Multiple‐Spike Discharge Evoking After‐Depolarizations in the Slowly Adapting Stretch Receptor Neuron of the Lobster

Abstract The extra impulses of the multiple‐spike discharges of the slowly adapting stretch receptor neuron of the lobster were evoked by different kinds of after‐depolarizations which appeared with rising temperature. Two of them, called labile and fast after‐depolarization, were studied by recording extra‐and intracellularly at different cell regions under varying experimental conditions. The labile after‐depolarization which succeeded small impulses of a type indicative of a conduction block was most prominent under conditions critical for the establishment of the block. It was able to initiate one extra spike.—With rising temperature the terminal step of the repolarizing phase of full‐sized action potentials became increasingly retarded in relation to the earlier phases of the spike. Therefore, it assumed the form of a fast after‐depolarization which was able to evoke one extra spike. The intrasomally recordable fast after‐depolarization coincided in time with the negative phase of extradendritically recorded impulses and with the peak of intradendritically recorded spike potentials. At any temperature the prominence of the fast after‐depolarization increased with decreasing safety factor for propagation of the active process along the somato‐dendritic membrane. The findings indicate that the labile after‐depolarization is caused by a partial activation of the membrane of the cell region behind a blocking zone, whereas the fast after‐depolarization reflects the active depolarization of the cell region beyond that impaled.