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Repetitive doublet firing in human motoneurons: evidence for interaction between common synaptic drive and plateau potential in natural motor control.

The firing behavior of spinal motoneurons (MNs) is a result of processing synaptic inputs by MN membrane properties, including plateau potentials, fundamentally explored in animals. However, there is much less data about a plateau potential role in human motor control. We explored human MN repetitive doublet firing during gentle isometric voluntary muscle contractions with the aim of revealing possible evidence for interaction between plateau potentials and common synaptic drive known as an important determinant of MN pool firing behavior. Single motor unit (MU) repetitive firing of trapezius and triceps brachii was analyzed. Subjects were asked to recruit MUs capable of firing repetitive doublets. The analysis of interspike intervals (ISIs) of background firing of simultaneously recorded MUs showed that beyond doublet series ISIs varied often in unison with significant correlation coefficients, demonstrating common synaptic drive. During doublet series, MUs showed persistent doublet ISIs (typically 4-7 ms) and the tendency to increasing number of doublets in series throughout the experiment. It was consistent with involvement of MN plateau potentials resulting in the persistent delayed depolarization (underlying each doublet) and occurring warm-up effect. Common synaptic drive 'started' doublet series; probably both mechanisms controlled post-doublet ISIs. However, convincing effects of plateau potentials on MU firing behavior during single firing were not found. Thus, our results suggest a plateau potential role in specifying the essential firing pattern, doubling, of some MUs rather than its effect on firing behavior of MN pool, on the whole, during voluntary muscle contractions in humans.