This chapter deals with the pathways from the general sensory receptors to the thalamus and thence to the cerebral cortex, where the sensations are appreciated subjectively. With an understanding of the anatomy of these pathways, an appraisal of sensory deficits provides information concerning the location of a lesion in the central nervous system (CNS).
Sensory axons that enter the spinal cord in dorsal roots of spinal nerves segregate in such a way that there are two main general sensory systems. The first of these includes one or more synaptic relays in the dorsal gray horn. Spinal neurons give rise to axons that cross the midline and ascend in the ventrolateral white matter to the thalamus. This, thespinothalamic system, carries signals that report the senses of pain; temperature; and the less discriminative tactile sensations, including light touch and firm pressure.
In the second system, primary afferent axons turn rostrally in the ipsilateral dorsal funiculus
of the spinal cord and do not end until they reach certain nuclei in the lower medulla. Axons from these nuclei cross the midline and then ascend as the medial lemniscus to the thalamus. Hence, this second pathway is called the medial lemniscus system. It is concerned primarily with discriminative aspects of sensation, especially the awareness of position and movement of parts of the body and the tactile recognition of shapes and textures and of changes in the positions of stimuli that move across the surface of the skin. The medial lemniscus system is often called the posterior column system, especially in clinical usage, because it includes the dorsal funiculi (“posterior columns”) of the spinal cord.
The spinoreticulothalamic pathway, which includes relays in the reticular formation of the brain stem, also conducts ascending signals generated by cutaneous sensation. It is therefore closely related to the spinothalamic system. The association is especially seen in central conduction for pain. In fact, the spinothalamic pathway and the less direct spinoreticulothalamic pathway, with their projections to the cerebral cortex, may be combined under the term ventrolateral (or anterolateral) system. The comparable termdorsomedial system is then used for the medial lemniscus system. The various names for the pathways for general sensation are summarized in Table 19-1. Unfortunately, all the terms are in fairly widespread use by anatomists, physiologists, and clinicians. The trigeminothalamic pathways serve the same functions as the spinothalamic and medial lemniscus systems, but for the head. They are also mentioned in Chapter 8 in connection with the central connections of the trigeminal, facial, glossopharyngeal, and vagus nerves.
The general sensory pathways are said to consist of primary, secondary, and tertiary neurons, with cell bodies in sensory ganglia, the spinal cord or brain stem, and the thalamus, respectively. The concept of a simple relay of three neurons is not accurate, however, because interneurons act on the secondary and tertiary neurons. In addition, the activity of the secondary neurons is influenced by descending axons from the cerebral cortex and the brain stem.
The spinothalamic or ventrolateral system is known also as the “pathway for pain and temperature” because these modalities of sensation are transmitted to the brain in the spinothalamic tract. It is also concerned with touch, as already noted.
The receptors for pain (nociceptors) are the unencapsulated axonal endings of the thinnest group A fibers (group Aδ) and of unmyelinated (group C) fibers. Pain may be felt as two waves separated by an interval of a few tenths of a second. The first wave is sharp and localized, with conduction by group Aδ fibers. The second wave, which is rather diffuse and more disagreeable, depends on group C fibers, with a slow conduction speed. The two waves are most easily noticed in the feet (as when treading on something sharp) because of the greater lengths of the axons in the nerves of the lower limb.
The mechanism of pain perception is inseparable from that of the initiation of inflammation, which is the response of living tissue to any kind of injury. Injured cells release several substances known as mediators, which act on venules and nerve endings. The venules dilate, causing redness of the affected area and become permeable to blood plasma, which leaks out to cause swelling of the tissue. Simultaneous stimulation of the nociceptive endings results in perception of pain. Action potentials do not pass solely to the CNS; they are also propagated antidromically along other peripheral branches of the afferent axon. In the case of cutaneous group C fibers, these impulses cause a peptide neurotransmitter known as substance P to be released into the interstitial tissues of the dermis. This acts upon arterioles and in the dermis, which dilate. Substance P also causes degranulation of mast cells, which release more mediators, thereby enhancing the dilatation of arterioles and sometimes also causing edema in the area surrounding the injury. In the skin, the total result constitutes the triple response (of Lewis): a red mark and a wheal, surrounded by a flare of neurogenic
arteriolar vasodilation. A neurally mediated phenomenon such as this, which does not involve any synapses, is called an axon reflex. The receptors for temperature are probably also morphologically nondescript free nerve endings. The axons are of similar caliber to those that conduct impulses for pain. The receptors for light touch are unencapsulated nerve endings, Merkel and peritrichial endings, and Meissner's corpuscles. Ruffini endings respond to firm pressure on the skin, especially when this causes the dermis to move on the underlying subcutaneous tissue. Conduction for light touch and pressure in peripheral nerves is by myelinated group A fibers of medium diameter. (Descriptions of the specialized sensory nerve endings can be found in Chapter 3.)
TABLE 19-1 Names and Components of the Somatic Sensory Pathways
ASCENDING CENTRAL PATHWAY
Synapses and Interneurons in the Dorsal Horn
Cell bodies of small and intermediate size in the dorsal root ganglia have central processes that constitute the lateral divisions of the dorsal rootlets. These axons conduct impulses from pain and temperature receptors (Fig. 19-1). Afferents for light touch and pressure enter the dorsal gray horn through the medial division of the dorsal rootlets. The pain and temperature fibers enter the dorsolateral tract (tract of Lissauer) of the spinal cord, in which ascending and descending branches travel, in most instances, for lengths that correspond to about one segment.
The terminals and the collateral branches of the axons in the dorsolateral tract enter the dorsal horn, where they branch profusely (see Fig. 5-8). The substantia gelatinosa, which is located near the tip of the dorsal horn, is an important region in which patterns of incoming sensory impulses are modified. The dendrites of the gelatinosa cells are contacted not only by primary afferent axons but also by reticulospinal fibers, notably those derived from the raphe nuclei of the medulla. (The descending pathways that modulate transmission in the ascending sensory pathways are discussed later in this chapter.) The axons of the cells in the substantia gelatinosa ascend and descend in the dorsolateral tract and in the adjacent white matter, mostly for about the length of one segment. Throughout its length, the axon of a gelatinosa cell gives off branches that end by synapsing with the dendrites oftract cells, whose axons constitute the spinothalamic tract.
The dendrites of the tract cells are contacted by excitatory primary afferent axons for pain and temperature, by inhibitory axons of the gelatinosa cells, and by excitatory primary afferents for light touch and pressure. These connections, shown diagrammatically in Figures 5-8 and 19-2, enable a tract cell to decide whether a potentially harmful stimulus is intense enough to initiate the onward transmission of a signal of pain perception. The neuronal circuitry for pain is discussed in more detail later.
Most of the tract cells have their cell bodies in the nucleus proprius, near the base of the dorsal horn. Large neurons at the tip of the dorsal horn also contribute a proportion of the spinothalamic fibers, notably those concerned with pain. The axons of the tract cells cross the midline in the ventral white commissure. Continuing through the ventral horn of gray matter, the axons ascend in the spinothalamic tract, situated in the ventral part of the lateral funiculus and in the adjoining region of the ventral funiculus (see Fig. 5-10). Proceeding rostrally, axons are continually being added to the internal aspect of the tract. At upper cervical levels, therefore, fibers from sacral segments are most superficial, followed by fibers from lumbar and thoracic segments. The fibers from cervical segments are closest to the gray matter.
The spinothalamic fibers continue into the medulla without appreciable change of position initially (see Figs. 7-2, 7-3, and 7-4). At the level of the inferior olivary nucleus, the tract is close to the surface of the medulla, between the inferior olivary nucleus and the spinal trigeminal nucleus (see Figs. 7-5, 7-6, and 7-7). At and above this level, the spinothalamic fibers constitute most of the spinal lemniscus, which also includes axons of the spinotectal (spinomesencephalic) tract destined for the superior colliculus. The spinal lemniscus continues through the ventrolateral region of the dorsal pons, and in the midbrain, it runs along the lateral edge of the medial lemniscus (see Figs. 7-8, 7-9, 7-10, 7-11, 7-12, 7-13, 7-14, and 7-15). In their passage through the brain stem, the spinothalamic axons give off collateral branches that terminate in the medullary and pontine reticular formation and in the periaqueductal gray matter of the midbrain. There are also spinoreticular fibers that go no farther rostrally than the pons.
Thalamus and Cerebral Cortex
Most of the spinothalamic axons end in the ventral posterior nucleus of the thalamus. This nucleus consists of two parts: the ventral posterolateral (VPl) division, in which spinothalamic axons and the medial lemniscus terminate and the ventral posteromedial (VPm) division, which receives trigeminothalamic axons. The somatotopic organization is such that the contralateral lower limb is represented
dorsolaterally, and the contralateral upper limb is represented ventromedially in the VPl; the opposite side of the head is represented in the VPm.
FIGURE 19-1 Spinothalamic system for pain, temperature, light touch, and pressure. The pathway from the lower limb is shown in red, and that from the upper limb is shown in blue.
FIGURE 19-2 Simple illustration of the gate control theory of pain. Whereas nonnociceptive primary sensory neurons stimulate the inhibitory interneurons, nociceptive afferents inhibit them. An increase in nonnociceptive input reduces the rate of firing of the spinothalamic tract neuron. Compare this diagram with Figure 5-8.
The thalamocortical projection consists of neurons in the ventral posterior nucleus, whose axons traverse the posterior limb of the internal capsule and corona radiata to reach theprimary somesthetic area in the parietal lobe. The contralateral half of the body, exclusive of the head, is represented as inverted in the dorsal two thirds of the primary somesthetic area (see Fig. 15-3). The cortical area for the hand is disproportionately large, providing for maximal sensory discrimination. The somatotopic arrangement at various levels of the sensory pathways forms the basis for recognition of the site of stimulation.
Some axons of the spinal lemniscus end in thalamic nuclei other than the VPl, notably those of the posterior and intralaminar groups and the mediodorsal nucleus. The posterior group projects to the insula and to the adjacent parietal cortex, including that of the second somatic sensory area, which is situated at the lower end of the postcentral gyrus. The intralaminar nuclei project diffusely to the frontal and parietal lobes of the cerebral cortex and to the striatum. They may be involved in the maintenance of a conscious, alert state (see Chapter 9). The mediodorsal nucleus is connected with the frontal lobes, especially their medial and orbital surfaces—cortical regions concerned with affect, decision making, and foresight (see Chapter 15). A projection of the mediodorsal nucleus to the anterior part of the cingulate gyrus is activated by painful stimuli.
Pain is a common complaint, and it is therefore necessary to become conversant with the anatomy, physiology, and pharmacology of this symptom. The mechanisms whereby peripheral nerve endings respond to injurious stimuli have already been reviewed. The central pathways concerned with pain are now discussed in further detail.
Perception of pain is thought to be modified by neural mechanisms in the dorsal horn. In addition to the influence of reticulospinal and corticospinal fibers, to be discussed later, the transmission of impulses for pain to the brain is altered by dorsal root afferents for other sensory modalities. Afferent axons of larger diameter, especially those for touch and deep pressure, have branches that synapse with the dendrites of the gelatinosa cells. Trains of impulses coming through the larger axons can stimulate the gelatinosa cells, causing these interneurons to inhibit the tract cells that are concerned with nociception. The inhibitory effect can be overcome by sufficient nociceptive input to the tract cells. This postulated mechanism, known as the gate control theory of pain (see Fig. 19-2), enables the neurons in the spinal cord to
determine, on the basis of all incoming sensory stimuli, whether a particular event should be reported to the brain as being painful. A similar mechanism is presumed to exist in the caudal part of the spinal trigeminal nucleus, which is the rostral continuation of the tip of the dorsal horn. The gate mechanism probably operates when pain arising in deep structures such as muscles and joints is relieved by stimulating sensory endings in the overlying skin (e.g., by rubbing or by applying warmth or a mild chemical irritant such as a liniment).
A simpler, direct pathway is provided by the large neurons (Waldeyer cells) at the tip of the dorsal horn. These are activated by nociceptive primary afferent fibers and have axons that travel in the spinothalamic tract to the ventral posterior and mediodorsal thalamic nuclei.
The simplest defensive reflex initiated by pain is the flexor reflex, which involves at least two synapses in the spinal cord (see Fig. 5-13) and causes flexion of a limb to withdraw it from the source of a sudden painful stimulus. In quadrupeds, there is also a crossed extensor reflex in which the withdrawal is assisted by extension of the contralateral limb. In normal humans, the crossed extensor reflex is largely suppressed as a result of activity in descending tracts of the spinal cord, but both it and the flexor reflex are conspicuous and, because of a lowered threshold, troublesome in paraplegic patients.
Impulses that signal pain are transmitted rostrally in the spinothalamic and spinoreticular tracts (Fig. 19-3). Additional axons with this function appear to be present in the dorsolateral funiculus. Tractotomy or surgical transection of the ventrolateral region of the spinal cord, which contains the spinothalamic and spinoreticular tracts, results in almost complete loss of the ability to experience pain on the opposite side of the body below the level of the lesion. The sensibility usually returns gradually over several weeks. The recovery is probably a consequence of synaptic reorganization and increased usage of intact alternative pathways. A surgical cut in the midline of the spinal cord (commissural myelotomy) causes prolonged analgesia in the segments affected by the lesion.
Pain is still felt, although poorly localized, after destruction of the primary somesthetic area. This clinical observation led to an early assumption that painful sensations reached the level of consciousness within the thalamus. It is more likely that spinothalamic and reticulothalamic afferents to the intralaminar and mediodorsal thalamic nuclei are responsible for the persistence of sensibility to pain after destruction of the primary somesthetic area. These thalamic nuclei are connected with most of the neocortex, including the prefrontal areas and the anterior part of the cingulate gyrus. A unilateral painful stimulus is associated with increased blood flow in both cingulate gyri. The ventral posterior nucleus of the thalamus and the primary somesthetic area are undoubtedly necessary for the accurate localization of the site of the painful stimulus.
Descending pathways modify the activity of all ascending systems; they are prominent in controlling the conscious and reflex responses to noxious stimuli. Both the subjective awareness of pain and the occurrence of defensive reflexes may be suppressed under circumstances of intense emotional stress. This effect may be mediated by corticospinal fibersthat originate in the parietal lobe and terminate in the dorsal horn (see Fig. 19-7).
Control of a subtler kind is exerted by certain reticulospinal pathways. The best understood of these is the raphespinal tract, which arises from neurons in the raphe nuclei of the medullary reticular formation, mainly those of the nucleus raphes magnus. The unmyelinated axons of this tract traverse the dorsal part of the lateral funiculus of the spinal cord (seeFigs. 5-10 and 19-7) and use serotonin as a neurotransmitter. The highest density of serotonin-containing synaptic terminals (observable by histochemical methods) is seen in the substantia gelatinosa. The nucleus raphes magnus is itself influenced by descending fibers from the periaqueductal gray matter of the midbrain. Electrical stimulation of the nucleus raphes magnus or the periaqueductal gray matter causes profound analgesia. This is reversed either by transection of the dorsolateral funiculus or by administration of naloxone or similar drugs that antagonize the actions of morphine
and related alkaloids of opium. Furthermore, the analgesic action of opiates is suppressed by transection of the dorsolateral funiculus.
FIGURE 19-3 Ascending pathways for pain. The spinothalamic system is shown in red, and the spinoreticular and reticulothalamocortical pathways are shown in blue. Interneurons in the spinal cord are green.
The actions of the opiates and their antagonists are attributable to selective binding molecules (opiate receptors) on the surfaces of neurons in several parts of the brain. The normal function of the opiate receptor is to bind naturally occurring opioid peptides, of which the best understood are two pentapeptides, known as enkephalins. These serve either as neurotransmitters or as neuromodulators. The analgesic action of morphine and related opiates can be attributed to simulation of the effects of endogenously secreted enkephalins on neurons that bear opiate receptors on their surfaces. Major anatomical sites of action include the dorsal horn, nucleus raphes magnus, periaqueductal gray matter, and probably the thalamus. Many other parts of the CNS contain enkephalins, mainly in local circuit neurons. These regions may be the sites of other pharmacological actions of the opiates, such as nausea, suppression of coughing, euphoria, and the development of addiction.
Information about the descending pathways that modulate pain has led not only to increased understanding of the sites of action of the opium alkaloids but also to a technique occasionally used for the relief of chronic pain. An electrode stereotaxically implanted into the periaqueductal gray matter enables a patient to relieve pain instantly by switching on an electrical stimulator.
Medial Lemniscus System
The set of sensory pathways known as the medial lemniscus system is for proprioception, discriminative touch, and (although not exclusively) vibration. In contrast to the spinothalamic system, in which ascending axons cross the midline at spinal segmental levels, the pathways that constitute the medial lemniscus system ascend ipsilaterally in the cord and cross the midline in the caudal half of the medulla.
The medial lemniscus (or dorsomedial) system is especially important in humans because of the discriminative quality of the sensations as perceived subjectively and their value in the learning process. The characteristics of fine or discriminative touch are that the subject can recognize the location of the stimulated points with precision and is aware that two points are touched simultaneously even though they are close together (two-point discrimination). These qualities accentuate recognition of textures and of moving patterns of tactile stimuli. Of the tactile receptors, Meissner's corpuscles, which have been found only in primates, have a special significance in discriminative touch (see also Chapter 3). These rapidly adapting receptors occur in the ridged, hairless skin of the palmar surface of the hands, which are moved over surfaces to feel texture and other small irregularities. Several additional touch receptors, noted in connection with the spinothalamic system, also produce sensations through the medial lemniscus system. Pacinian corpuscles are the principal receptors for the sense of vibration, although this modality, once believed to be served exclusively by the dorsal funiculi, is now known to also be carried in the lateral white matter of the spinal cord.
With respect to proprioception, the dorsomedial pathway provides information concerning the precise positions of parts of the body; the shape, size, and weight of an object held in the hand; and the range and direction of movement. The proprioceptors are neuromuscular spindles, neurotendinous spindles, and endings in and near to the capsules and ligaments of joints. For conscious proprioception (kinesthesia), input from muscle spindles probably is of greater significance than the input from other proprioceptors (see Chapter 3).
ASCENDING CENTRAL PATHWAYS
Identical pathways transmit discriminative touch and proprioception from the trunk and limbs. An additional pathway for proprioceptive signals from the lower limbs is also present.
The primary sensory neurons for discriminative touch and proprioception are the largest cells in the dorsal root ganglia, having large axons with thick myelin sheaths. The central
branches of these axons are medially located in each rootlet, and they bifurcate on entering the dorsal funiculus. Most of the ascending branches proceed ipsilaterally to the medulla (Fig. 19-4). Above the midthoracic level, the dorsal funiculus consists of a medial gracile fasciculus and a lateral cuneate fasciculus. The axons of the gracile fasciculus, which enter the spinal cord below the midthoracic level, terminate in the gracile nucleus; axons of the cuneate fasciculus, coming from the upper thoracic and cervical spinal nerves, end in thecuneate nucleus. More precisely, there is a lamination of the dorsal funiculus according to segments. Axons that enter the spinal cord in lower sacral segments are most medial, and axons from successively higher segments ascend along the lateral side of those already present.
Axons of neurons in the gracile and cuneate nuclei curve ventrally as internal arcuate fibers, cross the midline of the medulla in the decussation of the medial lemnisci (see Figs. 7-4and 19-4), and continue to the thalamus as the medial lemniscus. This substantial tract is situated between the midline and the inferior olivary nucleus in the medulla, in the most ventral portion of the tegmentum of the pons, and lateral to the red nucleus in the tegmentum of the midbrain. The medial lemniscus and spinothalamic tract intermingle in the dorsal region of the subthalamus before entering the lateral division of the ventral posterior nucleus of the thalamus. The fibers of the medial lemniscus, in contrast to those of the spinothalamic tract, all terminate in the VPl nucleus.
A topographic arrangement of axons is maintained throughout the medial lemniscus. In the medulla, the larger dimension of the lemniscus is vertical as seen in cross section; fibers for the lower limb are most ventral (adjacent to the pyramid), and fibers for the upper part of the body are most dorsal. On entering the pons, the medial lemniscus twists through 90°; from there to the thalamus, fibers for the lower limb are located in the lateral part of the lemniscus, and those for the upper part of the body are located in its medial portion. This pattern conforms with the representation of the body in the VPl nucleus of the thalamus. The pathway is completed by a projection from this nucleus to the primary somesthetic cortex of the parietal lobe.
The central pathways for conscious awareness of position and movement are similar to those for discriminative touch, but for the lower limb, an additional pathway is present (Fig. 19-5). The pathway for the upper limb corresponds exactly with the one just described. That is, the ascending branches of primary afferent fibers terminate in the cuneate nucleus, from which the impulses are relayed through the medial lemniscus to the ventral posterior nucleus of the thalamus and thence to the first somatic sensory area of the cerebral cortex.
An equivalent pathway exists for the lower limb, but by way of the gracile fasciculus and gracile nucleus. The accessory pathway for conscious proprioception from the lower limb is different, being a series of four populations of neurons:
FIGURE 19-4 Medial lemniscus system for discriminative tactile sensation. The pathway from the lower limb is shown in red, and that from the upper limb is shown in blue.
FIGURE 19-5 Pathways for conscious proprioception. The pathway from the upper limb is shown in blue. An equivalent pathway exists for the lower limb but is not shown. The accessory pathway from the lower limb is shown in red.
Dorsal Spinal Cord Lesions
The existence of an accessory pathway for proprioception from the lower limb has clinical implications. The dorsal funiculi conduct impulses concerned with proprioception in the upper and lower limbs. A lesion at a high cervical level that transects the dorsal funiculus but spares the dorsal spinocerebellar tract results in clumsiness and other symptoms of impaired position sense in the upper and lower limbs. Simple clinical testing in such cases shows loss of awareness of position and movement of the joints of the upper limb as well as preservation of these senses in the lower limb. The patient's daily experience, however, indicates quite severe proprioceptive impairment of the leg and foot. The pathway involving the dorsal spinocerebellar tract and nucleus Z is evidently sufficient to account for conscious proprioception when this modality is specifically tested in patients with dorsal funiculus lesions.
The short descending branches of the primary sensory axons in the dorsal funiculus enter the spinal gray matter, along with collaterals of the ascending branches. Some of the axons that enter the gray matter, especially those concerned with proprioception, establish connections for spinal reflexes, and the remainder terminate on tract cells. Axons of these tract cells ascend ipsilaterally in the dorsal and dorsolateral funiculi (see Fig. 19-4). All these axons terminate in the gracile and cuneate nuclei alongside the primary ascending axons. These spinomedullary neurons, especially those sending axons into the dorsolateral funiculus, convey some information for most modalities of cutaneous and deep sensation, including vibration and pain. This relatively small population of afferents to the gracile and cuneate nuclei broadens the role of the medial lemniscus system to some extent beyond that of a pathway for discriminative touch and proprioception.
Enhancement of Discrimination in the Gracile and Cuneate Nuclei
It is convenient to think of sensory signals being “relayed” through the gracile or cuneate nucleus and the VPl nucleus of the thalamus to the cerebral cortex. Simple interruptions in the pathway would only serve to retard transmission, however. The real purpose of the nuclei is to modify the message, increasing the sensitivity of the cerebral cortex to the tiny differences in shape, texture, or movement that stimulate the peripheral receptors. The way this happens is most easily understood by considering the circuitry of the gracile or cuneate nucleus in relation to stimulation of a point on the skin. This circuitry (Fig. 19-6) includes the excitatory synapses of the dorsal root ganglion neurons (blue) and a population of inhibitory interneurons (black) in the nucleus. Both are connected with the principal cells of the nucleus, whose axons (red) go to the thalamus.
Three principal cells (red) of the gracile or cuneate nucleus are shown receiving input that is strongest (highest frequency of action potentials) from the center of the area of skin represented at the bottom of the diagram. The inhibitory interneurons (black) that surround the principal cells receive more stimulation from the more active primary afferent (blue) neurons. The stimulated interneurons inhibit neighboring principal cells, thereby reducing the frequency of signals that relate to the area of skin surrounding the stimulus. Activation of inhibitory interneurons by collateral branches of afferent axons is called feed-forward inhibition. The same effect is also produced by recurrent collateral branches of the axons of the principal cells, also shown ending on interneurons in Figure 19-6. The action caused by recurrent collaterals is known as feedback inhibition. Both types of inhibition occur in the gracile and cuneate nuclei and are collectively known as lateral inhibition.
Lateral inhibition occurs at synaptic stations in all sensory pathways. It has been thoroughly studied in the retina (Chapter 22), and it occurs also in the thalamic “relay” nuclei (including the ventral posterior nucleus) and within the cerebral cortex.
Figure 19-6 also shows inhibitory interneurons being stimulated by a corticonuclear neuron (green). This arrangement provides distal inhibition
(also called remote inhibition), with the somatosensory cortex setting the sensitivity of the principal cells of the gracile and cuneate nuclei. Other examples of distal inhibition in sensory pathways include the raphespinal tract, mentioned earlier in this chapter, and the olivocochlear projection of the auditory system (Chapter 21). Descending tracts that influence general somatic sensation are summarized in Figure 19-7.
FIGURE 19-6 Amplification of contrast between neighboring parts of an area of skin in the overlapping territories of three primary sensory neurons (blue). The gracile and cuneate nuclei contain principal cells (red) and inhibitory interneurons (solid black). The activities of the principal cells that receive less excitation (left and right) are suppressed by feed-forward and feedback inhibition, mediated by the interneurons. Consequently, the thalamus receives input only from the neuron in the center, which is the one that was most strongly excited by the tactile stimulus. The diagram also shows a corticonuclear neuron (green), which is part of a descending system that uses distal inhibition to modulate the ascending flow of sensory signals in the medial lemniscus system.
Sensory Pathways for the Head
The back of the head and much of the external ear are supplied by branches of the second and third cervical nerves, whose central connections are with the spinothalamic and medial lemniscus systems. General sensations that
arise elsewhere in the head are mediated almost entirely by the trigeminal nerve. Small areas of the skin and larger areas of mucous membrane are supplied by the facial, glossopharyngeal, and vagus nerves, but the central connections of the general sensory components of these nerves are the same as for the trigeminal nerve (see Chapter 8).
FIGURE 19-7 Descending pathways that influence the transmission of sensory information to the cerebral cortex. Reticulospinal and raphespinal projections are blue, descending axons from the periaqueductal gray matter are black, and other descending pathways are red and green.
The cell bodies of primary sensory neurons of the trigeminal nerve, with the exception of those in the mesencephalic nucleus, are located in the trigeminal ganglion (see Fig. 8-10). The peripheral processes have a wide distribution through the ophthalmic, maxillary, and mandibular divisions of the nerve. The central processes enter the pons in the sensory root. Some of these axons end in the pontine trigeminal nucleus; many descend in the spinal trigeminal tract and end in the associated nucleus, and still others bifurcate, with a branch ending in each nucleus.
A spatial arrangement of axons in the sensory root and spinal tract corresponds to the divisions of the trigeminal nerve. In the sensory root, ophthalmic fibers are dorsal, mandibular fibers are ventral, and maxillary fibers are in between. Because of a rotation of the axons as they enter the pons, the mandibular fibers are dorsal, and the ophthalmic fibers are ventral in the spinal trigeminal tract. The most dorsal part of this tract includes a bundle of fibers from the facial, glossopharyngeal, and vagus nerves. The cell bodies of the primary sensory neurons are located in the geniculate ganglion of the facial nerve and in the superior ganglia of the glossopharyngeal and vagus nerves. Somatic sensory axons in the facial and vagus nerves supply parts of the external ear and tympanic membrane. The glossopharyngeal and vagus nerves supply the mucosa of the back of the tongue, pharynx, esophagus, larynx, auditory (eustachian) tube, and middle ear.
PAIN AND TEMPERATURE
Primary afferent fibers for pain and temperature end in the pars caudalis of the spinal trigeminal nucleus (see Chapters 7 and 8); the pars caudalis is located in the lower medulla and upper two or three cervical segments of the spinal cord. (There is some evidence that the pars interpolaris receives pain afferents from the teeth.) The part of the pars caudalis in the cervical cord receives sensory data from areas of distribution of the trigeminal nerve and upper cervical spinal nerves. The cellular characteristics of the pars caudalis are similar to those of the tip of the dorsal gray horn of the spinal cord.
Neurons in the reticular formation immediately medial to the pars caudalis correspond to the nucleus proprius of the spinal gray matter. The tract cells whose axons project to the thalamus are located in both the spinal trigeminal nucleus and the adjacent reticular formation. The axons of these second-order neurons cross to the opposite side of the medulla and continue rostrally in the ventral trigeminothalamic tract. The tract terminates mainly in the VPm, and thalamocortical fibers complete the pathway to the inferior (ventral) one third of the primary somesthetic area of cortex. The axons of the tract cells associated with the pars caudalis, similar to those of the spinothalamic tract, have branches that end in the intralaminar, posterior, and mediodorsal nuclei of the thalamus, thus providing for distribution of the sensory information to areas of cortex beyond the confines of the first somatic sensory area. From the foregoing description, it is evident that the pathway for pain and temperature from the head corresponds to the spinothalamic system.
The central pathway for tactile sensation from the head is similar to that just described for pain and temperature, differing mainly in the sensory trigeminal nuclei involved. For light touch, the second-order neurons are located in the pars interpolaris and pars oralis of the spinal trigeminal nucleus and in the pontine trigeminal nucleus. For discriminative touch, they are located in the pontine trigeminal nucleus and the pars oralis of the spinal trigeminal nucleus. The second-order neurons project to the contralateral VPm through the ventral trigeminothalamic tract. In addition, smaller numbers of axons, crossed and uncrossed, proceed from the pontine trigeminal nucleus to the VPm in the dorsal trigeminothalamic tract. The two sets of trigeminothalamic fibers often are named together as the trigeminal lemniscus.
The primary sensory neurons for proprioception in the head are unique in that most of their cell bodies are located in a nucleus in the brain stem instead of in a sensory ganglion. Constituting the mesencephalic trigeminal nucleus, they are unipolar neurons similar to dorsal root ganglion cells. The peripheral branch of the single process proceeds through the trigeminal nerve without interruption; these axons supply proprioceptors in the trigeminal area of distribution, such as those related to the muscles of mastication. Central branches of the single process go to the trigeminal motor nucleus for reflex action and join the dorsal trigeminothalamic tract. Some neurons of the mesencephalic trigeminal nucleus send peripheral branches to receptors in the sockets of the teeth. These receptors detect pressure on the teeth, a sense functionally related to muscle proprioception because it participates in the reflex control of the force of biting.
The only other type of sensation perceived by a tooth is pain, for which the sensory pathway has already been described. Pain may originate from the dentin, the pulp, or the periodontal tissues.
The standard method of testing for integrity of the pain and temperature pathway is to stimulate the skin with a pin and to ask whether it feels sharp or blunt. Light touch is tested with a wisp of cotton. Temperature perception does not usually need to be tested separately; if such testing is required, the method used is to touch the skin with test tubes containing warm or cold water.
Irritation of a peripheral nerve or dorsal root by external pressure or local inflammation stimulates pain and temperature fibers, causing painful and burning sensations in the area supplied by the affected roots or nerves. An example is pressure on a dorsal root of a spinal nerve by a herniated intervertebral disk. An effect opposite to that of irritation is produced by local anesthetic drugs. These are most effective in blocking the conduction of impulses along group C fibers, so that low doses may reduce pain perception while having little or no effect on tactile sensibility. Ischemia of a nerve, such as that resulting from a tight tourniquet, preferentially blocks conduction in group A fibers. Pain with a burning character is the only sensation that can be perceived before the failure of conduction in an ischemic nerve becomes complete.
Degenerative changes in the region of the central canal of the spinal cord interrupt pain and temperature axons as they decussate in the ventral white commissure. The best example is syringomyelia, a disease in which cavities slowly develop in the center of the spinal cord. When the process is most marked in the cervical enlargement, as is frequently the case, the area of anesthesia includes the hands, arms, and shoulders (i.e., yoke-like anesthesia). A classical presenting symptom is a burn that is not painful.
A lesion that transects axons in the ventrolateral part of the spinal cord on one side results in loss of pain and temperature sensibility below the level of the lesion and on the opposite side of the body. If, for example, the spinothalamic and spinoreticular tracts are interrupted on the right side at the level of the first thoracic segment, the area of anesthesia includes the left leg and the left side of the trunk. Careful testing of the upper margin of sensory impairment shows that cutaneous areas supplied by the first and second thoracic nerves are spared. Some signals from these levels reach the contralateral pathways above their interruption because of the ascending branches of dorsal root axons in the dorsolateral tract. Surgical section of the pathway for pain (tractotomy or chordotomy) may be required for relief of intractable pain. Tractotomy is most likely to be considered in later stages of malignant disease of a pelvic organ; interruption of the pain pathway may be unilateral or bilateral, depending on circumstances prevailing in the particular patient. It was pointed out earlier in this chapter that mobilization of alternative ascending pathways can lead to the return of pain several weeks after a tractotomy. An alternative analgesic procedure, effective for longer periods of time, is commissural myelotomy, in which decussating
spinothalamic and spinoreticular axons are cut by a median incision at and a few segments above the level of the source of the pain.
The spinal lemniscus may be included in an area of infarction in the brain stem. An example is provided by Wallenberg's lateral medullary syndrome; the area of infarction usually includes the spinal lemniscus and the spinal trigeminal tract and nucleus. The principal sensory deficit is for pain and temperature sensibility on the side of the body opposite the lesion but on the same side for the face (see also Chapter 7). The insensitivity to normally painful stimuli is sometimes accompanied by allodynia, a condition in which innocuous stimuli are felt as pain. This change may be caused by reorganization of connections in the thalamus. Allodynia is more frequently caused by injury or disease affecting the dorsal horn of the spinal cord. Avulsion of dorsal rootlets can result in severe pain that feels as if it comes from the affected dermatome.
MEDIAL LEMNISCUS SYSTEM
The usual test for proprioception is to move the patient's finger or toe, asking when the movement begins and what the direction of movement is. In the Romberg test, any abnormal unsteadiness is noted when the patient stands with the feet together and the eyes closed, thereby evaluating proprioception in the lower limbs. Another useful test is to ask the patient to identify an object held in the hand with the eyes closed. Proprioception is especially helpful in recognizing the object on the basis of shape and size (stereognosis) as well as weight. This is a sensitive test that the patient may perform unsuccessfully when he or she has a lesion in the parietal association cortex even though the pathway to the somesthetic area is intact.
For testing two-point touch discrimination, two pointed objects are applied lightly to the skin simultaneously. A suitable test object can be devised from a paper clip. Simultaneous stimuli are normally detected in a fingertip when the points are 3 to 4 mm apart, or even less. Thorough testing of two-point discrimination is a tedious procedure. A simpler test is for the examiner to ask the subject to identify simple figures “drawn” on the skin with the finger or with some other blunt object. This test relies on the ability to recognize the distance and direction of movement of the stimulus across the surface of the skin. It is highly specific for the dorsal funiculi of the spinal cord, provided there is no lesion in the cerebral cortex that is causing aphasia or agnosia.
Another sensory test is to ask the patient whether vibration as well as touch or pressure is felt when a tuning fork, preferably with a frequency of 128 Hz, is placed against a bony prominence such as an ankle or a knuckle. The sense of vibration is often reduced in elderly people, but even slight vibration should be felt in young people. For identifying the site of a lesion in the CNS, this test is less valuable than the examination of proprioception and discriminative touch. Diminished perception of vibration is often the first sign of disease affecting the largest myelinated axons in a peripheral nerve, some of which innervate pacinian corpuscles. Peripheral neuropathy is a term that embraces many disease processes that impair conduction in nerves, causing motor weakness or sensory deficits.
Defective proprioception and discriminative touch result from interruption of the medial lemniscus system anywhere along its course. For example, the dorsal and dorsolateral funiculi are sites of symmetrical demyelination in subacute combined degeneration of the spinal cord (see Chapter 5), and conduction may be interrupted at any level by trauma, infarction, or the plaques of multiple sclerosis. The medial medullary syndrome described in Chapter 7 is an instructive, albeit rare, example of unilateral transection of the medial lemniscus.
SENSATION FROM THE HEAD
The most common sensory abnormality affecting the face and scalp is herpes zoster. This disease is caused by a virus (the same one that causes chicken pox) that infects the neurons in sensory ganglia. Burning pain and itching, commonly in the field of distribution of one of the three divisions of the trigeminal nerve, is accompanied by a skin eruption. This can be a serious condition if corneal ulceration results from infection of the ganglion cells concerned with the ophthalmic division of the trigeminal nerve. Occasionally, the disability is prolonged, especially in elderly people, by postherpetic neuralgia. This may be particularly painful and recalcitrant to treatment. Relief can be obtained by applying capsaicin to the affected skin. Capsaicin first stimulates and then damages the terminal branches of nociceptive group C axons. Herpes zoster may also affect the geniculate ganglion or the superior vagal ganglion, causing an eruption
on the tympanic membrane and parts of the external auditory canal and concha of the auricle; this is classical clinical evidence for the anatomy of the dual cutaneous innervation of this region.
A less common condition that causes episodes of severe pain in the fields of distribution of one or more divisions of the trigeminal nerve istrigeminal neuralgia, described in Chapter 8. The more frequent types of headache, including migraine, are not caused by anatomically discrete lesions in sensory pathways.
Surgically or pathologically produced lesions in the VP nucleus of the thalamus cause profound loss of all sensations other than pain on the opposite side of the body. The intralaminar and posterior groups of nuclei in the thalamus are probably almost as important as the VP nucleus in the central pathway for pain.
Central neurogenic pain, which is not caused by activity in peripheral sensory axons, can be caused by lesions that interrupt the somatosensory pathways at any level. A destructive lesion that involves the VP nucleus of the thalamus may result in the thalamic syndrome, characterized by exaggerated and exceptionally disagreeable responses to cutaneous stimulation. This syndrome (see Chapter 11) may include spontaneous pain and evidence of emotional instability, such as unprovoked laughing and crying.
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