Physiology of Hearing
Every sound produces sound waves or disturbances in the air, which travel at about 332 metres (1088) feet per second. The auricle, because of its shape, concentrates the waves and directs them along the auditory meatus causing the tympanic membrane to vibrate.
Tympanic membrane vibrations are transmitted through the middle ear by movement of the ossicles. At their medial end the footplate of the stapes rock to and fro in the oval window, setting up fluid waves in the perilymph. These indent the membranous labyrinth and the wave motion in the endolymph stimulates the neuroepithelial cells of the organ of Corti. The nerve impulses produced pass to the brain in the cochlear portion of the eighth cranial nerve (VIII). The fluid wave is finally expended into the middle ear by vibration of the membrane of the round window. This nerve, the vestibulocochlear nerve, transmits the impulse to various nuclei in the pons varolii and midbrain. Some of the nerve fibres pass to the hearing area in the cerebral cortex where sound is perceived.
The semicircular canals have no auditory function although they are closely associated with the cochlea. They provide information about the position of the head in space, contributing to maintenance of equilibrium and balance.
There are three semicircular canals, one lying in each of the three planes of space. They are situated above and behind the vestibule of the inner ear and open into it.
Structure of the semircular canals
The semicircular canals, like the cochlea are composed of an outer bony wall and inner membranous tubes or ducts. The membranous ducts contain endolymph and are separated from the bony wall by perilymph.
The utricle is a membranous sac which is part of the vestibule and the three membranous ducts open into it at their dilated ends, the ampullae. The saccule is a part of the vestibule and communicates with the utricle and the cochlea.
In the walls of the utricle, saccule and ampullae there are fine specialized epithelial cells with minute projections, called hair cells. Amongst the hair cells there are the minute nerve endings of the vestibular part of the vestibulocochlear nerve.
Functions of the semicircular canal
The semicircular canals, utricle and saccule are concerned with balance. Any change of position of the head causes movement in the perilymph and endolymph which stimulates the nerve endings and the hair cells in the utricle, saccule and ampullae. The resultant nerve impulses are transmitted by the vestibular nerve to the cerebellum.
The cerebellum also receives nerve impulses from the eyes and the muscles and joints. Impulses from these three sources are coordinated and efferent nerve impulses pass to the cerebrum where position in space is perceived, and to muscles to maintain posture and balance.