Broadly hearing mechanism can be divided into:
- Mechanical conduction of sound (done by middle ear).
- Transduction of mechanical energy into electrical impulses (done by sensory system of cochlea)
- Conduction of electrical impulse to brain (i.e. auditory pathway)
- It is done mainly by middle ear. Middle ear not just simply conducts the sound but converts sound of great amplitude and less force to that of less amplitude and greater force. This function of the middle ear is called as impedance matching mechanism or the transformer action.
- Movements of the stapes footplate causes vibrations in scala vestibuli followed by scala tympani and is transmitted to the cochlear fluids which brings about movement of the basilar membrane.
- This sets up shearing force between the tectorial membrane and the hair cells.
- The distortion of hair cells gives rise to electrical nerve impulse.
PHYSIOLOGY OF HEARING
Conduction of sound
- Impedance matching mechanism or Transformer action of middle ear: conversion of sound of greater amplitude with lesser force to that of lesser amplitude but greater force
- It is accomplished by
- Lever action of ossicles
- Handle of malleus is 1.3 times longer than the long process of incur
- Mechanical advantage = 1.3
- Lever ratio = 1.3 : 1
- Hydraulic action of TM
- Effective vibratory area of TM = 55 sq.mm.
- Area of Foot Plate of stapes = 3.2 sq.mm.
- Hydraulic ratio = 17: 1
- The product of Lever ratio and Hydraulic ratio (17 x 1.3) gives the Transformer ratio of 22: 1
- Natural resonance and efficiency of auditory apparatus
- External auditory canal — 3000 Hz
- Tympanic membrane — 800 to 1600 Hz
- Middle ear — 800 Hz
- Ossicular chain — 500 to 2000 Hz
- Whisper — 30 dB
- Normal conversation — 60 dB
- Shout — 90 dB
- Discomfort of the ear — 120 dB
- Pain in the ear — 130 dB
- Frequency range in normal hearing — 20 Hz to 20000 Hz
- In routine audiometric testing — 125 to 8000 Hz are evaluated
- Function of Stapedius is Protects the ear from loud frequency sound.
- Exposure to noise above 160 dB causes permanent loss of hearing.
- Permanent privation of the hearing of either ear ,even though correctable by surgery,comes under Greivous injury.
- Psycho-acoustic index of loudness is indicated by the term 'Phon'.
- Sense gravity and Linear acceleration is associated with ‘otolith organs’.
- The limit of loudness expressed as decibels that people can tolerate without substantial damage to their hearing is 85 dB.
- Foetus starts hearing by what time in intrauterine life by 20 weeks.
- Sense organ for hearing is Organ of Corti.
- Bones of middle ear are responsible for Reduction of impedance to sound transmission.
- Impedance matching occurs at Difference of surface are of tympanic membrane and foot plate.
- Primary receptor cells of hearing is Hair cell
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