Mechanism of hearing

Functions of External Ear

Sound waves enter the external auditory meatus, pass along the external eustachian canal and fall on the tympanic membrane (TM). This causes the TM to vibrate. (Mechanism of hearing)

Functions of Middle Ear

The middle ear is an air filled cavity in the temporal bone which opens via the eustachian tube into the nasopharynx. The auditory tube opens during chewing, swallowing and yawning thus keeping the pressures on both sides of the tympanic membrane equal.

The three auditory ossicles are localised in the middle ear. Of these,
the malleus is attached to the TM, and the stapes to the membranous oval
window on the medial wall. Incus articulates with these two bones. Thus
vibrations of the TM are transmitted to the oval window. As the TM has an
area of 90mm2 and the foot plate of the stapes 3.2mm2 and the lever system
formed by the ossicles multiplies the force 1.3 times. (Mechanism of hearing)

The vibrations of the oval window generate pressure waves in the
fluid filling the vestibular canal. The pressure waves pass to the median canal
and vibrate the basilar membrane. The tympanic canal is connected to a
circular membrane called the round window just beneath the oval window.
This arragement allows the pressure waves to transmit through the cochlear
fluid. (Mechanism of hearing)

Functions of Cochlea

The inner ear (labyrinth) is made up of the cochlea and the vestibule. The cochlear portion of the labyrinth is a tubule coiled 2.75 times. Throughout its length the cochlea is divided into three compartments by the basilar and the Reissner’s membranes. (Mechanism of hearing)

The middle compartment (scala media) contains endolymph and the other two (scala vestibuli & scala tympani) contain perilymph.

Organ of Corti

Located on the basilar membrane is the Organ of Corti which
contains the auditory receptors. Four rows of hair cells arise from the basilar
membrane. (Mechanism of hearing)

Stimulation of hair cells

Movements of the foot plate of the stapes set up a series of waves in the perilymph of the scala vestibuli. This in turn causes vibrations of the vestibular membrane and hence of the endolymph in the scala media. These waves deflect the Reissner’s membrane and this in turn produces disturbances in the basilar
membrane which bend the hairs of hair cells in the organ of Corti. This leads
to development of action potentials in the related nerve fibres which are
transmitted along the auditory nerve. (Mechanism of hearing)

The site of maximum distortion in the organ of Corti is
determined by the frequency of sound, for high pitched sounds the
maximum height of the waves is near the base of the cochlea and for low
pitched near the apex. The extent of distortion is determined by the loudness
of the sound. Interpretation of these impulses is made in the auditory cortex.

Sound waves ® vibrate tymphanic membrane ® movements in ear ossicle
chain ® vibration of oval window ® waves in perilymph (scala vestibuli) ®
waves in endolymph ® deflection in the Reissner’s membrane ® basilar
membrane disturbed ® bending of hair cells ® action potential ®
transmission by auditory nerve. (Mechanism of hearing)