The ionic basis of the action potential in the large second-order neurones (L-neurones) of the ocellus of the cockroach, Periplaneta americana, was studied. L-neurones generated action potentials, usually once, at the off-set of hyperpolarizing light responses, or at the termination of hyperpolarizing current stimuli. The action potential was blocked by replacing saline Ca2+ with Mg2+ but maintained when Ba2+ was substituted. A block was produced by 2 mmoll l−1 Cd2+ or 20 mmol l−l Co2+. The peak amplitude of the action potential increased by 26 mV for a 10-fold increase in external Ca2+ concentration, at concentrations below 1.8 mmol l−1. The action potential was not affected by sodium-free saline or by 3×10−6mol l−1 tetrodotoxin (TTX). These observations suggest that calcium ions are the major carrier for the inward current of the action potential. This finding supports the suggestion that the off-set responses of hyperpolarizing visual neurones of both vertebrates and invertebrates have a common ionic mechanism, including voltage-sensitive calcium currents.

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