Mechanism of hearing and balancing
July 29, 2018
Introduction
- 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.
Hearing
- 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).
References:
i) https://www.ohniww.org/mechanism-general-overview/
ii) https://www.medicinenet.com/ear_anatomy_of_hearing_and_balance/views.htm