Ion conductance changes associated with spike adaptation in the rapidly adapting stretch receptor of the crayfish

The time course of the repetitive impulse discharges has been investigated for two high intensities of maintained depolarizing currents, 30 nA and 50 nA, for which the receptor adaptation was complete within 70 msec. The changes in sodium and potassium conductance associated with the decline in spike activity have been analyzed at different instances of time by interrupting in successive experiments the various action potentials in the pulse trains either at the early phase by holding the potential at about -60 mV and recording the inward current (upstroke-gNa) or by evaluating the delayed outward current flowing as the result of a depolarizing voltage pulse which at the end of the action potential re-increased the membrane potential by mV (after potentialgK). At the higher current intensity of 50 nA the discharge frequency was increased, while larger reductions in upstroke-gNa and after potential-gK during receptor adaptation became apparent. The progressive decrease in pulse amplitude from 99 mV to 63 or 55 mV is paralleled by a gradual reduction in upstroke-gNa from 97 mmho/cm-2 to 37 or 27.5 mmho/cm-2 and in after potential-gK from 11.5 mmho/cm-2 to about 7 mmho/cm-2. When under a stimulus of 30 nA the sodium conductance decreases to an average value of 37 mmho/cm-2 only a distorted spike can be elicited, while the spike activity was completely suppressed at upstroke-gNa equals 27.5 mmho/cm-2 was essentially the same under both conditions. The results have been interpreted in terms of the model for impulse generation formulated by Michaelis and Chaplain (1973). According to the model both sodium and potassium inactivation reduce the pulse amplitude. However, while Na-inactivation reduces the frequency of impulse discharge, the K-inactivation actually leads to an increase in spike frequency. As the frequency of the short train of pulses recorded under high-intensity current stimulation remained essentially unaltered, it is suggested that the coupling between Na- and K-inactivation actually leads to an increase in spike frequency. As the frequency of the short train of pulses recorded under high-intensity current stimulation remained essentially unaltered, it is suggested that the coupling between Na- and K-inactivation ensures a constancy of the information-carrying parameter, i.e. the average impulse density.