中耳肌(word 版）

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The middle ear muscles of humans and animals

Tiny muscles behind the eardrum contract involuntarily when a person vocalizes or is exposed to a loud noise. This neuromuscular control system prevents sensory overload and enhances sound discrimination.
Modern 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 common place. It was therefore considerate of nature to have equipped the human ear with a rather sophisticated noise-reduction system: two small muscles that are attached to the ossicles, the tiny bones that connect the eardrum to the cochlea (the structure 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 middle-ear muscles are not under voluntary 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 response serve as a basis for diagnosing various hearing disorders and neurological conditions.
The muscles of the middle ear contract not only in response to loud external sounds but also immediately before a person vocalizes. This prevocalization reflex operates even when one speaks, sings, or cries as softly possible. Yet most evidence suggests that it is meant to protect the inner ear from the fatigue, interference and potential injury caused by one’s own louder utterances, which can result in high sound levels in one’s head. The shouting and wailing of children or babies, for example, can reach their own ears with the same intensity as the sound of a train passing nearby.
The middle-ear muscles do more than just indiscriminately attenuate internal or loud external sounds in humans. The muscles muffle primarily a loud sound’s lower frequencies, which tend to overpower its higher frequencies. The net result of this frequency selectivity is to improve hearing particularly of those sounds that contain many high-frequency components, such as human speech. In fact, the middle-ear muscles are what enable one to hear other people talking even while one is speaking.
Perceived sounds regardless of their source are air-pressure waves that have been funneled to the tympanic membrane, or eardrum, causing it to vibrate. The vibrations are transmitted through the three ossicles in the middle ear (the malleus, incus and stapes) to the cochlea. The middle-ear mechanism the eardrum and ossicle linkage serves to convert the movements of low-density air into analogous movements of the higher-density fluid in the cochlea. The movements of the fluid are transmitted to the stereocilia: fine, hairlike protrusions of receptor cells on the cochlea’s basilar membrane. Mechanical forces on the stereocilia cause the cells to trigger electrical impulses in the auditory nerve that are then interpreted by the brain as sound. Attached to the ossicles are the two middle-ear muscles: the tensor tympani and the stapedius.
The tensor tympani is connected to the neck of the malleus and is anchored in the wall of the Eustachian tube (a ventilating tube connecting the throat, nasopharynx and middle ear). The stapedius oriinates in the wall of the middle-ear cavity (n. 腔；洞，凹处) and ends at the neck of the stapes, near its articulation (n. 关节；接合；清晰发音) point with the incus. The basic anatomy (n. 解剖；解剖学；剖析；骨骼) of the middle-ear muscles was described as early as 1562, by Bartolomaeus Eusta-chius (from whom the Eustachian tube is named). Yet the function of the muscles in human hearing was a subject of speculation until this century, when laboratory experiments on animals and clinical observation made a comparative analysis of their physiology (n. 生理学；生理机能) possible.
The middle-ear-muscle system is found in all classes of vertebrates, but it has distinctive features in certain species. In same species of frogs, for example, the hearing organ contains only a single ossicle that has a stapedi-uslike muscle attached to it. It is interesting to note that those frog species without a muscle or an ossicle in middle ear tend not to vocalize. Among lower vertebrates, birds possess the most elaborate systems for hearing and sound communication. In each ear they have a stapedius analogue, which is attached to both the tympanic (adj. 鼓膜的；鼓室的；鼓皮似的) membrane and a single ossicle, the columella. Because a bird’s stapedius muscle lies mainly outside the middle-ear cavity, it can be studied more readily than the stapedius of mammals without damaging the delicate middle-ear structures.
We have worked with common domestic fowl, such as chickens, in a series of experiments on the physiology of the stapedius at the Karolinska Institute in Stockholm and at Harvard University. By attaching a strain gauge to the tendon (n. [解剖] 腱) of a bird’s stapedius and then stimulating the muscle electrically, we wound that the stapedius was capable of contracting at rates in excess of 100 times a second.
The muscle’s inherent capacity for quick response and fatigue resistance is also evident from the microscopic appearance of its fibers. Electron micrographs show that the fibers contain abundant mitochondria (which provide energy), dense sarcoplasmic reticulum (n. 网状组织，网状质；网罟座（南天的星座）；[脊椎] 蜂巢胃) (which releases the calcium ions that trigger contraction) and numerous transverse tubules for the transmission of calcium ions.

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