IN­STANT EX­PERT: EARS

Science Illustrated - - CONTENTS -

Why you’re deaf and fall down a lot.

Our ears and the rest of the com­plex sense of hear­ing al­low our brains to dis­tin­guish be­tween 400,000 dif­fer­ent sounds. But the ears also help us keep our bal­ance, when we move. If the sen­si­tive hair cells in the ears are harmed, the brain is ad­vised of sounds which do not ex­ist.

Anor­mal, healthy ear can dis­tin­guish be­tween 400,000 sounds. It can also fil­ter out sound, if you are in a room full of peo­ple and only want to lis­ten to one con­ver­sa­tion. It is pos­si­ble due to 16,000 small hair cells that an­a­lyse the sound down to the slight­est de­tail on its way from the ex­ter­nal ear to the brain.

The hu­man ear is best at cap­tur­ing sound with fre­quen­cies of 1,0004,000 Hz, but it can cap­ture sound in the en­tire 20-20,000 Hz in­ter­val. The ear picks up the sounds, but it is the brain that hears a sound. Ba­si­cally, the ear's func­tion is to con­vert pres­sure waves, of which a sound is made up, into nerve im­pulses which the brain can cap­ture.

Our ears are made up of three parts. The ex­ter­nal ear, which con­sists of con­nec­tive tis­sue, is a type of satel­lite dish. The ex­ter­nal ear cap­tures sounds and con­cen­trates them through the au­di­tory canal to­wards the eardrum, which also makes up the bor­der on the mid­dle ear, where you will find the three tini­est bones of the hu­man body. The oval win­dow mem­brane marks the end of the mid­dle ear and the en­trance to the in­ter­nal ear, where you will find the cochlea. When the oval win­dow moves due to a sound, the hair cells also move, and via nerve fi­bres, the cells send sig­nals to­wards the brain.

The sig­nals pass through the au­di­tory nerve to end up in the au­di­tory cor­tex, i.e. the brain area re­spon­si­ble for sound. En route, some fi­bres cross over to the other side of the brain to be able to an­a­lyse where the sound came from. If the sound hits one ear first, it will reach the other about 700 mi­crosec­onds later. Nerve fi­bres also pass from the brain stem to the hair cells to re­duce their ac­tiv­ity and so their re­sponse to par­tic­u­larly harm­ful sounds. The hair cells are very sen­si­tive, and hefty noises such as loud rock mu­sic and plane en­gines could harm or de­stroy the cells.

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