Figure 2.

A, clockwise response of an mSOD1 motoneurone to a slow ramp (1 nA s⁻¹) of current. Aa, slow current ramp (bottom trace), voltage response (middle trace) and instantaneous firing frequency (top plot). The onset of the discharge is shown by the vertical dashed line (I_on, 3.7 nA) while the current of de-recruitment (I_off, 5.3 nA) is indicated by the vertical dash-dotted line. Ab, magnification of the voltage trace at the recruitment (region indicated in Aa). Spikes have been truncated to highlight the high-frequency oscillations (arrowheads) that appeared in the interspike intervals. Note the firing variability that characterizes the SPR. Ac, magnification of the region indicated in Aa. Increasing the injected current resulted in a PR without oscillation between spikes and with less variability. Ad, plot of the instantaneous firing frequency versus the intensity of the injected current for the ascending and descending branches of the ramp shown in Aa. Note that this F–I relationship displayed a clockwise hysteresis. Vertical dashed line indicates the transition between the SPR and the PR on the ascending branch. The gain of the F–I curve can be estimated by the slope of the linear regression (continuous line) in the PR. B, counter-clockwise behaviour of another mSOD1 motoneurone in response to a slow ramp (1 nA s⁻¹) of current. Ba, same organization as in Aa. Note that, in this motoneurone, the de-recruitment current (I_off, 1.2 nA) is lower than the recruitment current (I_on, 2.2 nA). Bb, magnification of the voltage trace at recruitment (region indicated in Ba). High-frequency voltage oscillations (arrowheads) were present between spikes as for the motoneurone illustrated in A. Bc, magnification of region indicated in Ba. Bd, same organization as in Ad. Note that the F–I relationship of this motoneurone displayed counter-clockwise hysteresis. PR, primary range; mSOD1, mutant superoxide dismutase 1; SPR, subprimary range.