Repetitive doublets of human motoneurones: analysis of interspike intervals and recruitment pattern

In order to study the probable mechanisms of repetitive doublets in human motoneurones, the firing patterns of single motor units (MUs) of the trapezius were analysed during a weak voluntary muscle contraction. The mean frequencies of MUs were 9.4-21.7 imp/sec (the mean interspike interval ranged from 46.0 to 106.7 msec). Repetitive doublets (up to 28 in succession) were recorded in 21 out of 120 MUs, mostly at the onset of a slow recruitment. These were followed by single discharges. Intradoublet intervals ranged between 2.5 and 20.0 msec. A significant difference between single spike firing and doublet firing was revealed by plotting interspike interval histograms, showing that two distinct mechanisms were involved. The analysis of interspike interval successions belonging to several MUs firing simultaneously showed that one of the MUs could start with doublets while the others went on firing single spikes with the regular mean frequency and interspike interval scatter. The results lead us to suggest that the intrinsic properties of motoneurones can be regarded as the main factor in the origin of repetitive doublets. It seems that a descending synaptic drive also contributes to the control of double firing since in a number of cases no doublets were produced at the beginning of MU activity. The findings are discussed with regard to the problems of regulating repetitive firing of human motoneurones by after-potentials. Steady delayed depolarization is assumed to be a possible mechanism of repetitive doublets.