Trigeminal Nerve

Cranial nerve V has only two components, GSA and SVE. The trigeminal nerve is named for its three divisions: The ophthalmic division provides sensory innervation to the upper face, including the forehead, upper eyelid, and cornea; the maxillary division to the region of the upper jaw, including the upper lip, jaw, and cheek, parts of the nose, and upper teeth; and the mandibular division to the region of the lower jaw, including the lower lip, jaw, cheek, lower teeth, and anterior two-thirds of the tongue. The mandibular division also provides motor innervation to the muscles of the jaw used for mastication (chewing). The trigeminal nerve emerges from the lateral aspect of the brain stem at a midpontine level.

The GSA fibers of the trigeminal nerve have most of their cell bodies located within the trigeminal ganglion and innervate the face and upper head region for fine (discriminative) touch, position sense, vibratory sense, pain, and temperature. GSA fibers for the modalities of fine touch, vibration, and position enter the brain stem and terminate in the principal sensory nucleus of V in the lateral part of the pons, which in turn projects to the ventral posteromedial nucleus (VPM) in the dorsal thalamus. The latter projects to the face representation within somatosensory cortex. GSA fibers for pain and temperature enter the brain stem and distribute to the spinal (or descending) nucleus of V, which also receives inputs from the GSA components of the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Like the principal sensory nucleus of V, the spinal nucleus projects to VPM for relay of its inputs to somatosensory cortex. (Trigem-inal innervation of the mucous membranes of the nose and anterior tongue region has traditionally been lumped with the GSA components, but since these membranes are embryologically derived from endo-derm, this component is technically general visceral afferent.)

A third trigeminal sensory nucleus, the mesence-phalic nucleus of V, is present along the lateral border of the central gray matter encircling the upper part of the fourth ventricle and the cerebral aqueduct of the midbrain. This nucleus comprises bipolar neurons that lie within the central nervous system rather than in the cranial nerve ganglion. The neurons' peripheral processes form the mesencephalic tract of V and carry pressure and position information from structures including the teeth, the palate, and the muscles of mastication. The mesencephalic V neurons project to multiple sites, including the motor nucleus of V for reflexive control of jaw position.

SVE, branchial motor fibers arise in the motor nucleus of V, which lies medial to the principal sensory nucleus and receives bilateral corticobulbar input. The axons exit the brain stem on its lateral surface in the trigeminal nerve head and distribute via the mandibular division to the jaw muscles used in mastication—the temporalis and masseter muscles and the medial and lateral pterygoids, which are embryologi-cally derived from the muscles of the branchial arches. Additional branchial arch-derived muscles innervated by the branchial motor component of V are the tensor veli palatini muscle of the palate, the tensor tympani muscle of the middle ear (which inserts on the malleus), and two suprahyoid muscles, the anterior belly of the digastric and mylohyoid.

Damage to the GSA components of cranial nerve V results in sensory loss to the face and loss of many reflexes due to lack of the sensory part of the reflex arc, including the corneal reflex (closing the eyes in response to light touch to the cornea) via the facial motor nucleus (via the reticular formation), the tearing reflex via the superior salivatory nucleus, the sneezing reflex via nucleus ambiguus, the vomiting reflex via vagal nuclei, salivatory reflexes via the superior and inferior salivatory nuclei, and the jaw-jerk reflex via the mesencephalic nucleus of V. The sensation for hot pepper (capseisin) on the anterior two-thirds of the tongue is also lost. Damage to the SVE component of V results in paralysis of the jaw muscles on the ipsilateral side, whereas a supranuclear lesion produces only moderate weakness due to the bilaterality of the supranuclear input. Changes in the loudness of sounds can also result from loss of innervation to the tensor tympani muscle.

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