Physiology of hearing (ENT)

i. Conduction of sound:

• 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.

ii. Transduction of mechanical energy to electrical impulse:

• 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.
•  A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on the basilar membrane, and stimulates that segment (traveling wave theory of von Bekesy).
• Higher frequencies are represented in the basal turn of cochlea and the progressively lower one toward the apex.

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

Intensity of various sound at a distance of 1 m

• 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

Exam Question

• 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