Mechanism of hearing and balancing

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.

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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).

Related image

  • 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

 

Mechanism of hearing and balancing