Key Points

• The auditory and vestibular systems both occupy the inner ear cavity and both rely on analogous cell types known as hair cells to convey information about sound or position and movement of the head.

• Transduction of mechanical energy into bio-electrical impulses in hair cells is achieved by bending stereocilia, opening potassium channels, and causing changes in membrane polarity in the form of generator potentials that trigger transmitter release.

• The coordinated response of the semicircular canals provides information about head position and movement, encoded in the pattern of increases and decreases in the tonic firing rates of vestibular neurons.

• Hair cells in the semicircular canals provide information about angular acceleration in three directions; cells in the utricle and saccule provide information about static head position and about linear acceleration in all directions.

• Information from the vestibular organs is sent to the vestibular nuclei and then to motor nuclei of the cerebellum and brain stem in order to coordinate eye movements and maintain posture.

A sense of balance is derived primarily from specialized receptors located in an area near the entryway or vestibule of the inner ear, hence the name vestibular organ (Fig. 1). The principal function of the vestibular system is to provide direct information not to the cortex, but to motor centers in the spinal cord, brain stem, thalamus, and cerebellum so that involuntary reflex movements can be initiated and the body can remain balanced.

Vestibular input is also important in coordinating head and body movements and in moving the eyes so that they remain fixed to a point in space when the head is moving. Only in unusual circumstances does information from the vestibular system reach conscious levels (e.g., when the sense of balance is disrupted). In this instance, relay pathways in the brain stem evoke sensations of dizziness and nausea.

The inner ear consists of a series of cavities that form a bony labyrinth in the petrous portion of the temporal bone (see Fig. 1). Within the bony labyrinth lies a series of fluid-filled sacks or tubes formed primarily from simple epithelial cells. Regions of the epithelium are specialized and contain receptors for hearing in the auditory portion of the inner ear and for balance in the vestibular portion. The vestibular organs consist of three semicircular canals and two otolith organs, the utricle and the saccule.

Auditory and vestibular systems share many functional properties. Both utilize hair cells to detect movement. As discussed in Chapter 53, hair cells in the auditory system respond to movement in the fluid of the auditory canal caused by sound waves. Hair cells in the vestibular system respond to movement in the fluid of the vestibular organs caused by head movement or gravity.

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