A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae

1. Post-synaptic potentials (PSPs) evoked by electrical stimulation of a variety of input systems have been compared in triceps surae motoneurones innervating slow and fast muscle units, the speed of contraction of which was also determined.2. Stimulation of high threshold afferents in both flexor and extensor muscle nerves, and of joint afferents, evoked polysynaptic PSPs which were predominantly hyperpolarizing in both fast and slow twitch motor units.3. Volleys in cutaneous afferents in the sural and saphenous nerves evoked polysynaptic PSPs composed of mixtures of inhibitory and excitatory components. The inhibitory components were predominant in slow twitch motor units, while in fast twitch units there was a trend towards excitatory predominance.4. Repetitive stimulation of the red nucleus caused predominantly inhibitory PSPs in slow twitch units and mixed or predominantly excitatory PSPs in fast twitch units. There was a correlation in the excitatory/inhibitory balance between PSPs of cutaneous and rubrospinal origin in those motoneurones in which both types of PSPs were studied.5. The amplitudes of group Ia disynaptic inhibitory PSPs were found to be correlated with motor unit twitch type: IPSPs in slow twitch units were larger than those in fast twitch units. Rubrospinal conditioning volleys were found to facilitate group Ia IPSPs in both fast and slow twitch motor units.6. The results suggest that there may be several basic patterns of synaptic input organization to motoneurones within a given motor unit pool. In addition to quantitative variation in synaptic distribution, there is evidence that qualitative differences in excitatory to inhibitory balance also exist in the pathways conveying input from cutaneous afferents and rubrospinal systems to triceps surae motoneurones. These qualitative differences are correlated with the motor unit twitch type.