The fifth nerve (Fig. 44-1) 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).
The spinal trigeminal nucleus in the upper cervical cord is a continuation of the spinal tract of Lissauer and substantia gelatinosa, while the main trigeminal sensory nucleus in the pons and medulla is a continuation of the nucleus of the medial lemniscus. From both the principal sensory and spinal trigeminal nuclei, second-order fibers cross to the opposite side and ascend to the thalamus in a system of fibers called the trigeminothalamic tract. These fibers come to lie in the most medial part of the spinothalamic tract and lateral part of the medial lemniscus. In addition, the secondary trigeminal neurons project to the facial and hypoglossal nuclei bilaterally, the salivatory nuclei, the cuneate nuclei of the upper cervical segments, and other cranial nerve nuclei. The principal sensory and spinal trigeminal nuclei receive fibers from the reticular formation, the thalamus, the nucleus tractus solitarius, and the somatosensory cortex.
The peripheral branches of the gasserian ganglion form the three sensory divisions of the nerve. The first (ophthalmic) division passes through the cavernous sinus and superior orbital fissure; the second (maxillary) division also passes through the cavernous sinus and leaves the middle fossa through the foramen rotundum; and the third (mandibular), does not traverse the cavernous sinus and instead exits Meckel’s cave inferiorly through the foramen ovale.