Multiple‐Spike Discharge Evoking After‐Depolarizations in the Slowly Adapting Stretch Receptor Neuron of the Lobster II. The slow after‐depolarization

The extra impulses of the slowly adapting stretch receptor neuron of the lobster were evoked by different kinds of after-depolarizations which appeared with rising temperature. One of them, called slow after-depolarization, was studied by recording extra-and intracellularly at different cell regions under varying experimental conditions. The slow after-depolarization developed after the first full-sized action potential of the multiple spike discharge. With rising temperature its amplitude increased from about 10 mV to 30–40 mV (from the resting membrane potential), and its duration from about 25 msec to several seconds. Depending on its length the slow after-depolarization was able to evoke between one and several hundred extra spikes. There was an inverse relationship between the safety factor for propagation of the active process along the somato-dendritic membrane and the duration and, to some extent, the amplitude of the slow after-depolarization. In any discharge a premature termination of the slow after-depolarization could be provoked by a short pulse of intracellularly injected anodal current or by activation of the inhibitory system of the stretch receptor organ. The membrane resistance was reduced during the course of the slow after-depolarization. The findings support the idea of an asynchronous activation and subsequent asynchronous activity in action current generating sites of the dendritic membrane.