Katz_1971_J.Physiol_216_503

1. Depolarization of the giant axon terminal of the squid causes local calcium influx which gives rise to transmitter release and post-synaptic response, and which under certain experimental conditions leads to a regenerative action potential in the presynaptic terminal itself.2. There has been conflicting evidence in the literature on the question whether the calcium permeability change in the terminal is rapidly inactivated, or whether it can persist with little diminution for hundreds of milliseconds during a depolarizing voltage step.3. Results are presented which show that there is little ;calcium inactivation', even when very large depolarizing steps are imposed on the terminal and maintained for periods of 1-2 sec.4. Contrary indications are examined and found to be attributable to an increase of potassium conductance, rather than direct inactivation of calcium conductance.