Vestibulocochlear nerve (CN VIII)

The vestibulocochlear nerve (eighth cranial nerve, CN VIII, Latin: nervus vestibulocochlearis) is a cranial nerve composed of two divisions, the vestibular and cochlear nerve, both purely sensory in function. The vestibulocochlear nerve carries special somatic afferent fibers from structures of the inner ear.

The cochlear nerve enables the sense of hearing, while the vestibular nerve is responsible for the sense of balance.

Cochlear nerve

The cochlear nerve (latin: nervus cochlearis) is one of the two divisions of the vestibulocochlear nerve (CN VIII). The cochlear nerve originates from the sensory receptors of the organ of Corti, travels through the internal ear to reach the brainstem, and the information is further transmitted to thalamus and to the auditory cortex of the temporal lobe. The cochlear nerve is involved in the sense of hearing.

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, the information is then 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.

After emerging from the cochlea of the inner ear the cochlear nerve enters the internal acoustic meatus where it 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).

In pons, the fibers of the cochlear nerve synapse with the posterior and anterior cochlear nuclei. Axons from the anterior 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 inferior colliculus and the medial geniculate bodies in the midbrain. Axons arising from the medial geniculate nucleus form the acoustic radiation that passes through the posterior crus of the internal capsule and terminates in the superior temporal gyrus and the transverse temporal gyrus region (Brodmann 41 and 42 area), where they synapse with cortical neurons.

Few axons of the inferior colliculus synapse with the superior colliculus stratum zonale and therefore is involved in the tectospinal tract.

Vestibular nerve

The vestibular nerve (latin: nervus vestibularis) is one of the two divisions of the vestibulocochlear nerve (CN VIII), which arises from the sensory receptors located within the membranous labyrinth. Most of the vestibular nerve fibers terminate in the brainstem in the vestibular nuclei, but some fibers run directly to the reticular nuclei of the brainstem without synapsing, and also to the cerebellar nuclei. The vestibular nerve is associated with the sense of equilibrium, spatial orientation and motion.

The vestibular nerve arises from the receptors of the maculae of utricle and saccule, and from the receptors from the ampullary crest of the membranous labyrinth. 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 auditory meatus to form the vestibulocochlear nerve.

The fibers of the vestibular nerve reach the vestibular area of the brainstem where they synapse with the vestibular nuclei. Axons from neurons of the vestibular nuclei travel in various directions: to the motor neurons of the anterior horn of spinal cord via the vestibulospinal tract, to the inferior olivary nucleus via the vestibulo-olivary tract, to the cerebellum via the vestibulocerebellar tract, and to the cerebral cortex via the ventral posterolateral nucleus of thalamus. These axons also join the ascending fibers of the medial longitudinal fascicle.