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| Anatomy of human ear |
Outer ear
Outer ear serves to gather sound waves and to lead them into external auditory canal, which is capped with drum called tympanic membrane. This membrane transfers vibrations further into middle ear.Middle ear
Tympanic membrane carries sound forward to three bones named: hammer (malleus), anvil (incus), and stirrup (stapes). They amplify the sound (nearly thirty times) and bring it to the snail-shaped organ called cochlea, which is already element of inner ear.Inner ear
Inner ear is neurosensory part of ear, which is literally “electronics” of ear as a whole – and it is highly sensitive to damage. Cochlea or cochlear apparatus or Organ of Corti is placed directly in cranial bone and it contains around 16000 neurosensory hearing cells called hair cells. These are of two types: inner hair cells (IHC) and outer hair cells (OHC). Function of IHC is to change sound waves to electric signal, which goes through auditory nerve or Cochlear nerve into brain. OHC serve as additional amplifier of those signal.Damaged hair cells (most often by acoustic trauma) can't accomplish their purpose and outcome of such state is hearing loss. IHC are in general more resistant to damage than OHC, which are dying sooner and thus amplifying of sound is not sufficient – it results in lower hearing sensitivity.
Here is very illustrative video on this issue:
On the following image we can see the difference between intact and damaged hair cells:
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| source: pnas.org |
