The acoustic nerve ahs two components-the cochlear nerve or the nerve of hearing, and the vestibular nerve which subserves equilibrium.
let's take a ride and analyse these two systems as we also find out some basic neurological test on the function of this nerve.
Cochlear system: The end-organs of the cochlear nerve are the hair cells in the organ of Corti, within the cochlea of the inner ear.
The central fibers from the cell bodies, pass as the cochlear nerve.
It traverses the internal auditory meatus, where it is inferolateral to the facial nerve and crosses the subarachnoid space at the cerebellopontine angel and enters the upper part of t he medulla to terminate in the dorsal and ventral cochlear nuclei.
Secondary acoustic fibers project via the trapezoid body and lateral leminscus to the primary auditory receptive areas in the transverse temporal gyri of Heschel through the auditory radiations.
vestibular system: The vestibular system consisting of the semicircular canals has the function of orienting the subject in space (all the 3 planes), and also maintain the relative position between the head and the body.
Changes in position set up electrical discharges in the neuroepithelium of the ampullae of the semicircular canals and the maculae of the Scarpas ganglion transmit these impulses to the vestibular nuclei of the same and opposite sides in the upper part of the medulla.
Vestibular nuclei are connected to the cerebellum through the inferior cerebellar peduncle, to the spinal centers through the vestibulospinal tracts and to the motor nuclei of the eye muscles, through the medial longitudinal bundle.
Disturbances of vestibular function lead to vertigo, inability t maintain posture, nystagmus and systemic disturbances such as nausea, vomiting, visual hallucination, feeling of rotation of the surroundings, sweating, teachycardiac, and hypotention.
In vertigo, there is a distinct sense of rotation.
The objects may seem to revolve around the patient 9objective vertigo) or the person may experience the sensation of spinning in the surrounds (subjective vertigo).
Testing Hearing is evaluated by noting the patient's ability to perceive the noise made when the examiner rubs his thumb and index finger or better still by a watch in front of the external auditory meatus.
The distance from the ear is noted, and the examiner compares the patient's acuity of hearing with his own.
Better information can be obtained by using a tuning fork.
Rinne test: Here, the patient's air conduction is compared with the bone conduction.
The tuning fork (512 Hz) is placed firmly against the mastoid and the patient is asked to indicate when the sound is no longer heard.
It is then placed in front of the external auditory meatus and the time during which it is heard is noted.
Normally, air conduction is better than bone conduction-Rinne positive.
In conductive deafness (Rinner negative) bone conduction is better than air conduction.
In sensorineural deafness, both bone and air conduction are diminished, but they retain the normal relationship and the Rinner is positive.
On the other hand, the RInner is negative in middle ear disease.
Weber test: Here, the tuning fork is placed over the forehead and the patient is asked to point to the ear he hears better.
Normally, it is heard equally well in both ears.
In conductive deafness, the sound is better heard by the involved ear, while in sensorineural deafness, it is better heard by the uninvolved ear.