Ear: Anatomy of Hearing and Balance (cont.)

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The bony labyrinth itself has three sections. 1) The cochlea is responsible for hearing, 2) the semicircular canals have function associated with balance, and 3) the vestibule which connects the two and contains two more balance and equilibrium related structures, the saccule and utricle.

The final structures of the inner ear are the round window and the eighth cranial nerve (cranial nerve VIII) which is composed of the vestibular nerve (balance) and the cochlear (hearing) nerve.

Picture of the Anatomy and Structures of the Ear
Diagram of outer, middle, and inner ear. The outer ear is labeled in the figure and includes the ear canal. The middle ear includes the eardrum (tympanic membrane) and three tiny bones for hearing. The bones are called the hammer (malleus), anvil (incus), and stirrup (stapes) to reflect their shapes. The middle ear connects to the back of the throat by the Eustachian tube. The inner ear (labyrinth) contains the semicircular canals and vestibule for balance, and the cochlea for hearing.

Hearing

We hear by funneling sound from the environment into the outer ear and causing the tympanic membrane to vibrate. Those sound waves vibrations are transferred into mechanical vibrations of the ossicles. Those mechanical vibrations cause the oval window to move back and forth causing the perilymph of the inner ear to begin wave-like motions. The perilymph fluid motion is transferred to the endolymph and the wave motion is transformed into electrical impulses picked up by the hairy cells of Corti and sent to the brain via the cochlear nerve. The round window is responsible for absorbing the fluid wave vibrations and releasing any increased pressure in the inner ear caused by the wave motion.

Balance

Balance is a choreographed arrangement that takes sensory information from a variety of organs and integrates it to tell the body where it is in related to gravity and the earth.

Information from the vestibular system of the inner ear (semicircular canals, the saccule and the utricle) is sent to the brainstem, cerebellum, and spinal cord. Potential balance abnormalities do not require conscious input from the cerebrum of the brain. Abnormal vestibular signals cause the body to try to compensate by making adjustments in posture of the trunk and limbs as well as making changes in eye movement to adjust sight input into the brain.

There are three semicircular canals in the inner ear positioned at right angles to each other like a gyroscope. They are able to sense changes in movement of the body. With such changes, endolymph waves within the canals cause hair cells located within their base to move. Position of the head is sensed by hair cells of the utricle and saccule which is stimulated when the head moves and the relationship to gravity changes.

Medically Reviewed by a Doctor on 12/1/2014