A low threshold K⁺ current determines the long refractory period.

<p>A) Sub-threshold conditioning stimulus (red trace, 0.8 thresholds) prevent neuron spiking in response to test stimuli. Spike is elicited by a control identical test stimulus in the absence of a conditioning one (black trace). B) Paired pulse stimulation with sub-threshold stimuli show that depolarization leads to an hyperpolarization after the end of the pulse (red arrow)and a decrease of excitability of the neuron evidenced by the reduction depolarization peak in a constant amplitude test pulse (blue arrow). Linear relationships between end conditioning depolarization (end voltage-black arrow), after-hyperpolarization (red arrow) and the amplitude of the hump evoked by the test pulse (blue arrow) are shown in C and D). E) Pair pulses applied at different delays shows a long lasting refractory period. F) As expected for a low threshold K⁺ conductance, refractoriness was shortened after the application of 50 microMolar 4-aminopyridine to the bath.