Effect of post-impulse depression on background firing of sympathetic preganglionic neurons.

(1) Many of the preganglionic neurons responsible for sympathetic tone in the cat exhibit a characteristic irregular background spike activity with a low repetition rate. The properties of this activity, described by the interspike interval histogram, can be explained as the result of the responses of the neurons to random synaptic imputs. (2) Serial interspike interval correlation was used to show that additive post-impulse depression in preganglionic neurons does not enter into the timing of typical low-rate, irregular background firing. However, if cells are accelerated by anitdromic tetanization, a depressive recovery process accumulates to cause a prolonged silent period after driving of the cells has ceased. If cells are accelerated, by the action of their synaptic inputs, to rates higher than their usual basal rates, serial post-impulse depressions overlap, and summate to cause a temporal interaction between neighboring pulses which is observable by serial interval correlation. (3) By observing the effect of antidromic responses occurring at various intervals after a background spike, we showed that the time course of the summative part of post-impulse depression is shorter than the interspike intervals typically encountered in background firing. (4) At higher-than-basal levels of sympathetic activity, occurring spontaneously or during antidromic stimulation, successive post-impulse recovery periods overlap and sum to impart a negative correlation of serial interspike intervals. At the levels of sympathetic activity existing in waking animals, the damping effect of cumulative post-impulse depression is probably an important factor in stabilizing sympathetic tone.