Dynamic properties of cockroach cercal "bristlelike" hair sensilla.

Responses of cercal "bristlelike" hair sensilla (BHS) on Periplaneta americana L. to movement were investigated by recording generator (GP) and spike potentials with an extracellular pipette electrode which held the bristle by its tip. BHSs had no resting discharge, were purely phasic, with sensitivity only to stimulus transitions. They were directionally sensitive. Sinusoidal analysis suggested, to a first approximation because of the important nonlinearities, the behavior of a first-order lead system with corner frequencies distributed between 8 and 20 Hz. Responses elicited by step- and ramplike displacements were roughly in accord with the above behavior. Nonlinearities occurred both at GP level and at the level of spike generation. The phasic and the nonlinear behaviors at GP level may have a mechanical origin. The lack of spontaneous activity and the threshold nature of the spike generator account for other linearities. The operation of BHS could be separated conceptually into a linear element followed by nonlinear elements. A computer simulation based on these concepts showed a close fit to the biological responses.