|MadSci Network: General Biology|
I agree with your ex-physician but will try to give an explanation. The eardrum is connected to a series of bones in the middle ear. This series of bones acts like a lever system to conduct sound from the ear drum to the inner ear where the sensation of hearing actually occurs. The neural structures in the inner ear can be permanently damaged by exposure to high level sounds. There are two small muscles in the middle ear behind our eardrums that are connected to this series of bones. One muscle is the tensor tympani and the other is the stapedius, the smallest muscle in our body. These two muscles contract involuntarily in the presence of various stimuli including high levels of sound in the case of the stapedius, and very high levels of sound in the case of the tensor tympani. For low level and moderate level sounds, up to about 90 dB sound pressure level, these muscles show no activity. For sounds greater than 90 dB, the stapedius muscle begins to contract involuntarily and contracts harder for sounds that are louder. At very high sound levels, around 120 dB or so, the tensor tympani contracts. When the muscles contract, they reduce the sound vibrations going through the series of bones which has the effect of reducing or damping the sound reaching the inner ear. Therefore, the most common explanation of the function of these involuntary muscle contractions is that they automatically protect the inner from loud sounds. Another explanation for these muscle contractions, particularly for the stapedius muscle, is that they automatically allow fine adjustments of the tension of the ear drum to optimize sound transmission. In either case, the contractions are involuntary and operate automatically without us being aware of them. Individuals in a small percentage of the population can contract these muscles voluntarily even though the muscles normally contract only involuntarily. The voluntary contraction of these muscles causes them to vibrate not unlike the small vibrations in any muscle that is contracted very hard, say the muscles in your arm. You perceive these contracted middle ear muscle vibrations as a "rumbling noise" because the vibrations are transmitted the middle ear bones and then to the inner ear. I suspect that you are among this small percentage of the population who can voluntarily contract the middle ear muscles. Why can't you hear the "rumbling noise" when the muscles contract involuntarily? First, the loud sounds that cause the involuntary muscle contractions are much louder than the quieter "rumbling noise" from the muscle contractions making the "rumbling noise" inaudible. Second, the involuntary muscle contractions are probably much weaker than voluntary contractions. You might be aware that a small percentage of the population can wiggle their external ears, the part of the ear on the side of the head. The muscles that contract to wiggle the ears also are activated involuntarily. This function is easily observed in a dog for example, when it automatically moves its ears to help determine from where a sound is coming. Even though this involuntary function has largely disappeared in humans, a small percentage of us still can voluntarily wiggle our ears. So, a few people can voluntarily contract any of the muscles of the ear though this ability has no particular usefulness nor does i
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