TEMPORAL CORRELATIONS IN SENSORY-SYNAPTIC INTERACTIONS - EXAMPLE IN CRAYFISH STRETCH RECEPTORS

1. The effects of inhibitory postsynaptic potential (IPSP) barrages on coding of sinusoidal length inputs were investigated in slowly and rapidly adapting stretch receptors of crayfish (RM1 and RM2, respectively). To examine the contribution of postsynaptic firings, a description was obtained of the temporal correlations between sensory stimuli, pre- and postsynaptic spikes, plus the relative position of the latter. In this analysis, the times between an IPSP and the closest spike preceding of following it are known, respectively, as phase (Φ) and cophase (θ). 2. Interactions were similar in RM1 and RM2. The usual effect was the expected increased postsynaptic interval durations, but special consequences were also evoked. With regular IPSPs there were epochs including inhibitory (S) and postsynaptic (R) spikes and fixed 'alternations' at S/R times the IPSP rate. S/R ratios were usually 1:1 and 1:2, more rarely 2:1. There were sudden postsynaptic interval duration changes, or 'jumps', between successive alternation ratios where inhibitory effects increased of decreased. With 1:1 and 1:2 ratios there was a gradual 'sliding', or Φ increase-θ decrease, at the pull and the opposite at the release. The θ and Φ roughly paralleled the control postsynaptic intervals and the inhibited-minus-control differences, respectively, when sliding was present. Sliding disappeared, and Φ and θ tended to become invariant, with 2:1 ratio. 3. Irregular IPSP increased postsynaptic interval irregularities and eliminated alternations. Phase values were irregularly distributed, but θ and control postsynaptic intervals were similar irrespective of pattern. 4. The elaborate operational properties, which occured in RM1 and RM2 with and wihtout self-sustained oscillations, respectively, were jointly determined by the sensory drive, the presynaptic rate, and the Φ dependence of IPSP effects. They could enable a much greater flexibility and a more precise control of sensory coding than the usually proposed summation of excitation and inhibition. The results provide new evidence indicating that IPSPs include effects other than slowings.