Figure 1. Firing was elicited from antidromically identified lumbar motoneurones by intracellular current injection, prior to and during MLR-evoked fictive locomotion.

A shows a trial for a SmAB motoneurone, where a 50 nA ramp of current was injected, after which the brainstem stimulation was started (indicated by the bar under the ENG trace). Fictive locomotion was evident as rhythmic activity that alternated between extensor and flexor ENGs (not illustrated). Discontinuous current clamp recording allowed accurate measurement of the membrane potential during simultaneous current injection. Bars labelled B and C denote the time periods expanded in panels B and C. B shows that the voltage threshold for production of action potentials (V_th) before fictive locomotion was -46.5 mV. C shows that during fictive locomotion, less current was required to fire the neurone (compare current at +) and the V_th was hyperpolarized compared to B. Note that the neurone fired at 29 Hz before locomotion, and at 59 Hz at the same membrane potential during locomotion (see bracketed areas). The Y-axes in B also apply to C, and the time bar shown below C also applies to B. V_m, membrane potential; I_m, membrane current.