Time characteristics and potential dependence of early and late adaptation in the crustacean stretch receptor.

Abstract

The receptor potential of the crustacean stretch receptor evoked by a ramp and hold stretch is depolarizing and consists of an initial peak followed by a static phase. The receptor current, defined as the stretch induced current change is inward and has a similar appearance. The adaptive fall of the receptor potential and receptor current occurs in two phases which can be separated by their different time characteristics. Using intracellular recordings including potential clamp an attempt has been made to fit experimental values of the adaptive fall of the potential and current response to a double exponential function. Ramp and hold stretches with a rise time of about 7-15 ms were used. The time constants were determined with the cell clamped or polarized to different holding potentials. It was found that the adaptive fall for both potential and current could be fitted reasonably well by double experimental functions with an initial fast phase and a second slow phase. For the potential response the time constant of the early adaptive fall, tau 1, varied with holding potential, having a minimum of approximately 3 ms at about -50 mV. tau 1 for the current response showed a similar variation with holding potential, although less pronounced than for the potential response. The minimum value was about 5 ms. The time constant, tau 2, of the late phase of adaptation both for the current and potential response was about 500 ms and did not vary in a systematic way with holding potential. The results are consistent with the idea that the first phase of adaptation is related to ionic mechanisms in the membrane of the receptor neuron while the second phase might be caused by mechanical factors.