How Hearing Happens
☛ The Hair Cell: When a young student heard that the lab next to mine studied frog hair cells, she exclaimed, “Oh? I didn’t know frogs had hair!” Actually, hair cells, so named because of the curious stacked arrangement of hair-like stereocilia emerging from their crowns (image a), are the cells that detect sound. About 16,000 of them line the snail shaped cochlea of our inner ear, picking up sound induced vibrations of the fluid inside our ears of less than 1 nanometer. The remarkable hair cell is what gives us humans the ability to detect sound of frequency ranging from 20 Hz to 20 kHz.
☛ Mechanosensation: How does a hair cell detect sound? The secret lies in the way the stereocilia are stacked. Each one is connected to an adjacent taller ‘hair” by a tenuous thread, known as tip link (image b). At the bottom end, the tip link controls the opening of an ion channel while at the upper end it is held taut by a motor protein (myosin) that moves along tracks (actin) inside the “hair”. When a mechanical disturbance in the surrounding fluid pushes against the hair bundle (image c), the tip link is stretched, yanking open the gate of the ion channel. Calcium ions flood the interior, changing the electrical potential of the cell and triggering a message to the nerves leading away from the cell. Immediately, however, the motor protein slips down, releasing the tension on the tip link and closing the ion channel to end the signal (image d). Later, the motor protein climbs up the cables again to re-establish tension in the tip link.
☛ Deafness, Eugenics and Alexander Bell: It may come as a surprise that the inventor of the telephone also had a profound impact on deaf culture. With both his mother and wife deaf, Alexander Bell became an avid proponent of “oralism” – teaching deaf people to articulate sounds in place of sign language. Given the uniqueness of deaf culture with frequent intermarriage among deaf people, Bell cautioned that the incidence of deafness could rise until there was a separate race of deaf people. Although his ideas on eugenics are not credited now, he was responsible for many changes made to education of the deaf. Deafness is the most common inherited sensory defect at 1-3 births per 1000. Interestingly, the most common inherited form of deafness has actually increased due to assortive mating (this is also seen in other disorders linked to ethnicity or race). Bell’s goals may yet be achieved, not by eugenics but by cochlear implants, which may restore hearing and abolish deaf culture in the future. Will that be a good thing?
Another installment in the #excyting series on cell types.
▶ Cardiomyocyte: http://goo.gl/uBL37G
▶ Adipocyte: http://goo.gl/S4fQFS
▶ Erythrocyte: http://goo.gl/R5R6Y0
▶ Astrocyte: http://goo.gl/SMpXMV
Image and Free Read: Corey, D. (2009) Cell biology of mechanotransduction in inner-ear hair cells.
Through Deaf Eyes (Alexander Bell): http://goo.gl/DY2ouS