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The Inner Ear

The inner ear lies beyond the ossicles. The oval window is the boundary between the middle and inner ear, just as the eardrum (tympanic membrane) is the boundary between the outer and middle ear. The inner ear is filled with fluid and encased in some of the hardest bone of the skull. The main organ of the inner ear is the cochlea, a snail-shaped structure.

What is the cochlea?

The cochlea contains long channels that are coiled into a snail-shell-like shape.

How does the cochlea act as a transducer?

The cochlea is a transducer, which means it is a system for converting energy from one form into another. The cochlea transduces mechanical vibrations of sound into nerve impulses. How does the cochlea transduce pressure waves into nerve impulses? Throughout its entire length the cochlea is divided into several parallel channels filled with fluid. In this picture the artist shows a single, empty channel to highlight the spiral shape of the cochlea. In reality, the channel is divided into several smaller chambers, and it contains some delicate structures.

How is frequency of sound related to location of the traveling wave?

Sound vibrations produce ripples in the fluid within the channels. Superimposed on tiny, fast vibrations of the fluid are larger, slower waves called traveling waves. Georg von Bekesy won the Nobel Prize in 1961 for research that showed that sounds of different frequencies cause traveling waves that peak at different locations on the cochlea.

The basilar membrane is a thin sheet of tissue that runs the length of the cochlea. It is surrounded by channels of fluid where traveling waves occur. Along the basilar membrane is an elaborate structure called the Organ of Corti. It contains tiny hair cells that touch an overhanging shelf, the tectorial membrane.

Where are the receptors for the sense of hearing? How are mechanoreceptors involved in the sense of hearing?

Tiny hair cells (cilia) are vibrated by the basilar membrane.

The hair cells are bent when the basilar membrane and surrounding fluids are distorted by traveling waves. When a hair is bent, it generates an electrical impulse that ultimately causes the firing of nerve impulses along the auditory nerve leading to the brain. Therefore, in a sense, the sense of hearing is based on touch. The receptor cells for the sense of hearing are mechanoreceptors (hair cells in the Organ of Corti).

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