Impulse dependent adaptation in Helix pomatia neurones: demonstration of the adaptation conductance.

By conventional voltage clamp methods an increase in membrane conductance after a depolarizing pulse is demonstrated in neurones of Helix pomatia. This increase decays exponentially with a time constant in the range of 5--20 s. The rise of the conductance during the depolarization can be represented by 3 exponentials with time constants from 12 ms to 1 s. The steady state value of the conductance depends on the membrane potential in a sigmoid manner. The conductance gives rise to an outward current, which appears to be carried by potassium ions. The firing pattern of the cell is governed by the conductance. A short, rectangular potential change, a pulse clamp, is used to disturb the firing pattern of the freely firing cell. The effect on the firing pattern depends on the height and duration of the clamp pulse in the same manner as the conductance increase. The normal firing produces similar changes in the membrane conductance.