Anatomy and Functions of the Middle Ear

Anatomy and Functions of the Middle Ear

The middle ear contains many important parts that necessary for hearing sounds. The middle ear consists of the structures enclosed in a space right behind the eardrum. They are the tympanic membrane or eardrum. The tympanic membrane is a thin membrane shaped like a drum that separates the outer ear from the middle ear. Next is a chain of three ossicles, or very small bones. The chain begins with the malleus, which is connected to the center of the tympanic membrane. The other end of the malleus is connected to the incus. The incus is the middle of the chain and attaches the malleus to the stapes. The stapes is the smallest and last of the three ossicles. It has one end attached to the incus with the other end almost fills the oval window which is a oval shaped membrane that begins the inner ear (As described in Figure 6.1). The chain of ossicles makes up the smallest bones in the body (Yost 71).

One of the middle ear's main operations is to take sound waves and turn them into vibrations. Once a sound wave reaches the tympanic membrane, the membrane vibrates like a drum and the vibration is transmitted to the malleus, which is transferred to the incus and in turn to the stapes. The vibration causes the stapes to act as a piston on the oval window of the cochlea. As the stapes goes in the oval window it pushes fluid until it gets to the round window. Then the round window moves outward or gets sucked in from the movement of fluid kind of like a release valve. The middle ear makes sure that the oval window and round window work together.

The middle ear also has two small muscles, the stapedius muscle and the tensor tympani muscle. The muscles attach to the bones of the ossicular chain. The tensor tympani connects to the top part of the malleus. The stapedial muscle is the smallest muscle in the body and connects at the top of the stapes (Yost 71). These muscles are very important in protecting our hearing. They will contract and hold the malleus, incus and stapes stiff so they are less likely to vibrate any time a noise of high intensity occurs. This is called the acoustic reflex. It is enacted every time a person opens their mouth to protect the ear from the intensity of their own voice. It takes 100 milliseconds for the acoustic reflex to happen and has about 10-20dB of attenuation. So a sound at a very high intensity for a short duration (e.g. gun shots) is very damaging to a person's hearing.

The cochlea is filled with water and once the sound wave reaches the cochlea it bounces off the water which creates a 30dB loss in sound. A function of the middle ear is an impedance matching between the air in the outer ear and the fluid in the inner ear. There are three things that help us deal with the 30dB loss. One of them is the area difference between the tympanic membrane and the ossicles