A prolongation of the postspike afterhyperpolarization following spike trains can partly explain the lower firing rates at derecruitment than those at recruitment.

The original motivation for this study was the observation in previous human experiments that the motor neuron firing frequency (recorded from the motor units in the EMG) was lower at derecruitment than that at recruitment, with slow linearly varying voluntary contractions. Are the lower firing rates at derecruitment correlated with a change in the postspike afterhyperpolarization (AHP) after preceding spike trains? This question was investigated by intracellular recordings from cat motor neurons in both unanesthetized and anesthetized preparations. The firing frequencies at recruitment and derecruitment were compared during slow triangular current injections mimicking the slow linearly varying voluntary contractions in humans. There was a lower frequency at derecruitment in almost all motor neurons (83 of 86 motor neurons; mean = 3.35 Hz). Thus intrinsic mechanisms play an important role for the lower frequencies at derecruitment. This was independent of whether the current injection had activated persistent inward current (PIC; plateau potentials, secondary range firing). It was found that a preceding spike train could prolong the AHP duration following a subsequent spike. The lower rate at derecruitment matches the prolongation of the AHP. However, a quantitative comparison between the lowest firing frequency and AHP duration for individual motor neurons reveal that the predicted lowest firing frequency does not match the absolute AHP duration; the lowest frequency is lower than that predicted from AHP duration in fast motoneurons and higher than expected in slow motoneurons. It is suggested that these deviations are explained by the presence of synaptic noise as well as recruitment of PICs below firing threshold. Thus synaptic noise may allow spike discharge even after the end of the AHP in "fast" motor neurons, whereas synaptic noise and PICs below spike threshold tend to give higher minimum firing frequencies in "slow" motor neurons than predicted from AHP duration.