http://cnx.rice.edu/content/m12365/latest/ Introduction The ear is the sense organ that picks up sound waves from the surrounding air and turns them into nerve impulses that can be sent to the brain. The sound waves carry lots of information - language, music, and noises - all mixed up together. The task of the ear is to turn the signals in these waves of bouncing air molecules into electrical nerve signals, while keeping as much of the information in the signal as possible. (Then it's the brain's job to sort the signals and make sense out of them.) It's not easy to turn one kind of signal into another kind without losing information, but the ear is well designed for the task. Note: The human ear also has some other functions not related to hearing; those won't be discussed here. When something vibrates, the vibrations can travel as waves through solids, liquids, and gases. Even animals that have no ears can often feel these vibrations. But in order to understand language and hear music, the brain has to be given more information than just "there's a vibration". It needs to know the frequency and amplitude of all the waves that the ear is collecting. Interestingly, the ear sends this information to the brain very accurately by turning the sound waves in the air (vibrations in a gas) into vibrations in bones (solid), and then into waves in a fluid in the inner ear (a liquid), before they become (electrical) nerve signals. This might seem like a lot of unnecessary translation, but it allows the sense of hearing to be both sturdy and very sensitive, as explained below. Parts of the Ear The ear has three main sections. In the outer ear, the sound waves are still moving in air. In the middle ear, the sound waves are being conducted by three small bones. In the inner ear, the waves are moving through the fluid-filled cochlea. The Ear Figure 1: The parts of the ear that aren't involved in hearing have been left out. The Outer Ear The part of the human ear that you can see is simply a sound wave collector. Its shape helps to funnel the sound waves into the auditory canal (or ear canal) so that you get plenty of signals from even soft sounds, particularly ones from the direction that you are looking at. At the other end of the ear canal is the eardrum (or tympanic membrane). This is a membrane that is stretched tight, like the membranes on the tops of drums (including tympani). And thin, taut membranes are very good at vibrating, which is why they can be found both on drums and inside ears. The eardrum picks up the vibrations in the ear canal and vibrates with them. The Middle Ear On the other side of the eardrum are the three tiny bones of the middle ear, the hammer, the anvil, and the stirrup. They are named for their shapes. Vibrations in the eardrum are passed to the hammer, which transmits them to the anvil, which makes the stirrup vibrate against the oval window of the cochlea. Bone is a very good conductor of vibrations, and the bones of the middle ear are specially arranged so that they can amplify (make louder) very quiet sounds. On the other hand, if things get too loud, tiny muscles in your middle ear can relax the eardrum a bit. A relaxed eardrum doesn't vibrate as much (think of a relaxed rubber band as opposed to a taut one), and this helps to keep things from getting damaged. The Inner Ear The cochlea is a fluid-filled spiral (shaped something like a snail shell) about the size of a pea. Vibrations in the stirrup make waves in the fluid that travel down the spiral. In the fluid, in a long strip following the spiral, is the organ of Corti. This organ is covered with (about 20,000) tiny, incredibly sensitive hairs that are waving around inside the Cochlear fluid. Each of these hairs is a nerve ending that is picking up specific information about the vibrations in the fluid. At the end of the organ of Corti, the nerves are bundled together as the auditory nerve, which brings the information to the brain. Note: The fragile, sensitive hairs on the organ of Corti would never stand up to the rough conditions in the ear canal. Even protected in the Coclear fluid, they don't last forever, especially the ones that can sense the highest-frequency vibrations. That is why most people begin to lose their sense of hearing as they grow older.
