Testing excitability of human motoneurones capable of firing double discharges

Single motor unit (MU) action potentials were recorded from the flexor carpi ulnaris during its weak voluntary contraction in 6 healthy subjects. At low frequencies of motoneuronal firing (the average interspike interval ranged from 70 +/- 8 to 200 +/- 22 msec), double discharges with interspike intervals from 5 to 20 msec were seen from about one-fourth of all MUs (62 out of 249). Monosynaptic testing of the excitability of firing motoneurones was carried out to study the probable mechanism of doublets. By stimulating the ulnar nerve against the background of firing MUs an H-reflex was elicited. The excitability of the motoneurones was evaluated by a firing index whose changes within an interspike interval were analysed (the step width was 10 msec). The testing of 47 MUs has shown that motoneurones capable of firing double discharges during voluntary muscle contraction have a higher excitability at the beginning of an interspike interval (the first 20 msec) as compared to those incapable of firing double discharges. Our data confirm that the leading part in the formation of double discharges is played by delayed depolarization occurring in a firing motoneurone after its successive discharge. The correlation between the characteristics of doublets and the properties of delayed depolarization are discussed. It is proposed that motoneurones with a delayed depolarization possess certain transduction properties as they produce a maximal instantaneous increase in discharge frequency on a minimal increase in synaptic input. This study has revealed a phase of increased excitability in the motoneurone axon during the first 15 msec after a regular rhythmic discharge has been propagated.(ABSTRACT TRUNCATED AT 250 WORDS)