Usefulness and limitations of linear approximations to the understanding of Ca++ signals.

Simple approximations to some limiting cases of Ca++ signalling provide insight into the complex problems of buffered diffusion and of Ca++ homeostasis in the presence of buffers. Three cases are presented, where the influence of Ca++ buffers can readily be understood in the limit of small signals: the return of global cellular [Ca++] following a short stimulus in a 'Single Compartment', buffered diffusion along a cylindrical axon in the 'Rapid Buffer Approximation', and nonequilibrium microdomains of elevated [Ca++] in the immediate vicinity of open Ca++ channels.