We hear thanks to auditory receptors – hair cells in the inner ear. Sound vibrations through the tympanic membrane and the auditory ossicles (hammer, anvil and stirrup) are transmitted to the liquid medium of the inner ear, and already the vibrations of this liquid medium cause the cellular stereocilia (hairs) to move. Hair cells release neurotransmitters that act on nearby neurons, and then a nerve impulse runs to the brain, reporting some kind of sound.
Until now, it was believed that hair cells have frequency specialization and that they cluster in certain zones. For example, there are cells that respond best to 5000 Hertz, and they sit in the inner ear all together. All neurons that carry information about a sound at 5000 Hz to the brain depart from such a “five thousandth” area. However, as written in Science Advances staff at the University of Oregon Health Sciences and Linköping University, the tonotopic (“frequency-zonal”) theory of hair cell distribution emerged after experiments with high frequencies. And now the researchers decided to check how the hair cells that respond to low frequencies are distributed.
Electron micrograph of mouse inner ear hair cells. (Photo: SNSF Scientific Image Competition / Flickr.com)
Experiments were carried out with guinea pigs, and it turned out that a lot of cells simultaneously respond to frequencies below 1000 Hz, that is, there is no specialization for such frequencies. (At frequencies below 1000 Hz, normal speech vowels and, for example, notes from subcontroctave to the second octave sound.) This means that these frequencies are heard better – the brain receives much more signals about them than about higher sounds. And if, with age, sensitive cells die and hearing deteriorates, then the audibility of low frequencies will not weaken as quickly as the audibility of high frequencies – again, because a lot of cells respond to low frequencies.
Guinea pigs were chosen for the experiments because they hear low frequencies in much the same way as humans, and the features of auditory hair cells in guinea pigs and in humans are largely similar. Probably, new data will be useful in the development of cochlear implants, whose principle of operation is to compensate for the death of auditory receptors by transmitting sound vibrations in the form of electrical signals directly to the auditory nerve.
Source: Автономная некоммерческая организация "Редакция журнала «Наука и жизнь»" by www.nkj.ru.
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