Vestibulocochlear Nerve

The eighth cranial nerve traditionally has been classified in one of the special categories, SSA. It comprises the two senses of audition and the relative position and motion of the head in space. The eighth nerve innervates the inner ear organs—the spiral-shaped cochlea, which transduces auditory stimuli via vibration of its perilymphatic fluid, and the vestibular labyrinth, which transduces the effects of gravity and acceleration on its receptor apparatus.

a. Cochlear Division The bipolar neurons of the cochlear division of VIII innervate the hair cells of the cochlear organ of Corti. The apical surfaces of the hair cells are studded with stereocilia and border the inner chamber of the cochlea, the scala media, which is filled with endolymph. A tectorial membrane overlies the stereocilia. An outer chamber, formed by the scala vestibuli and scala tympani, contains perilymph, and the scala tympani portion is separated from the organ of Corti by a basilar membrane. Sound waves cause vibration of the perilymph and resultant displacement of the basilar membrane, which in turn pushes the stereocilia against the tectorial membrane, bending them and opening ion channels. Influx of potassium from the endolymph through the opened channels causes depolarization of the receptor cell. The frequency of the auditory stimuli is tonotopically mapped along the length of the cochlear spiral, with best responses to high frequencies occurring at its base and those to lower frequencies at more apical locations. The bipolar neurons preserve the tonotopic map for relay to the cochlear nuclei and then throughout the ascending auditory pathway. They also encode intensity by their discharge rate.

Cell bodies of cochlear bipolar neurons lie within the spiral ganglion, named for the shape of the cochlea. Their central processes enter the lateral aspect of the brain stem at a caudal pontine level and terminate in the dorsal and ventral cochlear nuclei. The cochlear nuclei project to multiple sites, including the superior olivary nuclear complex in the pons and the inferior colliculus in the midbrain roof. The superior olivary complex (SO) receives bilateral input mainly from the so-called bushy cells of the ventral cochlear nucleus; SO neurons are coincidence detectors that utilize the time delay between the inputs from the two sides in order to compute the location in space of the sound source. The ascending auditory projections predominantly originate from pyramidal neurons within the dorsal cochlear nucleus and from the superior olivary complex and pass via the lateral lemniscus to the inferior colliculus, which in turn projects to the medial geniculate body of the dorsal thalamus. The latter projects to auditory cortex in the temporal lobe. Damage to the cochlear division of VIII results in dysfunction (such as experiencing a buzzing sound) and/or deafness.

b. Vestibular Division The bipolar neurons of the vestibular division of VIII innervate hair cells within several parts of the vestibular labyrinth. The fluid-filled labyrinth consists of three semicircular canals that transduce rotational movements (angular acceleration) of the head and two chambers, the saccule and utricle, that constitute the otolith organ and transduce linear acceleration and gravitational force. The three semicircular canals are arranged at right angles to each other for responses in the various planes of space, and each contains a dilated area, the ampulla, that contains a ridge, the crista ampullaris. Vestibular hair cells on the surface of the crista have stereocilia and a longer kinocillium that are displaced by movement of an overlying gelatinous cupula, which moves due to fluid displacement within the canal caused by rotational motion. Similarly, the saccule and utricle contain hair cells in an area called the macula. The cilia of these hair cells are displaced by movement of an overlying gelatinous mass, the otolith membrane, that contains the otoliths (or otoconia), which are small particles of calcium carbonate that are denser than the surrounding fluid and are displaced by gravity when the head is tilted or during linear acceleration.

Cell bodies of vestibular bipolar neurons lie within the vestibular (Scarpa's) ganglion. The central processes of these neurons enter the lateral aspect of the pons along with the cochlear nerve fibers; most terminate in the superior, inferior, medial, and lateral vestibular nuclei, which lie in the floor of the fourth ventricle. A small number of vestibular afferent fibers bypass the vestibular nuclei and project directly to the cerebellar cortex, particularly within its flocculonod-ular lobe, which is concerned with the maintenance of posture, balance, and equilibrium. The vestibular nuclei also receive a substantial input from the cerebellum via its deep nuclei.

Descending projections from the vestibular nuclei form the medial and lateral vestibulospinal tracts that innervate spinal cord neurons for extensor muscles involved in postural maintenance and related reflexes. Unlike other brain stem nuclei that give rise to descending spinal projections, the vestibular nuclei do not receive any direct input from the cerebral cortex. Other major vestibular connections are with the oculomotor nuclei of cranial nerves III, IV, and VI via the medial longitudinal fasciculus for the vestibu-loocular reflex—the stabilization of eye fixation on a target while the head is moving—and other vestibu-lar-oculomotor interactions. The ascending pathway for conscious perception of vestibular stimuli arises from neurons in the superior, lateral, and inferior vestibular nuclei and terminates in several dorsal thalamic nuclei, including part of the ventral poster-olateral (VPLc) nucleus and the ventral posteroinfer-ior (VPI) nucleus. Thalamocortical vestibular projections are to ventrally lying parietal cortical areas (designated 2v and 3a), which are adjacent to the head representation of somatosensory and motor cortices, respectively. Damage to the vestibular division of VIII results in dizziness, pathologic nystagmus (an involuntary, repeated eye movement pattern consisting of conjugate movement of the eyes to one side followed by a rapid return to the original position), nausea and vomiting, vertigo (the sensation of rotation in the absence of actual movement), and other related symptoms.

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