Theoretical simulation of the calcium action potential in squid giant synapse: Repetitive stimulation

A biophysical model of the experimentally observed calcium action potential (CAP) in squid giant synapse is proposed. Whereas the inclusion of the inward calcium current in the Hodgkin-Huxley model can generate the rising phase of CAP, to account for the observed termination of the action potential, a repolarizing process needs to be introduced. Adding a term representing Ca-activated K current, the observed features of CAP can be reproduced. However, one feature of CAP, namely the gradual shortening of the plateau duration on repetitive stimulation, cannot be simulated by this model. In this paper, it is proved that both the termination of the action potential and the gradual shortening of the plateau cannot be accounted for by inclusion of a single repolarizing process. One more repolarizing process, namely a slow voltage-dependent Ca-inactivation, is therefore proposed to account for all the observed features of CAP.