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The human ear is a sophisticated system that converts acoustic energy (sound) to an electrical impulse that is processed and interpreted by the brain.

 

The outer portion of the ear includes the pinna. The pinna lies on the outside of the head and works as a funnel that channels sound directly into the ear canal.The ear canal is a long tube that courses into the skull and carries sounds to the tympanic membrane (eardrum).

 

The tympanic membrane is a thin, opaque tissue that stretches tightly across the end of the ear canal. The acoustic energy hits the tympanic membrane, causing the tissue to vibrate. We have successfully transformed our acoustic energy, or sound, into mechanical energy.

 

As we pass the tympanic membrane, we enter the middle portion of the ear. The middle ear is home to the three smallest bones in the human body - the incus, the malleus, and the stapes. At one end, this chain on bones, collectively referred to as the Ossicular Chain, is attached to the tympanic membrane. Vibration of the eardrum causes mechanical movement of the Ossicular chain.

 

The transfer of mechanical energy through the chain of small bones is concentrated in the footplate of the stapes - the final bone in the chain. The stapes lies against a second opaque, tissue membrane called the round window. As the stapes moves, the round window vibrates - again transferring the mechanical energy to another medium.

 

As we move to the opposite side of the round window, we have entered a fluid filled cavern carved out by the cochlea. The cochlea is a snail shaped formation within the skull and is filled with endolymphatic fluid. As the round window vibrates with stapes movement, the fluid within the cochlea is displaced; mechanical energy has now been converted to tiny waves of hydraulic energy.

 

As we travel into the cochlea, we see the basilar membrane. This dense, skin-like membrane courses through the cochlea and is lined with four rows of small hair cells. Hair cells are tiny fibres that act like small protuding fingers that move in response to fluid displacement. As the fluid moves, the hair cells bend.As the fluid within the cochlea is displaced, the entire basilar membrane shifts up and down. The strength of these movements corresponds to the original sound and encodes pitch and volume information to the system. As the basilar membrane moves, the hair cells are displaced.

 

The displacement of the hair cells causes an electrical impulse to be generated. We have now converted hydraulic and mechanical energy into electrical energy. This electrical impulse travels from the hair cell to the auditory nerve. The auditory nerve passes this information to the auditory cortex in the brain for processing and interpretation.