- The mammalian ear performs two different and unrelated functions, i.e. hearing and balance.
- The external and middle ears are involved in hearing only whereas the inner ear functions in both hearing and balancing.
- The sound waves from the external source get collected by the pinna.
- These waves are conducted through the external auditory meatus to the tympanic membrane causing it to vibrate.
- These vibrations are then transmitted to the internal ear through the ear ossicles.
- The vibrations of the membrane of fenestra ovalis cause alternate increase and decrease in the pressure of perilymph of scala vestibule.
- The perilymph’s vibration produces simultaneous vibration of the endolymph in the cochlear duct.
- This causes the basilar membrane to vibrate.
- As this happens, the hair cells attached to the basilar membrane move in relation to the tectorial membrane, which remains stationary.
- This result in the bending of the hair cell microvilli embedded in the tectorial membrane.
- The sensory hair cells get stimulated and the nerve impulses are thus produced.
- These impulses are carried through cochlear branch of the auditory nerve to the brain where these are interpreted as sound.
- Cochlea of ear also analyzes the sound, which is very complex and not completely understood.
- The portion of the basilar membrane that vibrates as a result of endolymph vibration depends on the pitch of the sound that created the vibration.
- Higher frequencies seem to stimulate shorter fibres forming the basal part, while lower frequencies stimulate the longer fibres forming the apical part.
Balance or equilibrium
- The semicircular canals, utriculus and sacculus are concerned with balance and posture.
- Any change in the position of the head stimulates the sensory hair cells of maculae due to the movement of the endolymph and the otoliths.
- The nerve impulses, thus, produced are transmitted by the vestibular nerve to the brain via vestibulo-cochlear nerve.
- Finally, the brain responds by causing appropriate muscles to contract, thus, bringing the head back to its normal position (static equilibrium).
- The cristae present in the ampullae of semicircular canals respond only to the changes in the direction or rotational movements of the head.
- When a person is at rest, the cristae do not move.
- As a person begins to spin, the semicircular canals begin to move with the body, but the endolymph tends to remain stationary relative to the movement.
- Thus, the cristae are displaced by the endolymph in a direction opposite to the direction of spin.
- When the person stops spinning, the endolymph tends to continue moving for some more time stimulating the hair cells and sending false information to the brain (kinetic equilibrium).
Mechanism of hearing and balancing