Repetitive doublet firing of motor units: evidence for plateau potentials in human motoneurones?

Abstract

During voluntary muscle contraction, human motoneurones can exhibit specific discharge patterns: single and repetitive doublets. Delayed depolarization has been accepted as the mechanism underlying single doublets. Repetitive doublet firing has been studied much less and its controlling mechanisms remain obscure. The aim of the present study was to examine properties of repetitive doublets in human motoneurones and to consider their underlying potential mechanisms. It was found that 22 of 41 (53.7%) lower-threshold motor units (MUs) in the trapezius and 15 of 42 (35.7%) MUs in triceps brachii displayed repetitive doublets with the mean interspike intervals (ISIs) of 5.5 +/- 1.1 and 6.4 +/- 2.6 ms, respectively. Each doublet was followed by a prolonged post-doublet ISI. The analysis of properties of repetitive doublets showed that they were typically initiated in quiescent motoneurones rather than in firing ones (appearing just at recruitment in an all-or-none manner) and could only be maintained at a certain level of muscle contraction. Repetitive doublets were interrupted either voluntarily (by the subject), or spontaneously with sudden transition from doublet firing to single discharges-the firing behaviour that may be referred to as a firing-pattern "jump". The properties of doublet firing seem to be consistent with traits of motoneurone firing in the presence of plateau potentials reported in animal studies. It was suggested that the potential mechanisms underlying repetitive doublet firing could include a delayed depolarization as the primary determinant, which likely could become persistent probably due to a plateau potential activated in parallel with a common synaptic input.

Motoneuron firing in amyotrophic lateral sclerosis (ALS)

“…It might represent dysfunction of the LMN pool in ALS (Zalewska et al, 1998). The underlying mechanism should involve delayed depolarization (Weber et al, 2009; Kudina and Andreeva, 2010). …”

Afterhyperpolarization of human motoneurons firing double and triple discharges

“…Repetitive doublets, i.e., series of doublet-postdoublet ISIs, require some additional mechanism, which would support the DD hump, shown to disappear during MN rhythmic firing (Granit et al, 1963; Calvin and Loeser, 1975). It was recently hypothesized that repetitive doublet firing may be related to plateau potentials (Kudina and Andreeva, 2010, 2013a). …”

Single motor unit firing behavior in the right trapezius muscle during rapid movement of right or left index finger

“…It is therefore interesting that in the present study simulating free living conditions with no feedback to the subject, a MU presented a firing pattern with repetitive doublets transitioning from stable doublets to stable single spikes firing. Such a firing behavior was earlier described by Bawa and Calancie (1983) and later provoked with MU feedback by Kudina and Andreeva (2010). Repetitive doublets followed by a jump in firing rate could be considered as the bistable firing behavior characterizing a plateau potential.…”

“…Since the earliest description of doublets by Denslow (1948) the origin has been debated and so has the distinction between the occasionally evoked doublets and the repetitive doublets (Bawa et al, 2000; Kudina and Andreeva, 2010). However, as recently stated doublets are rarely recorded during voluntary muscle activity in human subjects (Piotrkiewicz and Kuraszkiewicz, 2014).…”