(A1) WT motoneuron displaying the delayed firing pattern in response to a 5 s pulse. The current intensity was the minimal intensity necessary to elicit firing in our searching protocol (rheobase). Bottom: injected-current (square pulses), middle: voltage-response and top: instantaneous firing frequency. The horizontal dashed line shows the voltage threshold for spiking (−50 mV). (A2) Response of a WT motoneuron displaying the immediate firing pattern. Same arrangement as in A1. (Voltage threshold for spiking: −59 mV). (B1) Single action potentials from a delayed (black line) and an immediate (gray line) firing WT motoneurons elicited by a short square pulse of current. The arrowheads point to the horizontal dotted bar drawn at half action potential amplitudes. (B2) Same records at a longer time base in order to show the after hyperpolarisation (AHP). Dashed lines are the exponential fits of the AHP relaxation. The relaxation time constants are 17 ms and 37 ms for the delayed firing and the immediate firing motoneurons, respectively. Note that the relaxation time constant is longer in the immediate firing motoneuron than in the delayed firing one. C Reconstructed dendritic trees of WT delayed (C1) and immediate (C2) firing motoneurons. The axon was not reconstructed in either case. Reconstructions are projected in the same plane as the slice.