The ear consists of three main divisions: the outer ear, middle ear, and inner ear. The outer ear is the portion that allows sound waves to be transmitted, via the tympanic membrane, or eardrum, to the middle ear. In the middle ear are three tiny bones, the ossicles, comprising the ossicular chain. Here, the sound waves are transduced into mechanical energy, the ossicular chain rocking back and forth. The two tiny muscles of the middle ear, the tensor tympani and the stapedius, have attachments to the ossicles. The ossicles interface with the inner ear, which includes the cochlea, a snail-shaped structure that ultimately contains the electrochemical mechanisms for changing the mechanical waves into nerve impulses traveling along the eighth cranial, or auditory (acoustic), nerve to the brain.
The function of the middle-ear muscles has been debated over the years. One of at least five theories has been termed the frequency-selection or accommodation theory. This theory presumes that contraction of the muscles increases the sharpness of hearing by acting as a damping mechanism that selectively absorbs acoustic energy at particular frequencies. The other theories are the intensity-control or protective theory, the fixation theory, the labyrinthine-pressure theory, and a less-accepted theory that the middle-ear muscles are involved in the formation of overtones.
The middle-ear muscles are usually not under voluntary control but contract in response to sound energy in what has been called the acoustic reflex. Electromyography has been an important laboratory technique for its study. The acoustic reflex alters the mechanical properties of the middle ear transmission system; acoustic impedence is the term given to the consequent mechanical resistance. It may be measured indirectly by audiologists and auditory researchers and has become a notable means of studying hearing in humans for both research and clinical purposes.
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