PERIODICALLY-MODULATED INHIBITION OF LIVING PACEMAKER NEURONS - III -THE HETEROGENEITY OF THE POSTSYNAPTIC SPIKE TRAINS, AND HOW CONTROL PARAMETERS AFFECT IT

Codings involving spike trains at synapses with inhibitory postsynapti c potentials on pacemakers were examined in crayfish stretch receptor organs by modulating presynaptic instantaneous rates periodically (tri angles or sines frequencies, slopes and depths under, respectively, 5. 0 Hz, 40.0/s/s and 25.0/s). Timings were described by interspike and c ross-intervals (''phases''); patterns (dispersions, sequences) and for ms (timing classes) were identified using pooled graphs (instant along the cycle when a spike occurs vs preceding interval) and return maps (plots of successive intervals). A remarkable heterogeneity of postsyn aptic intervals and phases characterizes each modulation. All cycles s eparate into the same portions: each contains a particular form and sw itches abruptly to the next. Forms differ in irregularity and predicta bility: they are (see text) ''p:q alternations'', ''intermittent'', '' phase walk-throughs'', ''messy erratic'' and ''messy stammering''. Pos tsynaptic cycles are asymmetric (hysteresis). This contrasts with the presynaptic homogeneity, smoothness and symmetry. All control paramete rs are, individually and jointly, strongly influential. Presynaptic sl opes, say, act through a postsynaptic sensitivity to their magnitude a nd sign. when increasing, hysteresis augments and forms change or disa ppear. Appropriate noise attenuates between-train contrasts, providing modulations are under 0.5 Hz. Postsynaptic natural intervals impost c ritical time bases, separating presynaptic intervals (around, above or below them) with dissimilar consequences. Coding rules are numerous a nd have restricted domains; generalizations are misleading. Modulation -driven forms are trendy pacemaker-driven forms. However, dissimilarit ies, slight when patterns are almost pacemaker. increase as inhibition departs from pacemaker and incorporate unpredictable features. Physio logical significance-(1) Pacemaker-driven forms, simple and ubiquitous , appear to be elementary building blocks of synaptic codings, present always but in each case distorted typically. (2) Synapses are prototy pe. similar behaviours should be widespread, and networks simulations benefit by nonlinear units generating all forms. (3) Relevant to perio dic functions are that few variables need be involved in form selectio n, that distortions are susceptible to noise levels and, if periods ar e heterogeneous, that simple input cycles impose heterogeneous outputs . (4) Slow Na inactivations are necessary for obtaining complex forms and hysteresis. Formal significance-(1) Pacemaker-driven forms and pre sumably their modulation-driven counterparts, pertain to universal per iodic, intermittent, quasiperiodic and chaotic categories whose formal properties carry physiological connotations. (2) Only relatively elab orate, nonlinear geometric models show all forms. simpler ones, show o nly alternations and walk-throughs. (3) Bifurcations resemble those of simple maps th;it can provide useful guidelines. (4) Heterogeneity po ses the unanswered question of whether or not the entire cycle and all portions have the same behaviours: therefore, whether trajectories ar e continuous or have discontinuities and/or singular points. (C) 1998 IBRO. Published by Elsevier Science Ltd.