Turning Air Movement Into Neural Energy.
Our sense of hearing allows us to detect several dimensions of sound. We can discern the loudness, the pitch, and the timbre of a sound as well as where it came from. What we perceive as loudness is the amplitude of the sound, or the difference between the high- and low-pressure parts of the sound waves. This corresponds to the height of waves at the ocean. Pitch indicates the frequency of the sound, or how far apart or close together the waves are. Timbre represents the complexity of the sound, or the number of simpler wave patterns of which one complex sound is made.
The auditory apparatus with which we detect sounds can be divided into three parts: the external, middle, and inner ears. The external ear is made of the pinna and the ear canal. The pinna is the visible part of the ear, and it helps us to focus and localize sounds. Sound is transmitted to the middle ear through the auditory canal.
The middle ear consists of the tympanic membrane, or eardrum, the malleus, the incus, and the stapes. The tympanic membrane covers the end of the auditory canal and is attached to the malleus. The incus connects the malleus to the stapes, which is connected to the inner ear. When the pressure of air in the external ear changes, the tympanic membrane moves in or out. This makes the stapes move in such a way as to convey sound vibrations to the inner ear.
The cochlea is part of the inner ear and is shaped a little bit like a snail's shell. It is filled with a fluid through which sound waves pass. Sound vibrations are transmitted into the cochlea when the stapes moves the oval window. The round window is covered by a flexible membrane which allows the fluid in the cochlea to move back and forth in response to sound vibrations. Because the oval window is much smaller than the tympanic membrane, small vibrations of the tympanic membrane at some frequencies result in relatively large vibrations of the oval window; in other words, the middle ear serves to amplify certain tones.
Inside the cochlea is a structure called the basilar membrane, which runs nearly the entire length of the cochlea. Attached to the basilar membrane are hair cells, which move when the fluid of the cochlea moves; the hair cells convert sound into signals that are sent to the brain. Different parts of the basilar membrane flex in response to different kinds of sound.
An unrolled cochlea
Hair cells at the area of the basilar membrane which is flexed are activated, and send messages through the cochlear nerve to the brain. At the top of each hair cell are a number of cilia, which are connected to each other by extremely fine fibers called of protein called tip links. Hair cells are activated by the tension that is placed on the tip links when the cilia are moved. Ordinarily, there is a little bit of tension on the tip links and, consequently, the hair cells are very slightly activated. Movement of fluid over the hair cells causes them to send impulses to the brain.
Cilia on Hair Cells