How an Ear Works - Hearing

The hearing mechanism

The middle ear is an air filled space between the outer ear and the inner ear. The air for this space comes from the eustachian tube. The eustachian tube connects the middle ear space with the back of the throat in the area directly behind the nose. This tube opens and closes many times during the day. Some activities that will open the tube include swallowing, yawning and chewing. In the middle ear space, there are three bones connecting the ear drum to the inner ear. The maleus connects to the incus which in turn connects to the stapes. The stapes is often referred to as a stirrup. The stapes moves in an opening to the inner ear. Associated with the middle ear air tracts enter the surrounding bone. These air cells are largest directly behind the ear and makeup the mastoid.

The inner ear (8 in cochlea.gif) is an extremely small structure that is totally encased in bone. The bone is very dense and develops early in life. The inner ear if removed from the head and placed on a dime would still show silver around the edges. The inner ear is composed mostly of fluid. This fluid is divided into several compartments. There are two main divisions to the inner ear. One division controls the sense of balance. Another division controls and provides hearing. The fluid filling these compartments includes perilymph and endolymph. Perilymph and endolymph are composed of differing salt solutions. The salt solutions must be properly balanced so that the electrical potentials of the inner ear are normal. Suspended in the fluid are tiny hair cells. The motion of these hair cells creates a signal sent back to the brain along the hearing nerve. The hair cells in the balance part of the inner ear, called the labyrinth, send their signal to the brain's balance centers.

The hair cells and nerve cells of the inner ear are permanent structures. This means that when you are born, you get one set of cells. If the cells die because of infection or trauma they are gone forever. At this time, we have no way to replace or regrow the cells. Numerous diseases can affect different areas of the ear. Frequently, your doctor will ask for a hearing test in order to better understand what might be causing a problem with the ear.

Hearing tests measure two important things. An audiologists will fit you with special ear phones that will enable you to hear very quiet sounds. You will be placed in a special quiet room. The audiologists will present sounds in each ear phone. They will vary the pitch and loudness. By performing this and other tests, the audiologists will be able to tell whether you have a hearing loss from the outer and middle ear (a conductive hearing loss) or an inner ear type of hearing loss (a sensorineural hearing loss).