DISTRIBUTION OF RUBROSPINAL SYNAPTIC INPUT TO CAT TRICEPS SURAE MOTONEURONS

1. We evoked steady-state synaptic potentials in triceps surae motoneu rons of the cat by stimulating the hindlimb projection area of the con tralateral magnocellular red nucleus at 200 Hz. We measured the effect ive synaptic currents (I(N)) underlying the synaptic potentials using a modified voltage-clamp technique. We also determined the effect of t he rubrospinal input on the discharge rate of some of the motoneurons by inducing repetitive discharge with long injected current pulses dur ing which the red nucleus stimulation was repeated. 2. At motoneuron r esting 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 receiv ed a net depolarizing I(N) from the red nucleus stimulation, whereas o nly 38% of the putative type S units did so. The mean values of I(N) w ere significantly different in the two groups [+4.1 +/- 5.0 nA (SD) fo r 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 esti mated, the distribution of I(N) from the red nucleus was quite differe nt. At threshold, all of the putative type S units received hyperpolar izing 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 accelera ted by stimulating the red nucleus (+5 to +14 imp/s), whereas the disc harge rates of cells that received hyperpolarizing currents were retar ded by the rubrospinal input (-4 to -21 imp/s). 5. The red nucleus syn aptic input reduced motoneuron input resistance by 40% on average. The effect on input resistance was most pronounced in those motoneurons t hat received hyperpolarizing I(N). 6. Our findings indicate that the r ed nucleus input may provide a powerful source of synaptic drive to so me high-threshold motoneurons while concurrently inhibiting low-thresh old cells. Thus this input system can potentially alter the gain of th e input-output function of the motoneuron pool as well as disrupt the normal hierarchy of recruitment thresholds.