Long-term, current-clamp recordings were made from the cell body of the fast coxal depressor motoneurone (Df) of the third thoracic ganglion of the cockroach Periplaneta americana. In freshly dissected preparations the response to shortduration, suprathreshold, depolarising current pulses was a graded series of damped membrane oscillations similar to those reported previously in this neurone. The response to current injection changed, however, with increasing time after setting up the preparation: cells developed the ability to exhibit all-ornone action potentials. Their amplitude, however, was usually insufficient to overshoot 0 m V. Our observations suggest that the enhancement in excitability is dependent on time following dissection rather than on time following impalement.
Recordings taken from neurone somata mechanically divided from their processes indicated that the time-dependent changes in excitability were not attributable to changes in synaptic input to the neurone and, moreover, that the cell body was involved in action potential genesis. The action potentials were resistant to treatment with the sodium channel blocker tetrodotoxin (up to 10−5 mol l−1), but were reversibly abolished when preparations were bathed in saline containing cadmium ions (1 mmol l−1) or manganese ions (20 or 40 mmol l−1) and, therefore, the inward current underlying these events was largely, if not entirely, carried by calcium ions.
These time-dependent action potentials can co-exist with plateau potentials. In neurones giving both plateau potentials and time-dependent action potentials, plateau potentials can drive action potentials in bursts.