Name: ___________________________________________         Date: __________

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?

_____ 1.  The author’s purpose in this article is to:

       a)  explain how hearing works                                                    b) convince readers to support deaf research
       c)  describe a link between Bell’s research and deafness         d)  connect deafness to loss of hair cells

_____ 2.  Where are stereocilia located
      a)  within the ion channels                  b)  within the tip links       
      c) on the surface of motor proteins     d) within the cochlea

_____ 3.  The author states that Bell’s goals might yet be achieved.  What can you infer from the article were Bell’s goals?
      a) to invent the telephone                                       b)  to improve lives of deaf people             
      c) to prevent deaf people from having children           d)  to develop a cochlear implant

4.   What is MECHANOSENSATION? 

A Balancing Act                                                                                                                               by Rajini Rao

http://bit.ly/HpvM3Z

✦ Nature's Gyroscope: Your ear does much more than hear. While the snail shaped cochlea of the inner ear (pictured below) is superbly adapted for picking up sound vibrations (by deflections of hair cells described in last week's post), the rest of the inner ear is a complex labyrinth of tubes and chambers that keeps our lives in balance.

✦ Up, Side and Down: Since we live in a three dimensional world, we have three fluid-filled semicircular canals arranged at right angles to each other, along the x, y and z planes. Each semicircular canal senses a different movement of our head: up and down, side to side, and tilt. When we move our head, the fluid inside the canal moves and presses on a tear shaped bulb at one end. The bulb (ampulla) has a collection of mechanically sensitive hair cells embedded in a jelly like matrix. Deflection of the "hairs" triggers a message to the balance center of our brain that is interpreted as a deflection of the head. Because we have a pair of ears, the deflections are mirror images so that when one side is stimulated the other is simultaneously inhibited by the movement. 

✦ Rolling Stones: Two other chambers sense horizontal and vertical accelerations of your body. The saccule detects changes in vertical movement (when you are in an elevator), and the utricle monitors horizontal movement (as when a car suddenly moves forward or stops). While these organs also have mechanically sensitive hair cells, what is different is a special overlaying membrane weighted down with tiny stones of calcium carbonate, around a protein core, called otoconia. A shearing effect of the membrane against the hair cells detects vertical and linear accelerations of your body.  Sometimes, the otoconia fall into one of the semicircular canals (see image) sending conflicting signals to the brain, resulting in vertigo. Fortunately, a series of head maneuvers can restore the rolling stones back into place. Ménière's Disease is a common cause of vertigo, accompanied by hearing loss and tinnitus. It is thought to be caused by disturbances in the fluid volume filling the inner ear. Future relief from vertigo may come from prosthetic devices, similar to a cochlear implant in the inner ear.

4.  Based on the context of the article, what do the following words mean?

            a)  gyroscope

            b)  labyrinth

            c)  deflection

            d)  vertigo

5.  What is the difference between the saccule and the utricle?

6.  In your own words, describe the role of otoconio in maintaining balance.