The stimulation of the hair cells results in depolarization and increased calcium (Ca) influx, leading to increased firing of afferent vestibular nerve fibers. The hair cells within the three semicircular ducts (lateral, superior, and posterior) respond to angular acceleration or head rotation. Hair cells within the utricle respond to horizontal acceleration, while the hair cells within the saccule respond to vertical acceleration. The utricle and saccule sense the positioning of the head in space. The utricle and saccule contain macula, while the semicircular canals contain crista ampullaris. The two types of neuroepithelium are the macula and crista ampullaris, which both contain sensory hair cells. The vestibular apparatus has five components: the utricle, saccule, and three semicircular ducts (contained within the semicircular canals). The membranous labyrinth contains the sensory neuroepithelium and is located within the bony labyrinth, suspended in perilymph. The peripheral vestibular apparatus is located within the temporal bone, and it consists of a bony and membranous labyrinth. The vestibular system involves coordinated communication between the vestibular apparatus (semicircular canals, saccule, utricle), ocular muscles, postural muscles, brainstem, and cerebral cortex. The vestibular nerve relays information related to motion and position. The Vestibular Nerve and Vestibular System Next, the fibers reach the thalamic medial geniculate nucleus before traveling to the primary auditory cortex, within the temporal lobe. Īfter the cochlear nuclei, the fibers cross and join the contralateral lateral lemniscus toward the midbrain inferior colliculus. This organization is preserved within the cochlear nuclei. The afferents from the apex of the cochlea transmit low frequencies. Afferent nerve fibers from hair cells at the base of the cochlea transmit high frequencies. Cochlear nerve fibers are characterized tonotopically. The anterior division innervates the AVCN, while the posterior division innervates the DCN and PVCN. Upon entering the brainstem, the cochlear nerve separates from the vestibular nerve and branches into anterior and posterior divisions. There are three divisions of the cochlear nuclei: anteroventral (AVCN), dorsal (DCN), and posteroventral (PVCN). The nerve fibers run past the CPA and enter the brainstem at the pontomedullary junction to innervate the cochlear nuclei within the rostral pole of the upper medulla. Within the IAC, the cochlear nerve joins the vestibular nerve to form CN VIII. Ĭentral projections from the spiral ganglion form the cochlear nerve before entering the IAC. Type I neurons project to the inner hair cells of the organ of Corti, while Type II neurons project to the outer hair cells. Type 1 neurons are comparatively larger, myelinated, and account for 90% of cochlear nerve cells. Type 2 neurons are smaller and nonmyelinated. Type I and Type II neurons populate the spiral ganglion, and both send peripheral processes to the ciliated hair cells of the organ of Corti and central processes that join together to form the cochlear nerve. The spiral ganglion is located at the spiral canal of the modiolus. The bipolar neurons making up the spiral (cochlear) ganglion create the link between the central nervous system (CNS) and the organ of Corti. The cochlea is a spiral, fluid-filled cavity in the bony auditory labyrinth that contains the organ of Corti along its basilar membrane.
The cochlear nerve is responsible for transmitting auditory signals from the inner ear to the cochlear nuclei within the brainstem and ultimately to the primary auditory cortex within the temporal lobe.
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