SENSORY TRANSDUCTION OF HEAD VELOCITY AND ACCELERATION IN THE TOADFISH HORIZONTAL SEMICIRCULAR CANAL

1. Sinusoidal mechanical indentation of the long-and-slender limb of t he horizontal semicircular canal and/or utricle was used to produce ad equate stimulation of the labyrinth. Indentation of the canal increase d, while indentation of the utricle decreased the afferent discharge r ate. This follows because indentation of the canal and utricle produce oppositely directed mechanical stimuli as defined by endolymph flow, transcupular pressure, and cupular deflection. Simultaneous in-phase i ndentations of both the canal and utricle, with amplitudes adjusted to produce equal(but opposite) magnitudes of afferent response modulatio n, generate destructive interaction that minimizes the afferent modula tion, whereas sinusoidal indentation 180 degrees out-of-phase generate s constructive interaction that maximizes the afferent modulation. Thi s observation correlates directly with analysis of the labyrinthine el asto-hydrodynamics which predicts that balanced in-phase indentations minimize macromechanical endolymph flow through the ampullary cross se ction and maximize the dilatational pressure within the ampulla acting equally on both sides of the cupula and across the labyrinthine wall. 2. Two groups of afferents are identified according to their response to balanced sinusoidal indentation of the canal limb and the utricle. In one group there is complete destructive interaction and the affere nt response can be effectively nulled by adjusting the relative amplit ude and phase of the two stimuli. In the second group a residual affer ent response remains that cannot be nulled. The residual is described in the model as unit-specific sensitivity to dilatational pressure act ing equally on both sides of the cupula. 3. The results are quantified using simple linear equations with terms derived from macromechanical endolymph flow, transcupular pressure and dilatational pressure. The model illustrates that the phase of the macromechanical pressure leads flow across the frequency bandwidth employed. This phase lead could a ccount for the diversity of afferent responses observed during the phy siological rotation of the head. Responses of afferents that are in ph ase with acceleration of the head at 1 Hz, and the commonly seen high- frequency phase and gain enhancements are described by the model in te rms of sensitivity to macromechanical pressure in addition to endolymp h flow. 4. Rotation of the head produces a differential pressure gradi ent across the cupula and crista, high on one side and low on the othe r. Macromechanical pressure is distinct from gross endolymph now assoc iated with displacement of the cupula and is manifested as a modulated isopressure surface through the sensory epithelium with pressure grad ients on either side. The apparent response to this macromechanical pr essure may account for much of the physiological diversity seen in pri mary afferent response dynamics during natural head movement.