Figure 7.

Bistable persistent activity prevented when spike afterhyperpolarizations slow initial discharge frequency. A, Truncating slow-feedback AHP decays (middle panel, τ = 1 s truncated to 0 pA after 250 ms) fails to convert bistable persistent firing into self-terminating discharges. Allowing the same feedback AHP to continue to decay exponentially generated self-terminating responses in the same neuron (bottom panel). Inset, Expansion of microcontroller-generated current waveforms when feedback AHPs were truncated at 250 ms (thin trace) and when feedback AHPs were allowed to exponentially decay until the next AP (thick trace) in the same neuron. B, Persistent discharges spontaneously terminate even when feedback AHP decay kinetics is accelerated to prevent steady-state hyperpolarizing current injection (τ = 500 ms; middle panel, vs 1 s τ used elsewhere in study). Self-termination does require temporal summation of feedback AHPs since injected current decayed to <5% of peak during interspike intervals (all with >3 decay time constants). Accelerating feedback AHP decay kinetics further (to 200 ms τ; bottom panel) generated long-lasting bistable persistent firing. Dashed lines indicate zero microcontroller-injected current level. C, Diagram illustrating hypothesized mechanism of self-terminating persistent discharges. See text for details.