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 are 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 diseases that affect the trigeminal (V) nerve. These are considered here.
The Fifth, or Trigeminal, Nerve
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 inferomedial part of the middle cranial fossa in a recess called Meckel's cave. The central axons of the ganglion cells form the sensory root of the nerve. These fibers, on entering the lateral mid pons, divide into short ascending and long descending branches. The former are concerned mainly with tactile and light pressure sensation and synapse with second-order neurons in the principal sensory nucleus. Proprioceptive afferents from facial muscles and the masseter also ascend to terminate in the mesencephalic nucleus. The fibers that mediate pain and temperature sensation do not end in these nuclei but form long descending branches of the spinal trigeminal tract. This pathway, which contains both facilitatory and inhibitory fibers, together with its adjacent 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).
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 trigeminal nucleus in the upper cervical cord is a continuation of the spinal tract of Lissauer and substantia gelatinosa; the main trigeminal sensory nucleus in the pons and medulla is a continuation of the nucleus of the medial lemniscus. From all parts of the principal ...