The main feature separating a living pacemaker from other oscillators
is the fact that the former has not only the oscillatory property but
also slow and fast changes of the membrane potential corresponding to
the sub- and suprathreshold regions. We propose a simple mathematical
model called mRIC and show that the model exhibited the essential feat
ure of pacemakers generating spikes at constant time intervals. The be
havior of the model driven by periodic pulse trains is analysed using
the phase transition curve.