PHYSIOLOGY OF HEARING
Sound travelling in the environment is collected by the pinna. It passes through external auditory canal and strikes the tympanic membrane. Vibrations of the tympanic membrane are transmitted to stapes footplate through a chain of ossicles (malleus and incus) coupled to the tympanic membrane.
Movements of stapes footplate cause pressure changes in the labyrinthine fluids, which move the basilar membrane. This results in stimulation of the hair cells of the organ of Corti.
The stimulated hair cells convert the mechanical energy into electrical impulses, which travel along the auditory nerve.
Mechanism of hearing can be broadly classified into:
- Mechanical conduction of sound (conductive apparatus).
- Transduction of mechanical energy to electrical impulses (sensory system of cochlea).
- Conduction of electrical impulses to the brain (neural pathways).
Conduction of sound: Impedance matching is one of the important functions of middle ear. The middle ear transfers the incoming vibration from the comparatively large, low impedance tympanic membrane to the much smaller, high impedance oval window. Middle ear is an efficient impedance transformer. This will convert low pressure, high displacement vibrations into high pressure of the air into, low displacement vibrations suitable for driving cochlear fluids. It is accomplished by:
- Lever action of the ossicles– Handle of malleus is 1.3 times longer than long process of the incus, providing a mechanical advantage of 1.3.
- Hydraulic action of tympanic membrane– The area of tympanic membrane is much larger than the area of stapes footplate. The average ratio between the two is 21:1. As the effective vibratory area of tympanic membrane is only two-thirds, the effective areal ratio is reduced to 14:1, and this is the mechanical advantage provided by the tympanic membrane.The product of areal ratio and lever action of ossicles is 18:1.
- Curved membrane effect– Movements of tympanic membrane are more at the periphery than at the centre where malleus handle is attached. This too provides some leverage.
- Sound waves striking the tympanic membrane do not reach the oval and round windows simultaneously. There is a preferential pathway to the oval window because of the ossicular chain. Thus, when oval window is receiving wave of compression, the round window is at the phase of rarefaction.
- This acoustic separation of windows is achieved by the presence of intact tympanic membrane and a cushion of air in the middle ear around the round window.
- Phase differential between the windows contributes 4 dB when tympanic membrane is intact.
- External and middle ear allow certain frequencies of sound to pass more easily to the inner ear due to their natural resonances.
- Natural resonance of external ear canal is 3000 Hz and that of middle ear 800 Hz.
- Frequencies efficiently transmitted by ossicular chain are between 500 and 2000 Hz while that by tympanic membrane is 800–1600 Hz.
- Hence sound transmission occurs between 500 and 3000 Hz.
- Movements of the stapes footplate is transmitted to the cochlear fluids.
- Movement of the basilar membrane occurs and it produces a shearing force between the tectorial membrane and the hair cells.
- Distortion of hair cells gives rise to cochlear microphonics, which trigger the nerve impulse.
- Sound wave depending on its frequency reaches maximum amplitude on a particular place on the basilar membrane and stimulates that segment (travelling wave theory of Von Bekesy).
- Higher frequencies are represented in the basal turn of the cochlea and the lower ones towards the apex.
- Hair cells get innervation from the bipolar cells of spiral ganglion.
- Central axons of these cells collect to form the cochlear nerve which goes to the ventral and dorsal cochlear nuclei.
- From there, both crossed and uncrossed fibres travel to the superior olivary nucleus, lateral lemniscus, inferior colliculus, medial geniculate body and finally reach the auditory cortex of the temporal lobe.