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Understanding How We Hear

Wednesday, April 25, 2012 - 15:33

Contributing Author: Guy Slowik FRCS

Hearing is a complex process involving the coordination of many different parts of the ear and nervous system.

The ear consists of five main parts:

  1. The outer ear
  2. The middle ear
  3. The inner ear
  4. Nerve pathways
  5. The brain

The Outer Ear

  • The outer ear consists of the auricle, the part of the ear that we can see, and includes the ear lobe and the external ear canal. These structures collect sound waves and carry them toward the eardrum.

The Middle Ear

  • The middle ear lies between the eardrum and the inner ear. It contains three tiny bones, commonly called the hammer, anvil, and stirrup or stapes. These bones are named for their distinctive shapes.
  • Sound, which is carried through the air like a continuously moving wave, makes the eardrum vibrate. These vibrations are then transmitted across the middle ear space by the tiny ear bones. Movement of the stapes bone produces waves in the liquid-filled inner ear.
  • The middle ear chamber is connected to the back of the nose by a small canal called the eustachian tube. This tube maintains equal pressure between the middle ear and outside environment.

A good example of how this mechanism works is to think of the popping sensation experienced during altitude changes such as while landing in an airplane or traveling at high altitudes. The popping sound is a result of pressure adjustments in the middle ear through the eustachian tube.

The Inner Ear

  • The inner ear is enclosed in dense bone and is composed of two specific parts:
    1. The organ of hearing (the cochlea)
    2. The organ of balance (the vestibular labyrinth)
  • The interior of the organ of hearing or cochlea is divided into three compartments by strands of tissue, as in a shell. The middle compartment contains the hearing hair cells that are bathed in special fluid. These special cells respond to the fluid waves produced by the movements of the tiny ear bones, the stapes.

The Nerve Pathways

  • Fluid waves in the cochlea are changed into electrical impulses, which travel rapidly along the auditory or hearing nerve to the brain.
  • The nerve pathways leading to the brain are enclosed in a small bony canal along the nerve responsible for balance, and the nerve that stimulates the movement of the facial muscles.

The Brain

  • The nerve hearing pathways divide as they reach the brain into an extremely complex intercommunication system.
  • Ultimately, nerve impulses are transmitted to a certain part of the brain behind the temple. There, these impulses are processed and interpreted as recognizable sounds.

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