Synaptic activity in alpha motoneurons consequent to their motor unit contractions

Abstract

Single motor units of the triceps surae muscle group were caused to contract by intracellular stimulation of their alpha motoneurons. Simultaneous intracellular recording and signal averaging revealed a series of low-amplitude synaptic depolarizations in the stimulated neuron. Since the depolarizations were abolished by section of the homonymous muscle nerve, it is concluded that the depolarizations arise from activity in homonymous primary muscle spindle afferents and reflect the underlying synaptic activity responsible for autogenetic facilitation. Depolarizations during the rising phase of the twitch were seen in 16 of 27 “fast” motor units and in 3 of 13 “slow” motor units. Depolarizations during the falling phase of the twitch were produced by 37 of 40 motor units; the later depolarizations were always preceded by a cessation of depolarizing synaptic activity. Hyperpolarizing activity was seen on subtracting postnerve-cut waveforms from prenerve-cut waveforms from four motor units. Hyperpolarization was associated with the rising phase and peak of the twitch tension; however, the experimental procedure did not allow localization of the mechanisms involved. The relationship between the patterned depolarizations and the patterned discharge of primary muscle spindle afferents during a muscle twitch is discussed.