Effectiveness of temporal pattern in the input to a ganglion: inhibition in the cardiac ganglion of spiny lobsters.

(1) The single inhibitor axon of one side, going to the cardiac ganglion of Panulirus in a preparation with the central nervous system removed, was stimulated at a constant mean frequency but with different temporal fine structure. In particular, inhibitory trains with uniform intervals are compared with trains of alternately short and long intervals (“paired pulses”). Different ratios of short to long interval were used and, besides pairs, groups of up to 8 shocks. (2) Sixteen different measures of the inhibitory effect upon various aspects of the electroneurogram of the heart beat are compared. Some are more sensitive, others are more consistent. (3) Pattern sensitivity is found, that is the heart beat slows to different extents for different temporal structures. Trains of uniform intervals were generally more effective than any other pattern. Bursts of 7 or 8 are less effective than shorter bursts with the same minimum and mean interval. (4) The same burst has a different effect according to the phase of the heart cycle at which it arrives; the later the arrival the greater the effect. At the optimal phase and impulse interval, the minimum number of impulses for a just noticeable inhibitory effect (5–10%) is about 4. (5) In some preparations aging was associated with a reversal from inhibition to acceleration and such changes were not necessarily in parallel for different stimulus patterns. (6) On present evidence it cannot be said that the pattern sensitivity is explained by the effect of conditioning shock-test shock interval on the synaptic response to the test shock. (7) Evidence does not yet permit evaluation of the possible role of pattern sensitivity under normal conditions. The significance at presetn is the finding that in a nearly ideal preparation for testing, a postsynaptic neuron can “read,” i.e., respond differently to the same mean frequency according to temporal fine structure.