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The cranial nerves are susceptible to a number of special diseases, some of which do not affect the spinal peripheral nerves. For this reason alone they deserve to be considered separately. Certain of the cranial nerves and their disorders have already been discussed: namely, disorders of olfaction in Chap. 12; of vision and extraocular muscles in Chaps. 13 and 14; of cochlear and vestibular function in Chap. 15; and craniofacial pain in Chap. 10. There remain to be described the disorders of the facial (VII) nerve and of the lower cranial nerves (IX to XII), as well as certain of the diseases that affect the trigeminal (V) nerve. These are considered here.

The Fifth, or Trigeminal, Nerve

Anatomic Considerations

The fifth nerve (Fig. 47-1) is a mixed sensory and motor nerve. It conducts sensory impulses from the greater part of the face and head; from the mucous membranes of the nose, mouth, and paranasal sinuses; and from the cornea and conjunctiva. It also provides the sensory innervation of the dura in the anterior and middle cranial fossae. The cell bodies of the sensory part of the nerve lie in the gasserian, or semilunar, ganglion. This, the largest sensory ganglion in humans, lies in the medial part of the middle cranial fossa at the base of the cranium. The central axons of the ganglion cells form the sensory root of the nerve. These fibers, on entering the mid pons, divide into short ascending and long descending branches. The former are concerned mainly with tactile and light pressure sense and synapse with second-order neurons in the principal sensory nucleus. Proprioceptive afferents from facial muscles and the masseter terminate in the mesencephalic nucleus. The fibers that mediate pain and temperature sensation do not end in these nuclei but form the unique anatomy of the long descending branches of the spinal trigeminal tract. The latter pathway, which contains both facilitatory and inhibitory fibers, together with its nucleus, extends from the junction of the pons and medulla to the uppermost segments (C2 or C3) of the spinal cord (as evidenced by the relief of facial pain after medullary trigeminal tractotomy).

Figure 47-1.

Scheme of the trigeminal nuclei and some of the trigeminal reflex arcs. I, ophthalmic division; II, maxillary division; III, mandibular division. (Originally from Ramon y Cajal S: La Textura del Sistema Nervista del Hombre y los Vertebrados, Madrid, Moya, as adapted from Carpenter MB, Sutin J: Human Neuroanatomy, 8th ed. Baltimore, Williams & Wilkins, 1982, by permission.)

The spinal nucleus is a continuation of the spinal tract of Lissauer and substantia gelatinosa; the main sensory nucleus is a continuation of the nucleus of the medial lemniscus. From all parts of the principal trigeminal sensory and spinal nuclei, second-order fibers cross to the opposite ...

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