The low hearing frequency is related to the cochlea shape

The shape is very important, even for hearing. Especially the shape of the cochlea - a shell-shaped organ in the inner ear that converts sound waves into nerve impulses so that the brain can decode - surprisingly important.

The recently published study in the online Proceedings of the National Academy of Sciences found a direct link between the cochlea curvature and the low hearing frequency limit of about 12 other mammal species. together.

This relationship will be very useful for conserving animals if estimating the effect of noise from human activities on animals such as the Xiberian tiger, polar bear and other animal species. Breasts living under water never stand still when conducting auditory experiments. It can also provide new information about the hearing ability of extinct mammals such as mammoths, tiger fangs; from there can give us more insight into the evolution of hearing.

Daphne Manoussaki - assistant professor of mathematics at Vanderbilt University, who led the research with Richard S. Chadwick, Dean of the National Institute of Hearing Impaired and Communication Disorders (one of the National Institutes of Health - NIH) - said: 'It turns out that the curvature of the cochlea is not the size associated with the low hearing frequency limit'.

The cochlear implant reconstruction is in the human inner ear using a high resolution tomography machine similar to the image used to measure cochlear radius in the species analyzed in the study.(Photo: Courtesy of Darlene Ketten)

The cochlea has a characteristic spiral in mammals. Birds and reptiles have a disk-shaped or slightly twisted cochlea that limits the time they can hear. Helical cochlear animals often have a greater range of auditory hearing, but previous attempts to link the effects of hearing to the structural features of the cochlea do not produce satisfactory results by not counting to sound effect.

In 2006, Manoussaki and colleagues NIH published an article that suggested that the spiral form of the cochlea enhances the low-frequency sound thanks to the same effect as the 'long tunnel effect' in which the The small, light sound when moving through the curved wall in a large room will become loud enough to hear clearly from the other side of the room.

When sound waves enter the ear, they strike the eardrum, causing the eardrum to vibrate. Small bones in the ear expand and transmit vibrations into the fluid in the cochlea, creating pressure waves that propagate along the narrow pipe of this tubular twist. The pipe is one of the two main chambers created from the elastic membrane that runs along the length of the cochlea. The mechanical properties of the basement membrane vary from very hard at the wide outer end and gradually increase the flexibility inside while the room becomes cramped. Selective characteristics of the membrane make the sound waves grow and disappear. Different frequencies reach the peak in different positions along the membrane.

Sensory cells attach to the membrane and there are small hairs called stereocilia attached to the adjustable structure in the tube. When the basement membrane moves, the sensing cell is tilted and the stereocilia fibers are bent. This movement generates an electromagnetic signal that travels along the auditory nerve to the brain. As a result, sensing cells near the outer end of the cochlea recognize high-frequency sounds such as metallic flutes; and the cells in the inner end of the cochlea recognize the lower frequency sounds such as echoes of the bass drum.

The order of mechanical reactions from high frequencies to low frequencies still works in the old fashion even though the cochlea is straight or spiral. But Manoussaki's calculations, the spiral shape caused the energy of low-frequency sound waves to accumulate at the outer edge of the room. In contrast, this irregular energy distribution makes the basement membrane toward the outer wall more room, increasing the curvature of stereocilia.Extremely curved curvature at the top of the helical path where the lowest frequency sounds are received. Manoussaki and colleagues calculated that the sound intensity can be increased by up to 20 dexiben equivalent to the difference between the sound environment of a quiet restaurant and a busy, crowded street.

Darlene R. Ketten - an experienced scientist at Woods Hole Oceanographic Institute and an assistant professor at Harvard Medical School - also participated in the study. He said: 'The idea that the curvature of the cochlea has a great effect on hearing has been controversial for many years. The curvature of the cochlea is usually not taken into account during the study or the hypotheses are not yet satisfactory. We now have a hypothesis proved by many specific examples of using real ear shape and hearing ability. '

Ketten supports Manoussaki and colleagues with a high-resolution tomography machine that captures the cochlea of ​​many different land animals and aquatic mammals. Together with fellow biophysics, Manoussaki analyzed cochlear shapes and found that the lower hearing frequency limits of species from rats, cats, cows to whales vary in proportion to the radius. of the curve from the bottom of the cochlea to the top . This ratio has a value from 2 to 9. The larger the ratio, the lower the frequency of sounds the animal hears. Manoussaki said: 'This is perfectly reasonable because the larger the ratio, the more cochlear the cochlea. The sound energy of low-frequency sounds must be rubbed against the cochlea more, making it easier to scratch. '

Animals like mice, which have a ratio of about 2, cannot hear clearly at frequencies below 1000 Hz. Species such as cows and elephants with a ratio of about 9 can hear sounds with a low frequency of about 20 Hz. The authenticity of the method is also demonstrated by cats, guinea pigs and sea lions. The cat's cochlea is longer than the hamster's cochlea. But guinea pigs have a ratio of 7.2 and can hear frequency sounds as low as 47 Hz. While cats with a smaller ratio of 6.2 can hear up to 55 Hz. Similarly, sea lions have a basement membrane three times longer than guinea pigs. But its radius of 5.2 is lower than that of cats and guinea pigs. so it cannot identify sounds smaller than 180 Hz. (This is the sea lion's hearing limit when on the shore, it can hear sounds at 200 Hz).

Manoussaki said: 'What interests me is a macroscopic characteristic of ears that is also very effective for our hearing. As colleagues have pointed out, there are now too many studies done at the genetic and cellular levels, so we often don't see cases like this, when only a simple geometric structure It is also very significant. '

Participating in the study were Emilios K. Dimitriadis (National Institute of Biomedical and Biomedical Engineering), Julie Arruda (Woods Hole Oceanographic Institution), and Jennifer T. O'Malley (Massachusetts Ear Hospital and Infirmary)
The research was funded by the National Institutes of Health, Naval Research, the University of Crete and Vanderbilt Technical University.