DISTRIBUTION OF RUBROSPINAL SYNAPTIC INPUT TO CAT TRICEPS SURAE MOTONEURONS

1. We evoked steady-state synaptic potentials in triceps surae motoneurons of the cat by stimulating the hindlimb projection area of the contralateral magnocellular red nucleus at 200 Hz. We measured the effective synaptic currents (I(N)) underlying the synaptic potentials using a modified voltage-clamp technique. We also determined the effect of the rubrospinal input on the discharge rate of some of the motoneurons by inducing repetitive discharge with long injected current pulses during which the red nucleus stimulation was repeated. 2. At motoneuron resting potential, the distribution of I(N) from the red nucleus within the triceps surae pools was qualitatively similar to the distribution of synaptic potentials: 86% of the putative type F motoneurons received a net depolarizing I(N) from the red nucleus stimulation, whereas only 38% of the putative type S units did so. The mean values of I(N) were significantly different in the two groups [+4.1 +/- 5.0 nA (SD) for putative type F and -1.6 +/- 3.1 nA for putative type S]. 3. However, when the values of I(N) at threshold for repetitive firing were estimated, the distribution of I(N) from the red nucleus was quite different. At threshold, all of the putative type S units received hyperpolarizing I(N) but so did nearly half of the putative type F units. 4. As would be expected from the wide range of I(N) at threshold (-20 to +12 nA), the red nucleus input produced dramatically different effects on the discharge of different motoneurons. The discharge rates of those motoneurons that received depolarizing I(N) at threshold were accelerated by stimulating the red nucleus (+5 to +14 imp/s), whereas the discharge rates of cells that received hyperpolarizing currents were retarded by the rubrospinal input (-4 to -21 imp/s). 5. The red nucleus synaptic input reduced motoneuron input resistance by 40% on average. The effect on input resistance was most pronounced in those motoneurons that received hyperpolarizing I(N). 6. Our findings indicate that the red nucleus input may provide a powerful source of synaptic drive to some high-threshold motoneurons while concurrently inhibiting low-threshold cells. Thus this input system can potentially alter the gain of the input-output function of the motoneuron pool as well as disrupt the normal hierarchy of recruitment thresholds.