The vestibulocochlear nerve is the eight cranial nerve (CN VIII) and it is a purely sensory nerve. It consists of two parts: the vestibular part which carrries nervous impulses from the vestibular system and the cochlear part which carries impulses from the cochlea. Both of this structures are part of the ear. This article explains the anatomy and pathway of the vestibulocochlear nerve .
The vestibulocochlear nerve is the eighth of the twelve cranial nerves. The fibers consist of only afferent sensory fibers. The word afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word efferent is the opposite of afferent, meaning away from the centre and toward the periphery; when the stimulus is carried back to the brain from a peripheral area.
The pathway for the afferent fibers of the vestibulocochlear nerve before and after its division is the following (from the Initiation in the periphery to the termination in the brain):
- The vestibule is filled with endolymphatic fluid and is responsible for the somatic balance of the human body. It is also innervated by small sensory branches that innervate the parts of the vestibule which include the saccule, the utricle, the anterior, posterior and lateral membrane of the ampulla.
- Those small branches merge into superior and inferior branches, before entering the vestibular ganglion.
- The cochlea is responsible for sensory hearing and is innervated by a small branch of postganglionic fibers, which exit the lamina spiralis ossea after innervating the inner and outer hair cells of the spiral organ of corti; upon the hair cells sits the tectorial membrane.
- The cochlear nerve synapses in the cochlea upon the spiral ganglia before running alone and eventually running adjacent to the motor root of the facial nerve, the intermediate nerve and the vestibular nerve.
- For a brief section, the vestibular nerve also runs as a single branch, known as the vestibulocochlear nerve, before it enters the skull accompanied by the facial nerve and the intermediate nerve (CN VII) and merges into one branch - the vestibulocochlear nerve.
- The sensory fibers of both sets of nuclei cross over one another as they exit their contralateral corresponding nerves before bundling together.
- Lateral to the inferior cerebellar peduncle, at the same level of the vestibular nuclei, sit the anterior and posterior cochlear nuclei.
- The vestibular nuclei are four and consist of the superior, inferior, medial and lateral nuclei. They are scattered in the brainstem at the level of the vestibular area and behind the medullary striae, medial to the inferior cerebellar peduncle.
The vestibulocochlear nerve is the eighth of the twelve cranial nerves and the fibers consist of only afferent sensory fibers.
The pathway starts from the vestibule, which is responsible for somatic balance. It is also innervated by small sensory branches which enter the vestibular ganglion. The second part of the nerve begins with the cochlea inside which the cochlear nerve synapses with the spinal ganglia. The vestibular nerve enters the skull together with the facial and intermediate nerves, merging into the vestibulocochlear nerve, which project onto the cochlear and vestibular nuclei.