The trigeminal nerve is primarily sensory, and covers a large area of the skin of the face, dura mater, major in-tracranial blood vessels, the teeth, and the oronasal mucosa. The nerve also supplies the masticatory muscles with both sensory afferents and motor efferents. The three main divisions of the trigeminal sensory nuclei are the principal, mesen-cephalic, and spinal nuclei. The trigemi-nal sensory (gasserian) ganglion lies near the apex of the petrous temporal bone and gives rise to three main sensory divisions: ophthalmic, maxillary, and mandibular. The motor root lies medial to the sensory root. The motor nucleus lies in the lateral pontine tegmentum and supplies the masticatory muscles and the tensor tympani and tensor veli palatini muscles. The su-pratrigeminal nucleus, which is a pattern generator for mastication rhythms, lies at its upper end. Both motor nuclei receive corticonuclear fibers, which mediate voluntary control over muscle movements. Where the trigeminal exits from the brain stem, it is close to two vessels, namely the superior petrosal vein and the superior cerebellar artery. Compression of the nerve root by these vessels causes the pain known as trigeminal neuralgia.
The ophthalmic division enters the orbit of the eye, and carries sensory inputs from the upper eyelid, cornea, supraten-torial dura, globe, the mucosa of the frontal, ethmoidal, and sphenoidal sinuses, and from areas of the scalp and face. The mandibular division is mixed, containing motor and sensory fibers. It exits the cranium through the foramen ovale, and innervates the gingiva and lower teeth, and the skin of the lower face, extending into the temporal area. A branch of the nerve, the lingual nerve, innervates the tongue. The maxillary division travels forward in a groove on the floor of the middle cranial fossa, and exits the cranial cavity through the foramen rotundum. It innervates the lower eyelid, the upper teeth and gingiva, the hard palate, the skin above the mouth, and the posterior region of the nasal cavity.
The motor nucleus of the trigeminal nerve lies in the lateral tegmentum of the pons, situated ventromedially to the principal sensory trigeminal nucleus. It consists of large multipolar neurons, which are lower motoneurons for skeletal muscles that are involved in reflexes mediated by the trigeminal nerve. These moto-neurons are controlled in turn by upper motoneurons projecting from the facial area of the precentral gyrus and other cortical motor areas. There are also smaller interneurons in the nucleus, and inputs from the reticular formation. The motor nucleus is innervated also by several diffuse inputs from the limbic system, and these, together with inputs from the reticular formation, enable the production of appropriate facial expressions as part of an emotional response.
The principal trigeminal nucleus projects afferents in the contralateral medial lemniscus to the ventral posterior nucleus of the thalamus. These carry both mechanical and tactile information. Other fibers travel ipsilaterally to the thalamus, and the afferents constitute the ventral and dorsal trigeminothalamic tracts, respectively. The spinal nucleus projects pain, temperature, and some touch information to the contralateral thalamus.
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