Three modes of repetitive firing and the role of threshold time course between spikes.

When rhythmic firing is elicited in cat spinal motoneurons by long-lasting depolarizing currents, the `threshold' voltage from which each spike arises may appear to be quite constant (Fig . IA, arrow) . If one probes for the threshold during the interspike interval (ISI), however, it is seen to fall well below this level, rising towards it later in the ISL Exceptionally large depolarizing afterpotentials may intersect this threshold time course shortly after a spike, causing an extra spike . The extra spike itself may also similarly produce another extra spike ; thus a regenerative cycle may produce a burst of spikes at a high firing rate . Rhythmic firing was produced by steps of current injected through the intra-