Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

19. Spinal Cord

19.1 Spinal Cord, Segmental Organization

A Development of the spinal cord

Transverse section, superior view.

a Early neural tube, b intermediate stage, c adult spinal cord. The spinal cord develops from the neural tube:

 Posterior horn: develops from the posterior part of the neural tube (the alar plate). It contains the afferent (sensory) neurons.

 Anterior horn: develops from the anterior part of the neural tube (the basal plate). It contains the efferent (motor) neurons.

 Lateral column: develops from the intervening zone. Present only in the thoracic, lumbar, and sacral regions of the cord, it contains the autonomic (sympathetic and parasympathetic) neurons. (Its longitudinal distribution is shown in C, p. 283.)

Neurons do not develop from the roof or floor plates. Viewing the spinal cord in transverse section, we see that it consists of gray matter that is arranged about the central canal and is surrounded by white matter. The gray matter contains the cell bodies of neurons while the white matter consists of nerve fibers (axons).

Note: axons that have the same function are collected into bundles called tracts. Tracts that terminate in the brain are called ascending, afferent or sensory tracts, while tracts that pass from the brain into the spinal cord are called descending, efferent or motor tracts.

В Structure of a spinal cord segment

Two main organizational principles are observed in the spinal cord:

1. Functional organization within a segment (viewed in a transverse section of the spinal cord). In each spinal cord segment, the afferent dorsal rootlets enter the back of the cord while the efferent ventral rootlets emerge from the front of the cord. The rootlets in each set combine to form the dorsal (posterior) and ventral (anterior) roots. Each dorsal and ventral root fuses to form a mixed spinal nerve, which carries both sensory and motor fibers. Shortly after the fusion of its two roots, the spinal nerve divides into various branches.

2. Topographical organization of the segments (viewed in a longitudinal section of the spinal cord). The spinal cord consists of a vertical series of 31 segments (see C), each of which innervates a specific area in thetrunkand limbs.

C Spinal cord and spinal ganglia in situ

Posterior view with the laminar arches of the vertebral bodies removed. The longitudinal growth of the spinal cord lags behind that of the bony vertebral column. As a result, the lower end of the spinal cord in the adult lies at approximately the level of the first lumbar vertebral body (LI, see D). Below L1, the spinal nerve roots descend from the end of the cord to the intervertebral foramina, where they join to form the spinal nerves. The collection of these spinal roots is called the cauda equina (“horse’s tail”).

D Spinal cord segments and vertebral bodies in the adult

a Midsagittal section, viewed from the right side. The spinal cord can be divided into four major regions: cervical cord (C, pink); thoracic cord (T, blue); lumbar cord {L, green); and sacral cord (S, yellow). The spinal cord segments are numbered according to the exit point of their associated nerves and do not necessarily correlate numerically with the nearest skeletal element (see b). The spinal cord generally terminates at the level of the L1 vertebral body, and the region be

E Simplified schematic representation of the segmental innervation of the skin

(after Mumenthaler)

Distribution of the dermatomes on the body. Sensory innervation of the skin correlates with the sensory roots of the spinal nerves in D. Every spinal cord segment (except for C1, see below) innervates a particular skin area (= dermatome). From a clinical standpoint, it is important to know the precise correlation of dermatomes with spinal cord segments so that the level of a spinal cord lesion can be determined based on the location of the affected dermatome. For example, a lesion of the C8 spinal nerve root is characterized by a loss of sensation on the ulnar (small-finger) side of the hand.

Note: There is no С1 dermatome because the first spinal nerve is purely motor.

Spinal cord segment

Vertebral body

Spinous process

C8

Inferior margin of C6,

C6

 

superior margin of C7

 

T6

T5

T 4

T12

T10

T9

L 3

Til

Til

SI

T12

T12

low this is known as the cauda equina. The cauda equina consists of dorsal (sensory) and ventral (motor) spinal nerve roots, and provides safe access for introducing a spinal needle to sample CSF (lumbar puncture), b Differential growth of the spinal cord and vertebral column may separate spinal cord segments from their associated skeletal elements, with progressively greater “mismatch” occurring at more caudal levels. It is important to know the relationship of the spinal cord segments to the associated vertebral bodies when assessing injuries to the vertebral column (e.g., spinal fracture and cord lesions, see p. 357). The parts in the table are only approximations and may differ slightly in individual cases.

Note: there are only seven cervical vertebra (C1-C7), but eight pairs of cervical nerves (C1-C8).

19.2 Spinal Cord, Organization of Spinal Cord Segments

A Gray and white matter of the spinal cord

Three-dimensional representation, oblique anterior view from upper left.

a Cray matter, b white matter.

This three-dimensional view shows how the gray matter is divided into three columns:

 Anterior column (anterior horn): contains motor neurons.

 Lateral column (lateral horn): contains sympathetic or parasym pathetic (visceromotor) neurons.

• Posterior column (posterior horn): contains sensory neurons. The gray matter partitions the white matter analogously into anterior, lateral and posterior funiculi. When the spinal cord is viewed in crosssection, the gray-matter columns are traditionally referred to as “horns."

В Principal intrinsic fascicles of the spinal cord (shaded yellow) Three-dimensional representation, oblique anterior view from upper left. Because most of the muscles have a plurisegmental mode of innervation, axons must be able to ascend and descend for multiple segments within the spinal cord in order to coordinate spinal reflexes (see p. 272).

The neurons of these axons originate from interneurons (see p.271 E) in the gray matter, which form the intrinsic reflex pathways of the spinal cord (see p. 273 C). These axons are collected into intrinsic fascicles known also as fasciculi proprii. Arranged chiefly around the gray matter, these bundles make up the “intrinsic circuits” of the spinal cord.

C Position of the spinal cord in the dural sac

a Anterior view with the vertebral bodies partially removed to display the anterior aspect of the spinal cord. The transverse sections (b-e) depict fiber tracts (left side, myelin stain) and neuron cell bodies (right side, Nissl stain) at different levels of the spinal cord. The areas of the cervical and lumbrosacral enlargements have been demarcated (a). In these areas, which provide innervation to the limbs, the gray matter is significantly expanded.

19.3 Spinal Cord:

Internal Divisions of the Cray Matter

A Organizational principles of the anterior column of the spinal cord

Motor neurons that innervate specific muscles are arranged into vertical columns in the anterior (ventral) horn of the gray matter of the spinal cord. Analogous to the brainstem motor nuclei, these columns can themselves be called nuclei, and are arranged in a somatotopic fashion (see В for a mapping of these nuclei to their target muscles). The motor columns innervating the trunk have a relatively simple arrangement that follows the linear segmental organization of spinal nerves and dermatomes. The cervical and lumbrosacral enlargments, which innervate the limbs, have a more complex pattern of innervation than the trunk muscles: during the migratory processes of embryonic development, muscle precursors “саггу” their original innervation with them, generating a motor column that sends its axons through multiple nerve roots from multiple spinal cord levels. The muscles innervated by such a column are accordingly called multisegmental muscles (see B, p. 272). Muscles whose motor neurons are situated entirely within one segment are referred to as indicator muscles; testing the function of indicator muscles is valuable in clinical assessment.

Note: although one muscle may be innervated by axons from multiple spinal segments, those axons arise from a single motor column.

В Somatotopic organization of nuclear columns of the anterior horn (after Bossy)

a Common pattern of organization in the spinal cord. More medial nuclear columns of the anterior horn innervate muscles close to the midline, while more lateral nuclear columns tend to innervate muscles outside the trunk.

b Enlargement of cervical cord. The same pattern of medial-to-lateral organization exists (see a) with medial nuclei innervating axial muscles and lateral nuclei innervating muscles at the extremities.

However, there is also an anterior-to-posterior segregation of motor columns. Neurons serving extensor muscles (shades of blue) are found in the most anterior parts of the anterior horn, while those serving flexor muscles (shades of pink) are found in the more posterior regions. These nuclei are further divided into:

 Medial nuclei: innervate nuchal, back, intercostal, and abdominal muscles

 Anterolateral nucleus: innervates shoulder girdle and upper arm muscles

 Posterolateral nucleus: innervates forearm muscles

 Retroposterolateral nucleus: innervates small muscles of the fingers

C Cell groups in the gray matter of the spinal cord

a Cervical cord, b lumbar cord.

Besides the somatotopic organization of the anterior horn, the gray matter contains a particular pattern of neuron clustering. When the motor columns described in A and В are shown in red and the neurons participating in the sensory pathways are shown in blue, an obvious pattern of functional sequestration can be seen. The larger anterior (ventral) horn contains the motor nuclei, and is the source of the ventral (motor) root of the spinal nerve, whereas the more slender posterior (dorsal) horn contains the cell bodies of secondary sensory neurons and receives the dorsal (sensory) root. The sensory neurons of the posterior horn receive synapses from entering processes of spinal (dorsal root) ganglion cells, and in turn send their axons to other, mostly more cranial, levels.

Note: some ganglion cell axons enter ascending tracts without synapsing locally.

D Synaptic layers in the gray matter

a Cervical cord, b thoracic cord, c lumbar cord. Motor neurons are shown in red, sensory neurons in blue.

The gray matter can also be divided into layers of axon termination, based on cytological criteria. This was first done by the Swedish neuroanatomist Bror Rexed (1914-2002), who divided the gray matter into laminae 1-Х. This laminar architecture is especially well defined in the posterior (dorsal) horn, where primary sensory axons make synapses in specific layers.

E Gray matter neurons of the spinal cord

Motor neurons (neurons which send axons in the ventral root to the spinal nerve and periphery):

 Somatic motor neurons (including alpha and gamma motor neurons)

 Visceral motor neurons: preganglionic neurons which innervate ganglion cells. At thoracolumbar levels these are preganglionic sympathetic neurons; at mid-sacral levels, these are preganglionic parasympathetic motor neurons.

Intrinsic neurons (neurons which send axons to other CNS locations):

 Secondary sensory neurons (tract cells): neurons which send their axons in ascending tracts (white matter). These neurons receive synapses from primary sensory neurons whose cell bodies are in spinal (dorsal root) ganglia.

 Local interneurons: neurons distributed through the gray matter whose axons remain in the local spinal cord (see C, p.273). These include:

- Intercalated cells: neurons whose axons remain at the same segmental level.

- Commissural cells: neurons whose axons cross in the spinal white commissure to the contralateral side.

- Association (intersegmental) cells: neurons whose axons interconnect different spinal segments.

- Renshaw cells: a specific type of inhibitory interneuron that is excited by axon collaterals from alpha motor neurons. The excited Renshaw cell inhibits the motor neuron that stimulated it, and also neighboring motor neurons, creating a negative-feedback loop that modulates the firing rate of the group of neurons. The Renshaw cell also synapses on other local inhibitory neurons, and receives input from descending pathways.

Some of these distinctions are not exact. Tract cells, for instance, have collaterals that synapse locally. Specific intrinsic neuron types like the Renshaw cell have been identified not only by their pattern of connections but also by pharmacological and electrophysiological behavior.

19.4 Spinal Cord:

Reflex Arcs and Intrinsic Circuits

A Integrative function of the gray matter of the spinal cord: reflexes

Afferent nerves are shown in blue, efferent nerves in red. Black indicates neurons of the spinal reflex circuit.

The gray matter of the spinal cord supports muscularfunction at the unconscious (reflex) level, holding the body upright during stance and enabling us to walk and run without conscious control. To perform this coordinating function, the neurons of the gray matter must receive information from the muscles and their surroundings; this information enters the posterior horn of the spinal cord via the axons of neurons in the spinal ganglia (see p. 328). Two types of reflex exist:

• Monosynaptic reflex (left): intrinsic reflex in which information from the periphery (e.g., on muscle length and stretch) comes from the muscle itself. Receptors in the muscle transmit signals to alpha motor neurons via neurons whose cell bodies are in the spinal ganglia. These afferent neurons release excitatory transmitters which cause the alpha motor neurons to stimulate muscle contraction (see D).

• Polysynaptic reflex (right): reflex mediated by receptors in the skin or other sites outside the muscle. These receptors act via interneurons (see C) to stimulate muscular contraction

В Clinically important monosynaptic reflexes

a Biceps reflex, b triceps reflex, c patellar reflex (quadriceps reflex), d Achilles tendon reflex.

The drawings show the muscles, the trigger points for eliciting the reflexes, the nerves involved in the reflexes (afferent nerves in blue, efferent nerves in red), and the corresponding spinal cord segments.

The principal monosynaptic reflex es are should be tested in every physical examination. Each reflex is elicited by briskly tapping theappropriate tendon with a reflex hammer to stretch the muscle. If the muscle contracts in response to this stretch, the reflex arc is intact. Although each test involves just one muscle and one nerve supplying the muscle, the innervation involves several spinal cord segments (= multiseg- mental muscles, see A, p. 270). The right and left sides should always be compared in clinical reflex testing, as this is the only way to recognize a unilateral increase, decrease, or other abnormality.

C Components of the intrinsic circuits of the spinal cord

Afferent neurons are shown in blue, efferent neurons in red. The neurons of the spinal reflex circuits are shown in black. Polysynaptic reflexes often must be coordinated at the spinal cord level by multiple segments. Interneurons, some of whose axons show a T-shaped branching pattern, convey the afferent signals to higher and lower segments along crossed and uncrossed pathways (types of interneurons are described in E, p. 271). These chains of interneurons, which are entirely contained within the spinal cord, make up the intrinsic circuits of the cord. The axons of the neurons in the intrinsic circuits pass to adjacent segments in intrinsic fascicles (fasciculi proprii) located as the edge of the gray matter (see B, p. 268). These fascicles are the conduction apparatus of the intrinsic circuits.

D Effects of the Renshaw cell on the alpha motor neuron

The afferent fibers in a monosynaptic reflex originate in neurons of the spinal ganglia. They terminate on the alpha motor neurons, where they release the excitatory transmitter acetylcholine. In response to this transmitter release, the alpha motor neuron transmits excitatory impulses to the neuromuscular synapse (the transmitter at the synapse is also acetylcholine). The excitatory alpha motor neuron has axon collaterals that enable it to exert a stimulatory effect on an inhibitory interneuron called a Renshaw cell. In response to this stimulation, the Renshaw cell releases the inhibitory transmitter glycine. This self-inhibiting mechanism serves to prevent overexcitation of the alpha motor neurons (recurrent inhibition). The clinical importance of the Renshaw cells is dramatically illustrated in patients with tetanus. The tetanus toxin inhibits the release of glycine from the Renshaw cells. Inhibition of the alpha motor neurons fails to occur, and so the patient experiences sustained (tetanic) muscle contractions.

E Effects of long tracts on the alpha motor neuron

The alpha motor neuron not only receives efferent fibers from the spinal cord itself, but is also strongly modulated by efferent fibers from long tracts that originate in the brain. Most of these efferent fibers have an inhibitory effect on the alpha motor neuron. If these effects are abolished due to a complete cord lesion, for example, the disproportionately strong influence of the spinal intrinsic circuits will lead to spastic paralysis (see p. 343).

19.5 Ascending Tracts of the Spinal Cord:

Spinothalamic Tracts

A Course of the anterior and lateral spinothalamic tracts in a transverse section of the spinal cord

See p. 284 for overview of ascending tracts. The axons of the anterior spinothalamic tract run in the anteriorfuniculus of the spinal cord, while those of the lateral spinothalamic tract run in both the anterior and lateral funiculi. (These two tracts are sometimes referred to collectively as the anterolateral funicular tract.) The anterior spinothalamic tract is the pathway for crude touch and pressure sensation, while the lateral spinothalamic tract conveys pain, temperature, tickle, itch, and sexual sensation. The cell bodies of the primary afferent neurons for both tracts are located in the spinal ganglia. Both tracts contain second neurons and cross in the anterior commissure. The somatotopic organization of the lateral spinothalamic tract is shown on the left side of the diagram. Starting dorsally and moving clockwise, we successively encounter the sacral, lumbar, thoracic, and cervical fibers. In older terminology a distinction is sometimes drawn between epicritic and protopathic sensation. According to this terminology, the anterior and lateral spinothalamic tracts are classified as protopathic pathways while the tracts of the posterior funiculus are classified as an epicritic sensory pathway. Today the original classification has been dropped because it does not correspond well to the assignment of sensory qualities to anatomically defined tracts.

В Anterior spinothalamic tract and its central connections

1 Impulses from tactile corpuscles and from receptors about the hair follicles are carried to the anterior spinothalamic tract by moderately large-caliber myelinated axons (dendritic axons).

2 The cell bodies of these axons are located in the spinal ganglia (first neuron, prim ary afferent neuron).

3 The axons pass through the dorsal roots and enter the gray matter, where they branch in a T-shaped pattern. These branches descend for 1-2 segments and ascend for 2-15 segments. The synapses of these axons terminate on neurons in the posterior column (second neuron).

4 The axons of the second neuron form the anterior spinothalamic tract. They cross at the anterior commissure and ascend in the opposite anterior funiculus.

5 In the mesencephalon, the tract runs in the medial lemniscus as the spinal lemniscus (the lemnisci are described in D) and terminates in the posterolateral ventral nucleus of the thalamus (third neuron, see A, p.218).

6 The axons of the third neurons terminate in the primary somatosensory cortex, which is located in the postcentral gyrus.

D Synopsis of the lemniscal tracts (lemnisci)

Cerain sensory pathways cross in the form of a lemniscal tract (Jemniskus = “ribbon”). The characteristics of the four lemnisci are reviewed below.

C Lateral spinothalamic tract and its central connections

1 Free nerve endings in the skin function as receptors for pain and temperature sensation.

2 The cell bodies of these free nerve endings are located in the spinal ganglia (first neuron).

3 The central processes of these neurons pass through the dorsal roots into the spinal cord, where they terminate on projection neurons in the substantia gelatinosa (second neuron).

4 The axons of the second neurons cross in the anterior commissure in the corresponding spinal cord segment and ascend in the anterolateral funiculus on the opposite side. They terminate in the thalamus (third neuron).

5 The axons of the third neurons terminate in the primary somatosensory cortex, which is located in the postcentral gyrus.

Lemniscus

Connection

Functional importance

Lateral lemniscus

Trapezoid body/superior olive with inferior colliculus

Auditory pathway

Medial lemniscus

Dorsal column nuclei (gracilis and cuneatus) with thalamus

Touch, conscious proprioception (see p. 284) of the trunk and limbs

Spinal lemniscus* (borders the medial lemniscus)

Posterior horns (lateral and anterior spinothalamic tract) with thalamus

Pain pathway for the trunk a nd limbs

Trigeminal lemniscus (borders the medial lemniscus)

Sensory trigeminal nuclei with thalamus

Sensory pathway for the head

* The spinal lemniscus is the portion of the anterior spinothalamic tract located in the mesencephalon. The course of the anterior spinothalamic tract in the brainstem is not fully understood, and therefore cannot be clearly depicted in these diagrams.

19.6 Ascending Tracts of the Spinal Cord: Fasciculus gracilis and Fasciculus cuneatus

A Ascending axons in the fasciculus gracilis and fasciculus cuneatus

See p.284 for overview of ascending tracts. The fasciculus gracilis (“slender fasciculus”) and fasciculus cuneatus (“wedge-shaped fasciculus”) are the two large ascending tracts in the posterior funiculus. Both tracts convey fibers for position sense (conscious proprioception, see p.284) and fine cutaneous sensation (touch, vibration, fine pressure sense, two-point discrimination). The fasciculus gracilis carries fibers from the lower limbs, while the fasciculus cuneatus carries fibers only from the upper limbs and is therefore not present in the spinal cord below theT3 level. The cell bodies of the first neuron are located in the spinal ganglion. Their fibers are heavily myelinated and therefore conduct impulses rapidly. They pass uncrossed (the level of the decussation is shown in C) to the dorsal column nuclei (nucleus gracilis and cuneatus, see C). Both nuclei are located in the caudal portion of the medulla oblongata. Thus, the fasciculi are soma- totopically organized.

В Descending axons

Besides the ascending axons contained in the fasciculus gracilis and fasciculus cuneatus (both shown in A), there are also descending axon collaterals that are distributed to lower segments. This pathway takes different shapes at different levels, appearing as the comma tract of Schultze (interfascicular fasciculus) in the cervical cord, the oval area of Flechsig (septomarginal fasciculus) in the thoracic cord, and the Philippe-Gom- bault triangle in the sacral cord. These tracts are concerned with sensorimotor innervation at the spinal cord level and are thus considered part of the intrinsic circuits of the spinal cord (see p. 273).

C Tracts of the posterior funiculus and their central connections

1 Muscle and tendon receptors, and Vater-Pacini corpuscles are receptors for conscious proprioception. Receptors about the hair follicles and additional receptors mediate the fine touch sensation of the skin.

2 The cell bodies of the neurons that relay this information are located in the spinal ganglia (first neuron).

3 The axons of these neurons ascend uncrossed in the posterior funiculi to the nucleus cuneatus and nucleus gracilis (second neuron) in the lower medulla oblongata.

4 The axons from the second neurons cross in the medial lemniscus (see D, p. 275) to the thalamus (third neuron).

5 The axons of the third neuron terminate in the primary somatosensory cortex, located in the postcentral gyrus.

19.7 Ascending Tracts of the Spinal Cord: Spinocerebellar Tracts

A Anterior and posterior spinocerebellar tracts

See p. 284 for overview of ascending tracts. Both the anterior and posterior spinocerebellar tracts are located in the lateral funiculus of the spinal cord. Their afferent fibers, which convey afferent impulses from muscles, tendons, and joints to the cerebellum, are involved in the unconsciouscoordination of motor activities (unconscious proprioception, automatic processes below the conscious level, such as jogging and riding a bicycle, see p. 284). The projection (second) neurons of both spinocerebellar tracts receive their proprioceptive signals from primary afferent fibers originating at the first neurons of the spinal ganglia. In the anterior spinocerebellar tract, the second neurons are located in the dorsal gray horn. Their projection fibers ascend both ipsiiaterally and contralaterally to the cerebellum via the superior cerebellar peduncle. The second neurons of the posterior spinocerebellar tract are located in the posterior thoracic nucleus of the posterior horn; this nuclear column spans segments C8-L2. The projection fibers from these second neurons ascend ipsilaterally to the cerebellum via the inferior cerebellar peduncle. Both the anterior and posterior spinocerebellar tracts have the same somatotopic organization from front to back: thoracic (T), lumbar (L), and sacral (S) fibers. Fibers of similar function from the cervical region pass through the fasciculus cuneatus to the accessory cuneate nucleus and continue as cuneocerebellar fibers to the cerebellum. However, these do not pass through the posterior spinocerebellar tract, which contains no fibers from the cervical cord.

В Anterior spinocerebellar tract and its central connections

1 Proprioceptive signals from muscle spindles and tendon receptors are carried by fast-conducting myelinated axons (IA fibers) to pseudounipolar first neurons in the spinal ganglia.

2 The signals then proceed to the second neurons (projection neurons of the anterior spinocerebellar tract) in the dorsal gray horn.

3 The axons of the second neurons ascend both ipsilaterally and con- tralaterally to the cerebellum and then pass through the floor of the rhomboid fossa to the midbrain.

4 Once in the midbrain, the axons change direction and pass through the superior cerebellar peduncle and superior medullary velum to the vermis of the cerebellum.

C Posterior spinocerebellar tract and its central connections

1 Muscle spindles and tendon receptors convey proprioceptive information via fast IA fibers that arise from pseudounipolar first neurons in the spinal ganglia.

2 The lAfibers proceed to the second neurons of the central gray matter. The second neurons are contained in the thoracic nucleus, which spans spinal cord segments C8 to L2.

3 The axons of the second neurons (projection neurons of the spinocerebellar tract) ascend ipsilaterally to the cerebellum, entering through the inferior cerebellar peduncle.

19.8 Descending Tracts of the Spinal Cord: Pyramidal (Corticospinal) Tracts

A Course of the anterior and lateral corticospinal tracts (pyramidal tract) in the lower medulla oblongata and spinal cord

The pyramidal tract, which begins in the motor cortex, is the most important pathway for voluntary motor function. See p. 285 for overview of descending tracts. Some of its axons, the corticonuclear fibers, terminate at the cranial nerve nuclei while others, the corticospinal fibers, terminate on the motor anterior horn cells of the spinal cord (see В for further details). A third group, the corticoreticular fibers, are distributed to nuclei of the reticular formation.

В Course of the pyramidal tract

1 The pyramidal tract originates in the motor cortex at the pyramidal cells (large afferent neurons with pyramid-shaped cell bodies, see C). The pyramidal tract has three components:

 Corticonuclear fibers for the cranial nerve nuclei

 Corticospinal fibers for the spina I cord

 Corticoreticular fibers to the reticular formation

2 All three components pass through the internal capsule from the telencephalon, continuing into the brainstem and spinal cord.

3 In the brainstem, the corticonuclear fibers are distributed to the motor nuclei of the cranial nerves.

4 The corticospinal fibers descend to the decussation of the pyramids in the lower medulla oblongata, where approximately 80% of them cross to the opposite side. The fibers continue into the spinal cord, where they form the lateral corticospinal tract, which has a somatotopic organization: the fibers for the sacral cord are the most lateral, while the fibers for the cervical cord are the most medial.

5 The remaining 20 % of corticospinal fibers continue to descend with out crossing, forming the anterior corticospinal tract, which borders the anterior median fissure in a transverse section of the spinal cord. The anterior corticospinal tract is particularly well developed in the cervical cord, but is not present in the lower thoracic, lumbar, or sacral cords.

6 Most fibers of the anterior corticospinal tract cross at the segmental level to terminate on the same motor neurons as the lateral corticospinal tract. The axons of the pyramid cells terminate via intercalated cells on alpha and gamma motor neurons, Renshaw cells, and inhibitory interneurons (not shown, see p. 273, C).

Lesions of the pyramidal tract are discussed on p. 343. Other motor tracts are closely applied to the pyramidal tract in the region of the internal capsule and will be described in the next unit. While the pyramidal tract controls conscious movement (voluntary motor activity), supplementary motor tracts are essential for involuntary muscle processes (e.g., standing, walking, running; see p. 342).

C Silver-impregnation (Golgi) method staining of pyramidal cell

This method produces a silhouette of the stained neurons. The axons of the pyramidal cells form the pyramidal tract. Approximately 40% are located in the motor cortex (Brod- mann area 4, see p. 202).

19.9 Descending Tracts of the Spinal Cord: Extrapyramidal and Autonomic Tracts

A Tracts of the extra pyramidal motor system in the spinal cord

See p. 285 for overview of descending tracts. Unlike the pyramidal tract, which controls conscious, voluntary motor activities (e.g., raising a cup to the mouth), the extrapyramidal motor system (cerebellum, basal ganglia, and motor nuclei of the brainstem) is necessary for automatic and learned motor processes (e.g., walking, running, cycling). The division into a pyramidal and extrapyramidal system has proven useful in clinical practice. A recent alternative classification divides the descending tracts into a lateral and medial system. Under this classification, the lateral system includes:

 Lateral corticospinal tract (= pyramidal tract, see p. 280)

 Rubrospinal tract (extrapy ramidal)

The lateral system projects predominantly to the distal muscles, particularly those of the upper limb, and thus critically influences fine, discriminating motor functions of the hand and arm. The medial system projects mainly to the neurons of the trunk and lower limb muscles and is thus concerned with the motor aspects of trunk position and stance. The medial system consists of three extrapyramidal tracts:

 Anterior reticulospinal tract

 Lateral vestibulospinal tract

 Tectospinal tract

The central connections of this system are illustrated in B. Because the pyramidal and extra руга midaI tracts are closely interconnected and run close to one other, lesions generally affect both tract systems simultaneously (see p. 343). Isolated lesions of either the pyramidal or extrapyramidal pathway at the spinal cord level are virtually unknown.

C Autonomic pathways of the spinal cord

Autonomic pathways have a somewhat diffuse arrangement in the spinal cord and rarely form closed tract systems. There are two exceptions:

 The descending central sympathetic tract for vasoconstriction and sweat secretion borders the pyramidal tract anteriorly and shows the same somatotopic organization as the pyramidal tract.

 The parependymal tract runs on both sides of the central canal and contains both ascending and descending fibers. Passing from the spinal cord to the hypothalamus, this tract is concerned with urination, defecation, and genital functions.

19.10 Tracts of the Spinal Cord, Overview

A Ascending tracts in the spinal cord

Transverse section through the spinal cord. Ascending tracts are afferent (= sensory) pathways that carry information from the trunk and limbs to the brain. The most important ascending tracts and their functions are listed below.

Spinothalamic tracts

- Anterior spinothalamic tract (coarse touch sensation)

- Lateral spinothalamic tract (pain and temperature sensation)

Tracts of the posterior funiculus

- Fasciculus gracilis (fine touch sensation, conscious proprioception of the lower limb)

- Fasciculus cuneatus (fine touch sensation, conscious proprioception of the upper limb).

Spinocerebellar tracts

- Anterior spinocerebellar tract (unconscious proprioception to the cerebellum)

- Posterior spinocerebellar tract (unconscious proprioception to the cerebellum)

Proprioception involves the perception of limb position in space (“position sense”). It lets us know, for example, that our arm is in front of or behind our chest even when our eyes are closed. The information involved in proprioception is complex. Thus, our position sense tells us where our joints are in relation to one another while our motion sense tells us the speed and direction of joint movements. We also have a “force sense” by which we can perceive the muscular force that is associated with joint movements. Moreover, proprioception takes place on both a conscious (I know that my hand is making a fist in my pants pocket without seeing it) and an unconscious level, enabling us to ride a bicycle and climb stairs without thinking aboutit. The table on p.327 gives a comprehensive review of all the ascending tracts.

В Descending tracts in the spinal cord

Transverse section through the spinal cord. The descending tracts of the spinal cord are concerned with motor function. They convey information from higher motor centers to the motor neurons in the spinal cord. According to a relatively recent classification (not yet fully accepted in clinical medicine), the descending tracts of the spinal cord can be divided into two motor systems:

 Lateral motor system (concerned with fine, precise motor skills in the hands):

- Pyramidal tract (anterior and lateral corticospinal tract)

- Rubrospinal tract

 Medial motor system (innervates medially situated motor neurons controlling trunk movement and stance):

- Reticulospinal tract -Tectospinal tract -Vestibulospinal tract

Except for the pyramidal tract, which may be represented as a monosynaptic pathway in a simplified scheme, it is difficult to offer a simple and direct classification of the motor system because sequences of movements are programmed and coordinated in multiple feedback mechanisms called “motor loops” (see p.341). There is no point, then, in listing the various tracts in a simplified table. While the tracts can be distinguished rather clearly from one another at the level of the spinal cord, their fibers are so intermixed at the higher cortical levels that isolated motor disturbances (unlike sensory disturbances) essentially do not occur at the level of the spinal cord.

19.11 Blood Vessels of the Spinal Cord: Arteries

A Arterial blood supply to the spinal cord (after Nieuwenhuys)

Anterior view, a Overview of the arterial supply system, b Vessels supplying the vertical system, c Watershed areas in the vertical system.

The arterial blood supply to the spinal cord is derived from both vertical and horizontal components. The vertical system consists of the unpaired anterior spinal arteries and the paired posterior spinal arteries. The spinal arteries typically arise intracranially from the vertebral arteries, though the posterior spinal arteries may arise from the posterior inferior cerebellar artery. The descending spinal arteries are small where they originate at the vertebral arteries, and would significantly decrease in caliber without reinforcing contributions from the anterior and posterior segmental medullary arteries. These segmental medullary vessels arise from spinal branches of the vertebral, ascending cervical, deep cervical, posterior intercostal, lumbar, and lateral sacral arteries, depending upon the level of the spinal cord. The segmental medullary vessels vary in both their level of origin and number (an average of 8 anterior, and 12 posterior arteries are seen). One of these arteries, the great anterior segmental medullary artery (of Adamkiewicz), is usually significantly larger than the others, and reinforces the blood supply to approxi mately two-thirds of the cord, especially in the thoracolumbar region. In 65% of individuals it arises from the leftside,typically atT12or L1, although it may arise anywhere between T7 to L4. At all other vertebral and spinal cord levels, small radicular arteries arise from the spinal branches and supply the ventral and dorsal nerve roots, as well as the peripheral portions of the anterior and posterior horns. The radicular arteries do not reach or contribute to the spinal arteries. Since the spinal arteries receive variable reinforce ment from segmental medullary arteries, certain regions of the spinal cord may receive their blood supply from multiple sources (see c). Restriction of blood supply at such a region may result in ischemic injury to the cord. TheT1-T4andthe LI cord segments are particularly vulnerable.

В Blood supply to the spinal cord segments

In each spinal cord segment, the anterior spinal artery gives off several (5-9) sulcal arteries which course posteriorly in the anterior median fissure. Typically, each sulcal artery enters one half of the spinal cord, supplying the anterior horn, base of the posterior horn, and the anterior and lateral funiculi (approximately two-thirds of the total area) in that half; the sulcal arteries tend to alternate direction (left or right) to supply both halves of the spinal cord segment. The paired posterior spinal arteries provide the blood supply to the posterior one-third of the cord, including the posterior horn and funiculus. All three spinal arteries contribute numerous delicate anastomosing vasocorona on the pial surface of the spinal cord which in turn send branches into the periphery of the cord. The sulcal arteries are the only end-arteries within the spinal cord, and their occlusion may produce clinical symptoms. Occlusion of the anterior spinal artery at segmental levels may damage the anterior horn and ventral roots resulting in flaccid paralysis of the muscles supplied by these segments. If the pyramidal tract in the lateral funiculus is involved, spastic paralysis will develop below the lesion level. An occlusion of the posterior spinal arteries in one or more segments will affect the posterior horn and funiculus leading to disturbances of proprioception, vibration, and pressure sensation.

C Blood vessels supplying the spinal cord

Thoracic vertebra viewed from above. The spinal branches arise from the posterior branches of segmental arteries and divide into an anterior and a posterior radicular artery. The radicular arteries supply the dorsal and ventral roots, and peripheral portions of the dorsal and ventral horns; they also communicate with the vasocorona. These arteries have a better- developed connection with the anterior spinal artery at some levels and with the posterior spinal artery at other levels.

19.12 Blood Vessels of the Spinal Cord: Veins

A Venous drainage of the spinal cord

(after Nieuwenhuys)

Anterior view. Analogous to the arterial supply, the venous drainage of the spinal cord consists of a horizontal system (venous rings, see B) and a vertical system that drains the venous rings. The vertical system is illustrated here. While the arterial blood supply is based on three vessels, the interior of the spinal cord drains through venous plexuses into only two unpaired vessels: an anterior and a posterior spinal vein (see B). The anterior spinal vein communicates superiorly with veins of the brainstem. Its lower portion enters the filum terminale (a glial filament extending from the conus medullaris to the sacral end of the dural sac, where it is attached). The larger posterior spinal vein communicates with the radicular veins at the cervical level and ends at the conus medullaris. The radicular veins connect these veins, which lie within the pia mater, with the internal vertebral venous plexus (see C). Blood from the cord drains into the vertebral veins, which open into the superior vena cava. Blood from the thoracic cord drains into the intercostal veins, which drain into the superior vena cava via the azygos and hemiazygos system. Radicular veins are present at only certain segments, as shown. Their distribution varies among individuals.

В Venous drainage of a spinal cord segment

Anterior view from upper left. A spinal cord segment is drained by the anterior and posterior spinal veins. These vessels are located within the pia mater and are interconnected by an anastomotic venous ring. Both veins channel blood through the radicular veins to the internal vertebral venous plexus (see C). Unlike the radicular veins, the veins inside the spinal cord have no valves. As a result, venous stasis may cause a hazardous rise of pressure in the spinal cord. A typical cause of increased intramedullary venous pressure is an arteriovenous fistula, which is an abnormal communication between an artery and vein in the spinal cord. Because the pressure in the arteries is higher than in the veins, arterial blood tends to enter the veins of the spinal cord through the fistulous connection. The fistula will remain asymptomatic as long as the intramedullary veins maintain an adequate drainage capacity. But if the flow across the fistula outstrips their drainage capacity, the functions of the spinal cord will be impaired by the increased pressure. This is manifested clinically by disturbances of gait, spastic paralysis, and sensory disturbances. Untreated, the decompensated fistula will eventually cause a complete functional transection of the spinal cord. The treatment of choice is surgical correction of the fistula.

C Vertebral venous plexus

Transverse section viewed obliquely from upper left. The veins of the spinal cord and its coverings are connected to the internal vertebral venous plexus via the radicular and spinal veins. Located in the fatty tissue of the epidural space, this plexus occupies the inner circumference of the vertebral canal. The internal plexus is connected to the external vertebral venous plexus by the /ntervertebral and bosh/ertebral veins. Anastomoses exist between the tributary regions of the anterior and posterior spinal veins. Oblique anastomoses are located in the interior of the spinal cord and may extend over several segments (not shown). These connections are particularly important in maintaining a constant intramedullary venous pressure.

D Epidural veins in the sacral and lumbar vertebral canals

(after Nieuwenhuys)

Posterior view (vertebral canal windowed). The internal veins of the spinal cord are valveless up to the point at which they emerge from the spinal dura mater. The internal vertebral venous plexus is connected by other valveless veins (not shown here) to the venous plexus of the prostate. It is relatively easy for prostatic carcinoma cells to pass along the veins of the prostatic venous plexus to the sacral venous plexus and destroy the surrounding tissue. For this reason, prostatic carcinoma frequently metastasizes to this region and destroys the surrounding bone, resulting in severe pain.

19.13 Spinal Cord, Topography

A Spinal cord and spinal nerve in the vertebral canal at the level of the C 4 vertebra

Transverse section viewed from above. The spinal cord occupies the center of the vertebral foramen and is anchored within the subarachnoid space to the spinal dura mater by the denticulate ligament. The root sleeve, an outpouching of the dura mater in the intraver- tebral foramen, contains the spinal ganglion and the dorsal and ventral roots of the spinal nerve. The spinal dura mater is bounded externally by the epidural space, which contains venous plexuses, fat and connective tissue. The epidural space extends upwards as far as the foramen magnum, where the dura becomes fused to the cranial periosteum (see p.191).

В Cauda equina at the level of the L 2 vertebra

Transverse section viewed from below. The spinal cord usually ends at the level of the first lumbar vertebra (L1). The space below the lower end of the spinal cord is occupied by the cauda equina and filum terminale in the dural sac (lumbar cistern, see p. 191), which ends at the level of the S2 vertebra (see C and p.267 D). The epidural space expands at that level and contains extensive venous plexuses and fatty tissue.

C Cauda equina in the vertebral canal

Posterior view. The laminae and the dorsal surface of the sacrum have been partially removed. The spinal cord in the adult terminates at approximately level of the first lumbar vertebra (LI). The dorsal and ventral spinal nerve roots extending from the lower end of the spinal cord (conus medullaris) are known collectively as the cauda equina. During lumbar puncture at this level, a needle introduced into the subarachnoid space (lumbar cistern) normally slips past the spinal nerve roots without injuring them.

D The spinal cord, dural sac, and vertebral column at different ages

Anterior view. As an individual grows, the longitudinal growth of the spinal cord increasingly lags behind that of the vertebral column. At birth the distal end of the spinal cord, the conus medullaris, is at the level of the L3 vertebral body (where lumbar puncture is contraindicated). The spinal cord of a tall adult ends attheT12/L1 level, while that of a short adult extends to the L2/L3 level. The dural sac always extends into the upper sacrum. It is important to consider these anatomical relationships during lumbar puncture. It is best to introduce the needle at the L3/L4 interspace (see E).

E Lumbar puncture, epidural anesthesia, and lumbar anesthesia

In preparation for a lumbar puncture, the patient bends far forward to separate the spinous processes of the lumbar spine. The spinal needle is usually introduced between the spinous processes of the L3 and L4 vertebrae. It is advanced through the skin and into the dural sac (lumbar cistern, see D) to obtain a cerebrospinal fluid sample. This procedure has numerous applications, including the diagnosis of meningitis. For epidural anesthesia, a catheter is placed in the epidural space without penetrating the dural sac (1). Lumbar anesthesia is induced by injecting a local anesthetic solution into the dural sac (2). Another option is to pass the needle into the epidural space through the sacral hiatus (3).