of the population, the patient cannot verbalize the projected word out loud because communication between the hemispheres has been interrupted at the level of the telencephalon (seat of speech production). But the patient is still able to feel the ball manuallyand pick it out from other objects. The function of the corpus callosum is to enable both hemispheres (which can function independently to a degree) to communicate with each other when the need arises. Because of the phenomenon of hemispheric dominance, the corpus callosum in humans is more elaborately developed than in other animal species.

В Hemispheric asymmetry (after Klinke and Silbernagl)

Superior view of the temporal lobe of a brain that has been taken apart (i.e., the frontal lobes have been removed) along the lateral fissure. The planum temporale, located on the posterior and superior surface of the temporal lobe, has different contours on the two sides of the brain, being more pronounced on the left side than on the right in two-thirds of individuals. The functional significance of this asymmetry is uncertain. We cannot explain it simply by noting that Wernicke’s speech area is located in that part of the temporal lobe, because while temporal asymmetry is present in only 67% of the population, the speech area is located on the left side in 97%.

C Language areas in the normally dominant left hemisphere

Lateral view. The brain contains several language areas whose loss is associated with typical clinical symptoms. Wernicke’s area (the posterior part of area 22) is necessary for language comprehension, while Broca’s area (area 44) is concerned with language production. The two areas are interconnected by the superior longitudinal (arcuate) fasciculus. Broca’s area activates the mouth and tongue region of the motor cortex for the articulation of speech. The angular gyrus coordinates the inputs from the visual, acoustic, and somatosensory cortices and relays them onward to Wernicke’s area.

22.29 Brain: Clinical Findings

The figures in this unit illustrate the correlations that have been discovered between specific brain areas and clinical findings. Studies of this kind have enabled us to link particular patterns of behavior, some abnormal, and particular clinical symptoms to specific areas in the brain.

A Neuroanatomy of emotions (after Braus)

a Lateral view of the left hemisphere, b Anterior view of a coronal section through the amygdala, c Midsagittal section of the right hemisphere, medial aspect.

Emotion is linked to specific regions of the brain. The ventromedial prefrontal cortex is connected primarily to the amygdaloid bodies and is believed to modulate emotion, while the dorsolateral prefrontal cortex is connected primarily to the hippocampus. This is the area of the cortex in which memories are stored along with their emotional valence. Abnormalities of this network are believed to play a role in depression.

В Spread of Alzheimer's disease through the brain

(after Braakand Braak)

Medial view of the right hemisphere. Alzheimer’s disease is a relentlessly progressive disease of the cerebral cortex that causes memory loss and, eventually, profound dementia. The progression of the disease can be demonstrated with special staining methods and can be divided into stages using the classification of Braak and Braak:

• Stages 1-М: the appearance of the nerve cells is altered in the periphery of the entorhinal cortex (= transentorhinal region), which is considered part of the allocortex (see p.204). These stages are still asymptomatic.

* Stages III-IV: the lesions have spread to involve the limbic system (also part of the allocortex), and initial clinical symptoms appear. These stages may be detectable by imaging studies in some cases.

 Stages V-VI: the entire isocortex is involved, and the clinical manifestations are fully developed.

Thus, the allocortex is important in brain pathophysiology as the site of origin of Alzheimer’s dementia, even though it makes up only 5 % of the cerebral cortex.

C MRI changes In the hippocampus in a patient with Alzheimer’s dementia

Comparing the brain of a healthy subject (a) with that of a patient with Alzheimer’s dementia (b), we notice that the latter shows atrophy of the hippocampus, a brain region that is part of the allocortex. We notice, too, that the lateral ventricles are enlarged in the patient with Alzheimer dementia (from D.F. Braus: Ein Blickins Gehim. Thieme, Stuttgart 2004).

D Lesions of certain brain areas and associated behavioral changes

(after Poeckand Hartje)

Medial view of the right hemisphere. Bilateral lesions of the medial temporal lobe and the frontal part of the cingulate gyrus (blue dots) lead to a suppression of drive and affect. This structural abnormality in the limbic system produces clinical changes that include apathy, a blank facial expression, monotone speech, and a dull, nonspontaneous mode of be havior. The condition may be caused by tumors, decreased blood flow, or trauma. On the other hand, tumors involving the septum pellucidum and hypothalamus (pink-shaded area) and certain forms of epilepsy may cause a disinhibition of anger, and the patient may respond to seemingly trivial events with attacks of “hypothalamic rage” accompanied by screaming and biting. This outburst is not directed against any particular person or object and persists for some time.


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