Figure 1. Method of V_th determination.

Upper trace: intracellular recording from an extensor motoneurone in the SmAB motor pool in discontinuous current clamp mode (10 kHz switching) and digitized at 10 kHz. The voltage threshold of the action potential (V_th) evoked by a rectangular current pulse (lower trace) was defined as the data point at which the change in membrane potential of subsequent points exceeds 1 mV (indicated by the arrow). Middle trace: the derivative of the intracellular trace shows that the V_th data point clearly corresponds to the fast rising portion of the spike where the voltage trajectory surpasses 10 V s⁻¹. The oscillating noise on the traces is the result of switching artifacts from the discontinuous current clamp.