Anatomy Medicine

The Inner Ear Anatomy

Next to the middle ear in the bone of the skull is a small compartment that contains the hearing and balance apparatus known as the inner ear. The inner ear has two main parts. The cochlea, which is the hearing portion, and the semicircular canals are the balance portion.

The cochlea is shaped like a snail and is divided into two chambers by a membrane. The chambers are full of fluid that vibrates when sound comes in and causes the small hairs which line the membrane to vibrate and send electrical impulses to the brain.

The semi-circular canals are also known as the labyrinthine. These little canals are lined up at right angles (90°) to each other. This allows the brain to know in which direction the head is moving. These semi-circular canals are filled with fluid and have some small calcium crystals embedded in the lining.

Coming from the inner ear and running to the brain is the eighth cranial nerve, the auditory nerve. This nerve carries both balance and hearing information to the brain. Along with the eighth cranial nerve runs the seventh cranial nerve. The seventh cranial nerve is also known as the facial nerve because it supplies nerve impulses to the muscles of the face.

How does the system work?

The outer ear captures sound waves. The sound travels down the ear canal and hits the ear drum. The ear drum vibrates which causes the ossicles (middle ear bones) to vibrate. A piston action of the ossicles creates a wave in the fluid in the inner ear. The fluid wave stimulates the hair cells in the cochlea and an electrical impulse is sent through the eighth cranial nerve to the brain.

The balance system works by sending continuous electrical impulses to the brain. Moving the head causes the fluid in the semi-circular canals to shift. This, in turn, changes the electrical impulses to the brain. The brain uses this information to make any adjustments the body needs for balance.

See Also

The Ear Anatomy