After-potentials and control of repetitive firing in human motoneurones.

Characteristics of motoneurone after-potentials in man were derived from the recovery curve of motoneurone excitability after a single discharge evoked by threshold stimulation of Ia afferents or by gentle voluntary muscle contraction. The motoneurone excitability was estimated by the firing index of a single motor unit whose potentials were recorded by needle electrodes. The soleus (a slow muscle) and the flexor carpi ulnaris (a fast muscle) were investigated. The duration of motoneurone after-hyperpolarization of the soleus evaluated by this method ranged between 145 and 255 msec; for the flexor carpi ulnaris it was 55-150 msec. In some motoneurones of the fast muscle, an early short-lasting recovery of excitability (within 5-20 msec after a discharge) was revealed. It was accounted for by delayed depolarization of the motoneurone. The relationship between after-potentials and the characteristics of repetitive firing of motoneurones activated by weak voluntary muscle contraction was analysed. It was observed that the motoneurones with early excitability recovery were capable of firing double discharges with a 5-15 msec interspike interval. It was found also that the minimal firing rate of motoneurones (up to 3.1-5.2 imp/sec in the soleus and 3.8-9.0 imp/sec in the flexor carpi ulnaris) was not correlated with the after-hyperpolarization duration. This differs from the results obtained for cat's motoneurones under intracellular stimulation. The findings suggest that after-hyperpolarization is not the only leading mechanism controlling the low firing rate of motoneurones under conditions of their natural activity in man.

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.