SENSITIVITY OF NEURONES IN APLYSIA TO TEMPORAL PATTERN OF ARRIVING IMPULSES.

Abstract

ABSTRACTExperiments were carried out on the isolated ganglia of Aplysia californica to discover whether the precise timing (i.e. the interspike interval distribution and sequence) of a group of spikes entering a neuronal system at a given mean frequency constitutes a variable that is significant in the control of its output. The input consisted of stimulation bursts with different timings but identical mean frequencies applied to one or more afferent trunks. The output (EPSPs or spikes) was monitored with an intracellular micro-electrode placed in the giant cell. Changes in the timing of the input produce definite changes in the magnitude of the output, both in terms of depolarization induced and of spikes evoked. Because of the critical nature of the spike firing threshold, a consistent small increment in depolarization due to favourable timing can be functionally important in the vicinity of the firing level. The following properties contribute to timing-dependence : (i) Temporal summation of successive EPSPs. (ii) Interaction. Each EPSP is characteristically augmented or, more frequently, decreased by those preceding it; linear summation is a special case. The modification in general cannot be predicted from the excitability cycle after one EPSP; interactions between EPSPs of different origins are often irreciprocal; sequence of heterologous inputs is influential, (iii) Post-spike excitability. Responsiveness changes which follow a post-synaptic spike depend on whether it is preceded by others, on their number and timing. Nerve cells were thus found to exhibit a sensitivity to timing that issued from elementary functional attributes. Such sensitivity to timing would be biologically advantageous, especially in areas of sensory convergence, for it provides an additional coding parameter complementing mean frequency modulation. It is still too early to decide whether or not this mechanism is actually important in natural operation, though it appears likely.