Vestibular System

Control of Eye Movements

Semicircular canal signals are important in the control of eye movements. They provide effective ocular stabilization for displacements of the head. For example, when a gaze shift is made in terms of a combined eye and head movement to look at a target located off to the side, the eyes will initially move and reach the target and then the head will start to move. As the head moves, the semicircular canals are activated and with very short latency they innervate eye muscles that counter-rotate the eyes in the direction opposite the movement of the head. In this way, stable fixation of the target is maintained during a shift in direction of both the eyes and head. Without this compensatory stabilizing mechanism, the movement of the head following the movement of the eyes would carry the eyes past the target. These so-called vestibulo-ocular reflexes (VORs) are quite exact for pitch (up-down) and yaw (left-right) movements of the head.

There are a variety of disorders involving the semicircular canals. For example, occasionally an otoconium composed of calcium carbonate crystals from the otolith organs can break loose from the otolith membrane and become lodged against the cupula of a semicircular canal. This creates an unusual situation because, normally, the functioning of a semicircular canal is independent of [Page 1026]the direction of gravity. The endolymph in the semicircular canal and the cupula, the little door within the canal, have the same density. As a consequence, the canals are not affected by static head orientation in relation to gravity because there is no turning couple related to gravity as the fluid and cupula are homogeneous in density. By contrast, when an otoconium with a specific gravity of 2.7 lodges against the cupula (which has a density of about 1.005), it renders the canal gravity dependent. Consequently, when the canal is oriented so that the force of gravity pushes the otoconium against the cupula, it displaces it and activates the receptors of the canal. In this circumstance, the affected individual will feel as if he or she is turning even when stationary. This quickly can lead to disorientation, loss of balance, and motion sickness.

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