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.