Time coupling of skeletomotor discharges in response to pseudo-random transsynaptic and transmembrane stimulation

Firing pattern of skeletomotor neurones innervating triceps surae muscles in response to pseudorandom muscle stretching and white noise modulated transmembrane current stimulation was investigated in decerebrate cats. Pseudo-random muscle stretching (upper cut-off frequency 60 Hz, amplitude (standard deviation) ranging from 18.5 m to 40 m) was applied to triceps surae muscles. Membrane potential changes and action potentials of skeletomotor neurones were recorded intracellularly. White noise modulated current was applied through the same (recording) microelectrode. Sequences of ten identical 5 s periods of either muscle stretching or transmembrane current stimulation were applied. Skeletomotor neurones belonging to slow motor units (rheobase less than 8.5 nA) generated action potentials in response to both pseudo-random muscle stretching and transmembrane current stimulation, while firing threshold of those belonging to fast motor units could not be reached by the muscle stretches applied. Peri-spike averaging of muscle length and injected current records showed that the action potentials appeared at the peak of either depolarizing current wave or muscle stretching both preceded by a change in opposite direction (the spikes coinciding with the peak in muscle length PSA being actually elicited by muscle spindle action potentials triggered at the moment of the peak stretching velocity). Time coupling of action potentials occurred during both muscle stretching and transmembrane stimulation, being more tight in the latter case as well as when larger amplitudes of the stimuli were applied. It is supposed that discharges from muscle spindle primary endings phase-locked to small pseudo-random muscle length changes may, due to the time coupling of skeletomotor action potentials, provoke a synchronous firing of skeletomotor neurones, mostly of those belonging to slow motor units. Possible effects of such a firing pattern on the resulting muscle reflex contraction and the stretch reflex stability as well as a possibility of it being provoked by fusimotor discharges are discussed.