How do we hear?

1. How do we hear? Ms. Cox

2. Do you remember the parts of the ear?

3. Parts of the Ear Anvil Hammer Oval Window Stirrup

4. Steps in hearing. Step 1 • The pinna, the outer part of the ear, serves to "catch" the sound waves. • Your outer ear is pointed forward and it has a number of curves. This structure helps you determine the direction of a sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind you, above you or below you.

5. Step 2 • Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth • The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave.

6. Continued Step 2 • This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a greater distance. • The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. The tensor tympani muscle and the stapedius muscle suddenly contract. This pulls the eardrum and the connected bones in two different directions, so the drum becomes more rigid. When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, so the loud noise is dampened. • In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks loud, low-pitch background noise so you can focus on higher-pitch sounds. Among other things, this helps you carry on a conversation when you're in a very noisy environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a lot of the other sounds around you. • The eardrum is the entire sensory element in your ear.

7. Step 3 • This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include: • The malleus, commonly called the hammer • The incus, commonly called the anvil • The stapes, commonly called the stirrup

8. Continued Step 3 • The malleus is connected to the center of the eardrum, on the inner side. When the eardrum vibrates, it moves the malleus from side to side like a lever. The other end of the malleus is connected to the incus, which is attached to the stapes. The other end of the stapes -- its faceplate -- rests against the cochlea, through the oval window.

9. Step 4 • The cochlea is by far the most complex part of the ear. Its job is to take the physical vibrations caused by the sound wave and translate them into electrical information the brain can recognize as distinct sound. • The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes. In reality, these tubes are coiled in the shape of a snail shell, but it's easier to understand what's going on if you imagine them stretched out. It's also clearer if we treat two of the tubes, the scalavestibuli and the scala media, as one chamber. The membrane between these tubes is so thin that sound waves travel as if the tubes weren't separated at all.

10. Step 4 Illustration

11. Step 5 • The organ of corti is a structure containing thousands of tiny hair cells. It lies on the surface of the basilar membrane and extends across the length of the cochlea. • Until a wave reaches the fibers with a resonant frequency, it doesn't move the basilar membrane a whole lot. But when the wave finally does reach the resonant point, the membrane suddenly releases a burst of energy in that area. This energy is strong enough to move the organ of corti hair cells at that point.

12. Step 5 • When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The cochlear nerve sends these impulses on to the cerebral cortex, where the brain interprets them. The brain determines the pitch of the sound based on the position of the cells sending electrical impulses. Louder sounds release more energy at the resonant point along the membrane and so move a greater number of hair cells in that area. The brain knows a sound is louder because more hair cells are activated in an area. • The cochlea only sends raw data -- complex patterns of electrical impulses. The brain is like a central computer, taking this input and making some sense of it all. This is an extraordinarily complex operation, and scientists are still a long way from understanding everything about it.

13. Accessory Structures • The eustashian tube equalizes the air pressure in your ears with the air pressure out side your ears. • The vestibule contains the sense organs responsible for balance, the utricle and saccule