A mechanism for spike frequency adaptation.

1. Spike frequency adaptation was studied in large neurones of the marine molluscs Archidoris montereyensis and Anisodoris nobilis. These cells respond to a current step with a rapid rise in spike frequency followed by a gradual decline to a new steady level.2. An exponentially declining current, I(s), was measured when the cell was voltage clamped following an adapting spike train. The initial amplitude of this current depended on the preceding number of spikes and on the voltage to which the cell was clamped. A reversal potential (V(s)) for this current was obtained by clamping to various potentials following a spike train. The time constant (tau(s)) of decay of the current was dependent upon the clamping potential.3. Clamping the membrane potential to a constant test level from various initial levels initiates an exponentially decaying current of similar time constant. The voltage dependence of the steady-state conductance (g(s)a(s)(V, infinity)) associated with this current was determined using this technique.4. Equations for neural repetitive firing (Connor & Stevens, 1971c) were modified by the addition of a term describing these slow membrane currents: [Formula: see text]. The solution to the modified equation was in good agreement with the spike frequency adaptation observed in these cells.