This nerve consist of two sets of sensory fibers; Vestibular and Cochlear
The vestibular fiber which are concerned with equilibrium, represent the central processes of nerve cells of the vestibular ganglion. This is located in the outer part of the internal acoustic meatus.
The cochlear fibers which are concerned with hearing represent the central process of nerve cells of the spiral ganglion of the cochlea.
FUNCTIONAL COMPONENTS
The Cochlear nuclei (dorsal and ventral) that receive impulses of hearing through cochlear nerve.
The Vestibular nuclei (superior, inferior, medial and lateral) that receive fibers from the semicircular canals, the utricle, and the saccule through the vestibular nerves.
ORIGIN
The two parts of the nerve leave the anterior surface of the brain between the lower border of the pons and the medulla oblongata. They cross the posterior cranial fossa and enter the internal acoustic meatus with the facial nerve.
On reaching the bottom of the internal acoustic meatus the nerve divides into vestibular and cochlear portions.
VESTIBULAR GANGLION
The vestibular nerve is expanded to form the vestibular ganglion. The branch of the nerve then pierces the internal acoustic meatus and gain entrance to the membranous labyrinth where they supply the utricle, the saccule and the ampullae of the semicircular ducts
SPIRAL GANGLION
The cochlear nerve divides into branch which enters the foramina at the base of the modiolus. The sensory ganglion of this takes the form of an elongated spiral ganglion that is lodged in a canal winding around the modiolus in the base of the spiral lamina.
The peripheral branch of this nerve passes from the ganglion to the spiral organ of corti.
Auditory Receptors
The auditory receptors are the hair cells of the organ of corti, situated in the cochlear duct of the internal ear. They are innervated by the peripheral processes of bipolar neurons of the spiral ganglion.
CLINICAL CORELATES
Lesions of this nerve cause hearing defects. Hearing power can be tested by means of a watch, one ear at a time. If there is any impairment of hearing one must determine whether it is really due to disease of the nerve (nerve deafness), or merely due to disease of the middle ear (conductive deafness). This is done by Rinne’s test and Weber’s test.
Principle of the Tests
The tests are based on the principle that normally aerial conduction of sound is better than bony conduction. In conductive deafness bony conduction becomes better than aerial conduction. In nerve deafness both types of conduction are lost.
Rinne’s test
In Rinne’s test a vibrating tuning fork is held opposite the ear and then placed on the mastoid process. The patient is asked to compare the loudness of the fork in the two instances.
Weber’s test
In Weber’s test the vibrating tuning fork is placed on the centre of the forehead. The fork is heard better on the side of middle ear disease than on the normal side.
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