DISTRIBUTION OF EFFECTIVE SYNAPTIC CURRENTS IN CAT TRICEPS SURAE MOTONEURONS - VI - CONTRALATERAL PYRAMIDAL TRACT

We measured the effective synaptic currents (I-N) produced by stimulat ing the contralateral pyramidal tract (PT) in triceps surae motoneuron s of the cat. This is an oligosynaptic pathway in the cat that generat es both excitation and inhibition in hindlimb motoneurons. We also det ermined the effect of the PT synaptic input on the discharge rate of s ome of the motoneurons by inducing repetitive firing with long, inject ed current pulses during which the PT stimulation was repeated. At res ting potential, all but one triceps motoneuron received a net depolari zing effective synaptic current from the PT stimulation. The effective synaptic currents (I-N) were much larger in putative type F motoneuro ns than in putative type S motoneurons [+4.6 +/- 2.9 (SD) nA for type F vs. 0.9 +/- 2.4 nA for putative type S]. When the values of I-N at t he threshold for repetitive firing were estimated, the distribution wa s markedly altered. More than 60% of the putative type S motoneurons r eceived a net hyperpolarizing effective synaptic current from the pyra midal tract stimulation as did 33% of the putative type F motoneurons. This distribution pattern is very similar to that observed previously for the effective synaptic currents produced by stimulating the contr alateral red nucleus. As would be expected from the wide range of I-N values at threshold (-4.8 to +8.7 nA), the PT stimulation produced dra matically different effects on the discharge of different triceps moto neurons. The discharge rates of those motoneurons that received depola rizing effective synaptic currents at threshold were accelerated by PT stimulation (+1 to +8 imp/s), whereas the discharge rates of cells th at received hyperpolarizing currents were retarded by the PT input (-2 to -7 imp/s). The change in firing rates produced by the PT stimulati on was generally approximated by the product of the effective synaptic currents and the slopes of the motoneurons' frequency-current relatio ns. Our findings indicate that the contralateral pyramidal tract may p rovide a powerful source of synaptic drive to some high-threshold moto neurons while concurrently inhibiting low-threshold cells. Thus this i nput system, like that from the contralateral red nucleus, can potenti ally alter the gain of the input-output function of the motoneuron poo l as well as disrupt the normal hierarchy of recruitment thresholds.