The motor portion of the fifth nerve, which supplies the masseter and pterygoid muscles, has its origin in the trigeminal motor nucleus in the mid pons; the exiting fibers pass underneath (but not through) the gasserian ganglion and become incorporated into the mandibular nerve. The masseter and pterygoid muscles are used in chewing and are implicated in a number of brainstem reflexes, the best known of which is the jaw jerk. Tapping the chin with the jaw muscles relaxed stimulates proprioceptive afferents that terminate in the mesencephalic nucleus of the midbrain, which sends collaterals to the motor nucleus of the fifth nerve and causes the masseters to contract. This reflex is enhanced in spastic bulbar (pseudobulbar) palsy. Another pontine reflex that uses afferent trigeminal sensory nerves is the blink reflex. Tapping of the brow or bridge of the nose evokes bilateral blink through activation of the orbicularis oculi muscles (facial nerve efferents). Touching the eyelids and cornea (corneal reflex) does the same.
Because of their wide anatomic distribution, complete interruption of both the motor and sensory fibers of the trigeminal nerve is rarely observed. In contrast, partial dysfunction of the trigeminal nerve, particularly of the sensory part, is common, the main symptoms being facial numbness and pain. The various cranial nerve and brainstem syndromes in which the fifth nerve is involved are listed in Tables 30-5, 33-5, and 44-1 the last in relation to stroke syndromes of the brainstem that affect the nerve in its fascicular course or in its nucleus.
Diseases Affecting the Fifth Nerve
A variety of diseases may affect the peripheral branches of the trigeminal nerves, the gasserian ganglion, and the roots (sensory and motor). Hughes has summarized them and the main ones are described below. The role of the nerve in migraine is discussed in Chap. 9.
Trigeminal neuralgia (See also “Trigeminal Neuralgia” in Chap. 9) The frequent and important disease of the fifth nerve is trigeminal neuralgia (tic douloureux). This condition has been known since ancient times, having been described by Arateus in the first century A.D., by John Locke in 1677, by Nicolaus Andre in 1756, and by John Fothergill in 1776 (according to Katusic et al). The overall incidence rate for both sexes combined is 4.3 per 100,000 persons per year, but it is higher for women than for men (in a ratio of 3:2) and is much higher in the elderly. The mean age of onset is 52 to 58 years for the idiopathic form and 30 to 35 years for the symptomatic forms, the latter being caused by trauma or vascular, neoplastic, and demyelinative diseases. In the last decade it has become apparent, mainly from the work of Jannetta, that a proportion of cases result from compression and secondary demyelination of trigeminal nerve rootlets by small branches of the basilar artery (see Love and Coakham).