Hearing is yet another variation on a theme. As with the previous senses described, something in the external world stimulates modified neurons, and the stimulation initiates an action potential that passes along a series of neurons until it reaches the brain. In this case, the stimuli from the outside world are sound waves, tiny fluctuations in air pressure, collected and amplified by the ears, which then pass information to the brain. While tastes and smells are detected by chemoreceptors and sight is made possible by photoreceptors, hearing is a result of the stimulation of mechanoreceptors, specialized neurons with receptors that respond to mechanical pressure.

Although the details of how hearing works vary across different species, a general, six-step hearing model is consistent (FIGURE 23-26).

Figure 23-26The process of hearing.

• 1. The outer part of the ear collects sound waves and, because of its shape, funnels them down the ear canal, a channel that conducts sound waves.
• 2. At the end of the ear canal, the sound waves bang into the eardrum—a thin membrane that divides the outer ear from the middle portion of the ear—causing it to vibrate.
• 3. Small bones on the other (inner) side of the eardrum pass the vibrations on to the inner ear membrane.
• 4. The vibrations are conducted by fluid inside the inner ear, which consists of fluid-filled canals in the bones of the skull. The semicircular canals function in balance, and the coiled cochlea is involved in hearing. This fluid movement bends hair-shaped receptor cells, which release neurotransmitters. (The more vigorous their bending—in response to stronger sound waves—the greater the amount of neurotransmitter released.)
• 5. Auditory neurons respond to the neurotransmitters released by the hair cells by firing an action potential.
• 6. The signal continues along a path of neurons to the brain, where it is interpreted as sound.

The fluid in the semicircular canals of the inner ear also acts like an inner motion detector, telling your body about its orientation, speed, and direction. It’s not a foolproof system, though. Sitting in an IMAX theater watching a movie filmed from the seat of a roller coaster, your eyes tell your brain that you are moving, while your inner ear senses no motion at all. The conflicting signals can confuse your brain and lead to feelings of nausea. Some people experience similar effects when playing certain videogames.

Long-term exposure to loud noises, including music, can be damaging to hearing, because such stimulation can wear out the cochlea of the inner ear. The hair cells in the inner ear are very fragile and are irreplaceable, so chronic over-stimulation due to loud noises can damage them. This reduces their ability to release neurotransmitters and stimulate auditory neurons, causing hearing loss (FIGURE 23-27).

Figure 23-27Loud noises can cause hearing loss.

Are ears necessary for hearing? Not really. Most insects, for example, don’t have ears but still can hear a wide range of sounds, often with even greater sensitivity than humans. They do it with delicate hairs on their antennae, or on other parts of the body, that bend slightly in response to sound waves, much like the inner-ear hair cells described above. These insects often produce noises (that is, communicate by sound) by rubbing parts of their body together, rather than using their mouth. Male mosquitoes identify females by the vibrations of their wings as they fly. Cricket chirping, on the other hand, requires a bit more work; like miniature violinists, crickets rub their wings together to create their special mating calls.

Among the mammals, bats have a unique system for hearing, called echolocation. Most bats spend nearly all of their time in the dark, and such a sensitive call-and-response system of hearing has evolved that they can sense where everything is in their environment, even with terrible sight. They do this by emitting high-pitched squeaks (that are generally inaudible to humans) and then waiting to hear the sounds as they bounce off surfaces around them. This system is so sensitive that bats can detect even a small moth as it flies in their vicinity (FIGURE 23-28).

Figure 23-28Hearing through echolocation.