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Vestibulocochlear nerve (CN VIII)
The vestibulocochlear nerve (Latin: nervus vestibulocochlearis), the eighth cranial nerve (CN VIII) is a nerve composed of two divisions - vestibular and cochlear, which are both purely sensory in function. The vestibulocochlear nerve carries special somatic afferent fibers from structures of the inner ear. The cochlear nerve enables the hearing sense, while the vestibular nerve is responsible for the sense of balance.
Cochlear nerve and auditory pathway
The cochlear nerve arises from the organ of Corti, which lies in the cochlea of the inner ear. The receptor cells (hair cells) in the organ of Corti receive the primary stimuli, and then the information is transmitted to pseudounipolar neurons located in the spiral ganglion, which lies in the modiolus (center) of the cochlea. The axons of these neurons (special somatic afferent fibers) travel to the brainstem via the cochlear nerve. The spiral ganglion contains the first-order neurons of the auditory pathway.
After emerging from the cochlea, the cochlear nerve enters the internal acoustic meatus located in the petrous part of the temporal bone. In the meatus the nerve joins with the vestibular nerve, and they form the vestibulocochlear nerve. The cochlear nerve then travels along the vestibular nerve through the internal acoustic opening to the posterior cranial fossa, through the pontine cistern, and then enters the brainstem in the cerebellopontine angle along with the facial nerve (CN VII).
The cochlear nerve has two nuclei known as the posterior (dorsal) and anterior (ventral) cochlear nuclei located in the lateral corners of the rhomboid fossa. These nuclei house the second-order neurons. In the pons, fibers of the cochlear nerve synapse with the posterior and anterior cochlear nuclei. Axons from the anterior cochlear nucleus form horizontal fibers called the trapezoid body. Most of these fibers decussate and synapse with the superior olivary complex, but some fibers run ipsilateral and then synapse with the superior olivary complex.
The axons arising from neurons of the superior olivary complex and those of the posterior cochlear nucleus form the lateral lemniscus, which travels via the trigone of the lateral lemniscus and enters the nuclei of the inferior colliculus and the medial geniculate bodies in the midbrain. Few axons of the inferior colliculus synapse with the superior colliculus stratum griseum, and later axons of these neurons decussate in the tegmentum of the midbrain. Therefore, these fibers are involved in forming the tectospinal tract.
Axons arising from the medial geniculate nucleus form the acoustic or auditory radiation that passes through the posterior crus of the internal capsule and terminates in the superior temporal gyrus (corresponding to Brodmann 41 and 42 areas), where they synapse with cortical neurons.
Vestibular nerve and vestibular pathway
The vestibular nerve arises from the receptors of the maculae of utricle and saccule and from the receptors located in the ampullary crests of the membranous labyrinth. The first-order neurons of the vestibular pathway form the vestibular ganglion located deep in the acoustic internal meatus.
The receptor cells receive the primary stimuli, and the neurons of the vestibular ganglion transmit the information further from the receptors via their dendrites. The axons arising from the neurons in the vestibular ganglion form the vestibular nerve, which joins the cochlear nerve in the internal acoustic meatus to form the eighth cranial nerve - the vestibulocochlear nerve.
The fibers of the vestibular nerve reach the vestibular area of the brainstem, where they synapse with the vestibular nuclei, which contain the second-order neurons. The vestibular nuclei are located in the lateral corners of the rhomboid fossa, and they form two columns - lateral and medial. The lateral column contains the inferior, lateral, and superior nuclei, while the medial column contains the medial vestibular nuclei. Axons from neurons of the vestibular nuclei travel in various directions:
- To the motor neurons of the anterior horn of the spinal cord via the vestibulospinal tract;
- To the inferior olivary nucleus via the vestibulo-olivary tract; axons from the olivary nucleus via the inferior cerebellar peduncles reach the cerebellum (olivocerebellar tract) and also via the olivospinal tract goes to the motor nuclei of the anterior horns of the spinal cord;
- To the cerebellum via the vestibulocerebellar tract;
- Finally, to the cerebral cortex via the ventral posterolateral nucleus of the thalamus; these axons also join the ascending fibers of the medial longitudinal fascicle.