![]() ![]() 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. CT with and without contrast is an alternative for those who cannot undergo MRI.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.Recommended in patients with abnormal audiometric testing or high clinical suspicion of acoustic neuroma ( cerebellopontine angle syndrome).Contrast MRI (imaging modality of choice). ![]() Facial nerve ( CN VII): ipsilateral facial twitching or weakness.Trigeminal nerve ( CN V): ipsilateral decreased corneal reflex.More cost-effective but less sensitive than an MRI.Can be used as an additional test in patients with asymmetric audiometry findings.Brainstem-evoked audiometry: delay in cochlear nerve conduction time on affected side finding.Rinne test : air conduction > bone conduction in both ears.Weber test : lateralization to the normal ear.Best initial test : > 90% of patients will have some type of hearing loss.Hearing loss with a greater deficit for higher frequencies.Cochlear nerve ( CN VIII): sensorineural hearing loss.On average, the prognosis is favorable, as acoustic neuromas are usually benign, slow-growing tumors with low recurrence rates. However, observation with follow-up may be appropriate for patients with smaller tumors and minimal hearing loss. ![]() For patients with large tumors or significant hearing loss, the treatment of choice is surgical removal or radiation therapy. Diagnosis of acoustic neuroma involves audiometry that demonstrates ipsilateral sensorineural hearing loss and MRI with contrast to confirm the tumor. ![]() The most common symptom is unilateral sensorineural hearing loss. Symptoms are related to compression of cranial nerves VIII, V, VII, and the cerebellum. Bilateral acoustic neuromas strongly suggest the genetic condition neurofibromatosis type II. The tumor forms within the internal acoustic canal with variable extension into the cerebellopontine angle. Acoustic neuromas (also known as vestibular schwannomas) are benign tumors that arise from Schwann cells and primarily originate within the vestibular portion of cranial nerve VIII. ![]()
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