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Sensory Nerves and Sensory Roots

nuclei, terminal, nerve, impulses, columna, medial and fibers

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SENSORY NERVES AND SENSORY ROOTS Nervus Terminalis.—The terminal nerve has been found in fishes, am phibia, reptiles and mammals; it is present but vestigial in man. It is a plexiform nerve coursing along the surface of the gyrus rectus, medial to the olfactory bulb and tract; and, continuing through the cribriform plate, it descends in the septal mucosa as far as the vomero-nasal remnant and anterior to it. It possesses a ganglion, more or less distributed along its course, called the ganglion lerminale. Its point of attachment to the brain is !doubtful: Johnston, McKibben, McCotter and others claim that it enters the infero-medial part of the fore-brain in the uncinate region or along the medial border of the trigonum olfactorium; Hardesty suggests that it may be continuous with the cephalic sympathetic nerves. The gan glion cells are small and multipolar in character; the nerve fibers are very slender and non-medullated in man; these facts and the plexiform nature of the nerve indicate that its function is autonomic. The fibers are collected into bundles surrounded by sheath cells, like the fila of the olfactory nerve (Huber and Guild).

Many investigators have studied the terminal nerve in the lower forms. According to Johnston and others it develops like an ordinary sensory nerve in many selachians; its ganglion is derived from the neural crest; its fibers are medullated; its function is apparently cutaneous sensibility; its central processes enter the brain in the uncinate region and terminate in a nucleus, which is of vast importance in the development of the fore-brain. Whether through a long phylogenesis the nerve lost its primitive function and acquired a new and very different function remains to be determined, but it appears to be a sympathetic nerve in man.

Terminal Nuclei.—The terminal nuclei of the first and second cerebral nerves are peculiar and cannot as yet be classified with the nuclei of other sensory nerves and sensory roots (Figs. 31, 55 'and 86). The terminal nuclei of the fifth, the intermediate, the eighth, ninth and tenth nerves may be called the posterior columna series because they are formed by masses of cell-bodies representing the upward prolongation of the posterior columna of gray substance in the spinal cord. The posterior columna,

as pointed out by Herrick, contains two functional columns of cells, a somatic and a visceral. The somatic column com prises all the posterior columna except its medio-basal part, called the dorsal nucleus of Clark, that is visceral. The somatic nuclei receive sensory impulses as well as impulses exciting reflexes, while under normal conditions the visceral nuclei receive only non-sensory excito-reflex impulses. Terminal nuclei are composed of the bodies of afferent neurones of the second order; they receive the axones of first order neurones which constitute the peripheral nerves. Leaving the olfactory and optic terminal nuclei out of consideration for the present, all terminal nuclei give off three systems of axones: (r) the corticipetal system, which runs toward the cerebral cortex and may be divided into two or more bundles; (2) the simpler reflex system which directly or through the medial longitudinal bundle terminate in motor nuclei and (3) the coordinating reflex system, which runs through the restiform body to the cerebellar cortex and produces impulses of coordination. The corticipetal fibers from common sensory terminal nuclei form several specific strands, according to Head and Holmes, each of which carries only one particular variety of impulse, as tactile locali zation, tactile discrimination of two or more simultaneous contacts, muscle-sense (sense of posture and sense of move ment), pain, pleasure, hot, cold, etc. However, these distinct specific strands are collected into two groups or strung on two sets of poles; those carrying impulses of the muscle-sense and tactile discrimination form a part of the medial fillet; while the strands bearing pain, temperature and localizing tactile im pulses join the spino-thalamic tract. Both groups of corti cipetal axones terminate in the lateral nucleus of the thalamus. Of course the terminal nuclei of nerves of special sense form specific tracts; they may or may not be divided (Brain, Vol. 34). Terminal nuclei are common sensory and special sensory.

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