Gain of spinal motoneurons measured from square and ramp current pulses

The gain of motoneurons (MNs) characterizes how variations in synaptic input are transformed in to variations in output firing and muscle contraction. Experimentally gain is often defined as the frequency-current relation observed in response to injected suprathreshold square current pulses or current ramps during intracellular recording. The gain of MNs is strongly affected by adaptation: transient gain in response to depolarization is usually higher than steady state gain measured during sustained depolarization. The transient and the stationary gain of neurons are separate entities that can be selectively modified. Here we investigated how the transient and the stationary gain of spinal MNs obtained from responses to square current pulses are related to gain estimated from the responses to the current ramps. We found, that the gain in response to current ramps is identical to the steady state gain during sustained depolarization. Therefore, gain modulation is more fully characterized with square current pulses than with current ramps.