Sense of Rotational Acceleration

How Semicircular Canal Neurons Sense Angular Accelerations

Like those that innervate the otolith organs, the vestibular fibers originating from bipolar neurons in Scarpa's ganglion that innervate the semicircular canals exhibit a high and steady spontaneous firing rate. As a result, they can transmit information by either increasing or decreasing their firing rate. As already suggested, such bidirectional responses enable the afferent nerves to faithfully follow the receptor potentials, and thus to more effectively encode head movements. The bidirectional responses of fibers innervating the hair cells of the semicircular canal have been studied by recording the axonal firing rates in a monkey's vestibular nerve. Seated in a chair that could be rotated, the monkey was first rotated at an accelerated rate, then at constant velocity for several seconds, and finally the chair was decelerated to a stop (Figure 14.9). The maximum firing rates observed correspond to the period of acceleration; the maximum inhibition corresponds to the period of deceleration. During the constant-velocity phase, the response adapts so that the firing rate subsides to resting level; after the movement stops, the neuronal activity decreases transiently before returning to the resting level. Neurons innervating paired canals have a complimentary response pattern. Note that the rate of adaptation (on the order of tens of seconds) corresponds to the time it takes the cupula to return to its undistorted state (and for the hair bundles to return to their undeflected position); adaption therefore can occur even while the head is still turning, as long as a constant angular velocity is maintained. Such constant forces are rare in nature, although they are encountered on ships, airplanes, and space vehicles, where prolonged acceleratory arcs are sometimes described.