Displacement of the semicircular canal cupula during sinusoidal rotation

Abstract

The semicircular canal cupula has been described as a gelatinous partition of the ampulla which is displaced by the endolymph as if it were hinged at its receptor cell base. Subsequent studies have shown that during the artificial displacement of the endolymph, the cupula remains in position around its perimeter and is functionally displaced as a diaphragm. In this study the configuration and the magnitude of movement of the cupula during sinusoidal rotation was analyzed. Horizontal semicircular canal cupulae of bullfrogs were marked by microinjection of darkened oil droplets. Variable amplitudes of cupula displacement were achieved by sinusoidal rotation in the plane of the horizontal semicircular canal. The course of displacement was determined by analysis of individual dye droplets in cinematographic frames and compared to the turntable movement. Three findings demonstrate features of cupula movement and function. First, select regions of the cupula moved through peak amplitudes of 7.0 μm (peak to peak displacement, 14.0 μm) during sinusoidal rotation with peak angular velocities of 200 deg./s. The displacement of individual droplets was sinusoidal and followed the turntable position with a mean phase lag of about 64°. Second, the amplitude of displacement was dependent on the location of the droplet within the cupula as well as the amplitude of the rotation. Maximum displacement occurred just above the crista, while regions closer to the apex were displaced less. Third, cupula displacement was not equal for two different regions along the length of the crista. Droplets in the thin ventral region of the cupula had a mean amplitude of 6.6 μm for rotations with peak angular velocities of 200°/s. In contrast droplets in the region of transition between the thick and thin portions had amplitudes of only 4.1 μm for the same stimulus. We propose on the basis of these results that, during rotation, the cupula functionally behaves as a diaphragm with its greatest mechanical sensitivity adjacent to the crista. Hence, a minimal volume of the endolymph must be displaced to produce a given deflection of cilia. This diaphragm arrangement, along with the non-uniform thickness of the cupula and the profile of the endolymph flow through the canal, suggests a graded response along the length of the crista. The cupula may thus behave as a primary information filter which ‘pre-processes’ and separates rotational information (related to angular velocity and angular acceleration) through a ‘vestibular place phenomenon’.