The Middle-Ear Muscles

M odern industry has produced a noisy world. The din of jack­ hammers, the whine of jet engines and the blare of amplified electric guitars have become all too commonplace. It was therefore con­ siderate of nature to have equipped the human ear with a rather sophis­ ticated noise-reduction system: two small muscles that are attached to the ossicles, the tiny bones that connect the eardrum to the cochlea (the struc­ ture that houses the sound-receptor cells). When the muscles contract, they dampen the vibrations of the ossicles, thereby reducing the acoustic signal that ultimately reaches the inner ear. Although they are skeletal muscles (in fact they are the smallest skeletal muscles in the human body), the mid­ dle-ear muscles are not under volun­ tary control. They contract reflexively about a tenth of a second after one or both ears are exposed to loud external sounds. Indeed, the characteristics of the reflex have become so well known that deviations from the normal re­ sponse serve as a basis for diagnosing various hearing disorders and neuro­ logical conditions. The muscles of the middle ear con-