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The inner ear

The sound energy transmitted through the middle ear is applied on the oval window. The oval window is the entrance to the fluid filled cochlea. The cochlea is also closed by another membrane, the round window, which faces the middle ear cavity. Scala media and scala tympani within the cochlea are separated by the basilar membrane, on which the outer and inner hair cells are located.

A force on the oval window creates a wave motion in the fluid, generating a motion of the basilar membrane. This motion stimulates the hair cells. The maximum amplitude of the travelling wave occurs at different locations at the basilar membrane depending on the frequency of the sound. High frequencies stimulate basal parts closest to the oval window and low frequencies stimulate the apical parts.

Stimulation of an inner hair cell results in release of a neurotransmitter inducing electrical activity in the nerve fibres in the auditory nerve. In addition to the inner hair cells, outer hair cells are also stimulated by the motion of the basilar membrane. The main function of the outer hair cells is to mechanically enhance the motion of the tectorial membrane, in order to increase the stimulation of the inner hair cells. This mechanism increases the audibility of low level sounds, but also contributes to the frequency resolution.

THE CENTRAL AUDITORY SYSTEM
The nerve signals from auditory stimuli propagate from the auditory nerve through the auditory pathways in the brainstem to the auditory cortex. On the way from the cochlea to the cortex the signals pass through different nerve nuclei. The number of nerve fibres increases and the signal analyses become more complex for each level in the central auditory pathways. The primary auditory cortex represents perception and sensation of sounds and the associative auditory cortex processes linguistic stimuli and information of spoken language and other information-carrying sounds.

Both outer and inner hair cells are associated with fibres of the auditory nerve. Inner hair cells are mainly connected to afferent nerve fibres, transmitting signals to the brainstem. The outer hair cells are mainly associated with efferent nerve fibres transmitting signals to the cochlea. The efferent signals influence the length of the outer hair cells, which affects the stimulation of the inner hair cells.




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Magnus Johansson, Peltor AB







 THE AUDITORY SYSTEM

  How does sound occur
  The auditory system 
  The outer ear 
  The middle ear
  The inner ear
  Hearing
  Exposure to noise 
  Hearing impairment
  The concept of equal energy
  How to measure noise levels
  Choose right hearing protection
  Do not over attenuate
  Noise regulations