This nerve begins in the inner ear and terminates in the cochlear nucleus in the medulla. From there, second order neurons carry auditory information to the superior olivary nucleus in the medulla, inferior colliculus in the midbrain, the medial geniculate body in the thalamus and, finally, to the temporal lobe of the cerebrum. The efferent sensory fibers in this nerve carry commands from the CNS designed to the receptors in the inner ear. The CNS, therefore, has the limited ability to modify the response characteristics of the receptors such the organ of Corti. Involvement of the acoustic nerve results in impairment of hearing and disturbance of postural equilibrium.
Diagnosis requires fairly sophisticated audiometric, neurological and vestibular tests.
The motor fibers supply the stylopharyngeus and the superior pharyngeal constrictor muscles. An involvement of the the glossopharyngeal nerve results in the loss of or a reduction in the strength of the gag reflex and may also affect swallowing. The gag reflex may be tested by touching, with a sterile cotton swab, the posterior wall of the pharynx. The reflex consists of the elevation of the velum, retraction of the tongue, and the contraction of the pharyngeal musculature. The gag reflex may be absent in some people without the involvement of the nerve, particularly, in heavy smokers and drinkers.