A mathematical model of stationary activity in single neurons is developed to describe interspike intervals (ISI) and the influence of calcium-dependent accommodation on action potential generation. Unlike conductance-based Hodgkin-Huxley-type models, this approach focuses on the neuron’s firing responses rather than reproducing individual ionic currents. The model introduces a balance equation for intracellular calcium dynamics, incorporating calcium influx during action potential generation and its subsequent removal via transport mechanisms. The relationship between inward current and ISI is examined under different stimulation protocols, revealing a rational dependence in ramp protocols and an exponential saturation function in step protocols. This framework provides a simplified yet effective method for analyzing neuronal excitability and adaptation to calcium-dependent regulation.