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In This Chapter, We Will Examine CHAPTER 6 SOMESTHESIA - CENTRAL MECHANISMS n this chapter, we will examine the Magendie, who originally stated that most pathways that somatic activity can take fibers in the ventral roots were motor. Iin the central nervous system, and then we will consider the mechanisms by which different sensations can occur. Somatosensory pathways. Once the primary afferent fibers leave the skin, they group together into larger and larger bundles in the cutaneous nerves. At some point in their course they are joined by muscle nerves, containing sensory and motor fibers of the muscles. Nerves that innervate the skin are termed cutaneous nerves, and those that contain both skin and muscle fibers are termed mixed nerves. The sciatic Figure 6-1. A cross section through the cervical nerve is an example of a mixed nerve. spinal cord showing the entry of the two divisions Nerve fibers that carry impulses to the CNS of the dorsal roots and the routes that fibers take into the major ascending tracts. (Crosby EC, are called afferent; those that carry them Humphrey T, Lauer, EW: Correlative Anatomy of from the CNS are efferent. The afferent the Nervous System. New York, Macmillan, 1962) and efferent fibers are separate from each other at the skin or muscle, but they are The dorsal roots are often divided into two usually mixed together along most of the parts, a lateral division consisting mainly of course of the nerve on the periphery. Near unmyelinated fibers and a medial division the spinal cord, the afferent and efferent consisting mainly of larger, myelinated fibers separate again, the efferent fibers fibers (see Fig. 6-1). Upon entering the going to the ventral (or anterior) roots and spinal cord, some of the afferent fibers the afferent fibers, after passing their cell bifurcate, with one collateral descending bodies in the dorsal root ganglia, going toward the caudal regions of the cord and the through the dorsal (or posterior) roots. This other ascending toward the rostral regions. separation of afferent fibers in the dorsal Both myelinated and unmyelinated fibers root and efferent fibers in the ventral root is branch extensively and have terminals near the so-called Bell-Magendie Law. Lately the dorsal root entry zone. The collaterals of this concept of a rigid separation of dorsal, the unmyelinated fibers go only a short sensory and ventral, motor fibers has been distance, one or two segments, before challenged by findings that, in the cat, up to 30% of the ventral root fibers are sensory. This probably would not have surprised 6-1 terminating in the dorsal horn, but a fiber the nuclei themselves. As illustrated in may put down several collateral branches Figure 6-2, the axons of these second-order that terminate in many places in the gray neurons immediately cross over to the other matter. Descending branches of the side (decussate) of the brain stem, forming a myelinated fibers generally terminate within bundle of fibers, called the medial one segment from their entry; however, lemniscus. They are soon joined by fibers of many of the ascending branches can travel the trigeminal system, serving the face, and uninterrupted for long distances in the dorsal also by fibers from the spinothalamic tract. white matter, the dorsal columns, even Those fibers that send a branch into the dorsal columns at the segment of entry also send branches down into the gray matter. There they join the branches of other myelinated fibers and unmyelinated fibers to participate in reflexes and other spinal cord activities. Some of these fibers terminate on cells of the gray matter (second order cells), the axons of which cross in the anterior commissure to the contralateral ventral gray columns at or near the segment of entry and proceed into the ventrolateral white columns. In this position, they ascend the spinal cord as the spinothalamic tract, until they join the fibers of the already decussated medial lemniscus in the brain stem. It is wrong to think of the spinothalamic tract, and perhaps any other central nerve tract, as being a compact bundle of fibers all of which have the same origin, destination and function. In fact, within the region occupied by the spinothalamic tract there are axons of many other ascending and descending tracts, making it virtually impossible to transect spinothalamic fibers and only spinothalamic tract fibers. Figure 6-2. The course and major relay points of Primary afferent fibers from the region of the dorsal column system. (Crosby EC, Humphrey T, Lauer, EW: Correlative Anatomy of the Nervous the face in front of the ears group together System. New York, Macmillan, 1962) into the trigeminal nerve and have their cell bodies in the semilunar ganglion. From the reaching the lower medullary levels where semilunar ganglion the fibers enter the brain they terminate in the dorsal column nuclei, stem as a part of the fifth cranial nerve. As the nuclei gracilis (lower limb) and cuneatus in the spinal cord, many of the sensory fibers (upper limb). bifurcate immediately upon entering the In the dorsal column nuclei, these fibers brain stem, sending an ascending branch to form connections with cells that live within 6-2 the main sensory nucleus of the trigeminal subtle and less well-understood role in and a descending branch to the nucleus of sensation, and consequently there is less we the spinal tract of the trigeminal, a structure can say about them. analogous with the dorsal aspect of the It has been customary to think of the dorsal spinal cord gray matter. Though most of the column-medial lemniscus pathway as the fibers arising from the second order neurons mediator of touch, pressure, vibratory of the trigeminal spinal nuclei decussate in (pallesthesia), and joint position senses. the brain stem and soon join the medial The spinothalamic tract was usually thought lemniscus, a few do not cross, but join the to carry information about pain, deep ipsilateral medial lemniscus. pressure, temperature, and crude touch. The The fibers of the medial lemniscus, difference between the information in the along with those from the trigeminal nuclei two pathways was thought to be the and the spinothalamic tract, proceed directly sensitivity or threshold (the dorsal columns to the ventral part of the thalamus, being more sensitive) and the contralateral to the primary afferent fibers discriminability of the spatial qualities of the associated with them. Fibers originating in stimulus (the ability to locate the site of the main sensory nucleus and the rostral stimulus being much greater for the dorsal parts of the spinal nucleus of the trigeminal columns). In the trigeminal system, the terminate in the nucleus ventralis caudal part of the spinal nucleus (some posteromedialis (VPM) of the thalamus, people still maintain the whole spinal whereas those of the caudal part of the nucleus) was thought to carry information spinal nucleus terminate in the intralaminar for pain, temperature and perhaps also crude nuclei and the medial geniculate, and a few touch, whereas the main sensory nucleus end in VPM. Fibers originating in the dorsal was thought to relay information from the column nuclei and most fibers of the face about touch, pressure and vibration. spinothalamic tract terminate in the nucleus There is an increasing body of evidence ventralis posterolateralis (VPL) of the that has cast doubt on the dichotomy of thalamus. Some spinothalamic fibers function of the systems outlined above. terminate outside of VPL, notably in the Some of the evidence comes from laboratory medial geniculate body and suprageniculate experiments. When recordings were made nucleus. of the discharges of single identified The third-order fibers arising from the spinothalamic tract cells, it was found that thalamic nuclei can then be traced out some of them did, in fact, respond to through the internal capsule to the somatic noxious or thermal stimuli, in much the sensory cerebral cortex. In man, the primary same way as nociceptors or thermoreceptors somesthetic cortex is the region of the discussed ln the last chapter. In the monkey, postcentral gyrus, the region immediately these made up 30% of the fibers studied. posterior to the central sulcus, from the Other fibers responded to light touch, Sylvian sulcus to a place within the pressure, or hair movement, in much the midsagittal sulcus. same manner as the mechanoreceptive fibers There are other pathways that carry found in the dorsal columns. These made up somatic information and other areas of the 40-60% of those fibers studied. Most cortex that receive it, but they play a more spinothalamic tract fibers (60%) responded 6-3 to both light tactile stimulation and noxious fibers, but tabes dorsalis is a dorsal root stimulation (Trevino DL, Coulter JD, Maunz disease and more than just dorsal column RA, Willis WD: Localization and fibers are affected. functional properties of spinothalamic cells Additional evidence that casts doubt on the in the monkey. In Bonica JJ (ed): Advances dorsal column-spinothalamic dichotomy in Neurology, Vol. 4 Pain. New York, comes from the clinical literature. Because Raven Press, 1974). the spinothalamic tract and the dorsal Conversely, some dorsal column fibers columns serving one side of the body are on respond to stimulation that is frankly different sides of the spinal cord and because noxious. These fibers are located primarily the dorsal columns are on the dorsal aspect deep within the columns and are second- and the spinothalamic tract is on the more order, rather than primary afferent neurons. ventral aspect of the cord, it is possible to These second-order neurons make up 10- transect one without cutting the other1. In 30% of the dorsal column fibers by some some patients with intractable pain, pain that estimates, and are comprised of fibers that is not diminished by drug treatment or even respond to light tactile as well as noxious by cutting the dorsal roots (dorsal stimulation (77%), noxious stimulation only rhizotomy), the spinothalamic tract was cut (6%), and light tactile stimulation (17%) (spinothalamic tractotomy) at the (Angaut-Petit D: Exp Brain Res 22:471- appropriate level on the side opposite the 493, 1975).
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