Figure 2.

The discharge of a minority of motoneurons displays a counterclockwise hysteresis. A, Response of another motoneuron to a 1 nA/s triangular ramp of current (6.8 nA amplitude). Same arrangement as in Figure 1A. The resting potential was −76 mV and the voltage threshold for the first spike (−53 mV) was reached for a current intensity of 3.4 nA (recruitment current). Note that spike amplitude decreased much less during the ascending ramp than in Figure 1. The derecruitment current on the descending ramp was 2.4 nA, i.e., smaller than the recruitment current. B1, Magnification of the voltage trace near recruitment (truncated spikes). Voltage oscillations (arrowhead) appear before the first spike (asterisk) and remain present in the first few interspike intervals, giving rise to a small subprimary range (SPR). B2, Magnification of the voltage trace near the derecruitment current. B3, I–F relationship. Note the counterclockwise hysteresis. The transition from SPR to primary firing range (PR) occurred at 3.7 nA (dashed line). Note that a few subthreshold oscillations reappear. The input resistance of this motoneuron was 5.3 MΩ.