An idiopathic pure unilateral trigeminal motor neuropathy is known but is a clinical rarity. Chia described 5 patients in whom an aching pain in the cheek and unilateral weakness of mastication were the main features. Electromyography (EMG) showed denervation changes in the ipsilateral masseter and temporalis muscles. The outcome was favorable.
Postcontrast MRI with thin slices can disclose enhancement at the trigeminal root entry zone in a variety of inflammatory, demyelinating, and neoplastic disorders. The function of the nerve may be studied by the recording of blink reflexes. A few laboratories have developed an evoked potential test specifically of the trigeminal nerve.
The Seventh, or Facial, Nerve
The seventh nerve is mainly a motor nerve, supplying all the muscles concerned with facial expression on one side.
The sensory component is small (the nervus intermedius of Wrisberg); it conveys taste sensation from the anterior two-thirds of the tongue and, variably, cutaneous sensation from the anterior wall of the external auditory canal. The taste fibers at first traverse the lingual nerve (a branch of the trigeminal mandibular) and then join the chorda tympani, which conveys taste sensation via the facial nerve to the nucleus of the tractus solitarius. Secretomotor fibers originate in the superior salivatory nucleus and innervate the lacrimal gland through the greater superficial petrosal nerve and the sublingual and submaxillary glands by traveling through the chorda tympani and then forming the lingual nerve (Fig. 44-3).
Several other anatomic facts are worth noting. The motor nucleus of the seventh nerve lies ventral and lateral to the abducens nucleus, and the intrapontine fibers of the facial nerve partly encircle and pass dorsolaterally to the abducens nucleus before emerging from the lower pons, just lateral to the corticospinal tract. The impression made by these looping fibers of the seventh nerve is visible in the floor of the upper fourth ventricle as a protuberance, the facial colliculus. In this region of the pons, infiltrative lesions affect the sixth and seventh nerves simultaneously.
The facial nerve enters the internal auditory meatus with the vestibulocochlear nerve bundle and then bends sharply forward and downward around the anterior boundary of the vestibule of the inner ear. At this angle (genu) lies the sensory ganglion (named geniculate because of its proximity to the genu). The nerve continues in its own bony channel, the facial canal, within which, just distal to the geniculate ganglion, it provides a branch to the pterygopalatine ganglion, that is, the greater superficial petrosal nerve, which exits the skull through the vidian canal and innervates the lacrimal, nasal, and palatine glands. Somewhat more distally, it gives off a small motor branch to the stapedius muscle and is then joined by the chorda tympani, which carries parasympathetic fibers that become the lingual nerve, project to the submandibular ganglion, and innervate the submandibular and sublingual glands. The motor root of the facial nerve exits the skull at the stylomastoid foramen and then passes through the parotid gland and subdivides into 5 branches that supply the facial muscles, the stylomastoid muscle, the platysma, and the posterior belly of the digastric muscle.