COMPUTER-SIMULATIONS OF MOTONEURON FIRING RATE MODULATION

1. As a human subject slowly increases the amount of force exerted by a muscle, the discharge rates of low-threshold motor units saturate at a rather low level, whereas higher-threshold units continue to be rec ruited and undergo increases in their discharge rates. The presently k nown intrinsic properties of motor units do not produce this ''rate li miting.'' 2. Using computer simulations of a model motoneuron pool, we tested the hypothesis that rate limiting can be accounted for on the basis of the known distributions of synaptic input from different sour ces. The properties of the simulated motor units and their synaptic in puts were based as closely as possible on the available experimental d ata. A variety of simulated synaptic input organizations were applied to the pool, and the resulting outputs were compared with the data on rate limiting in human subjects. 3. We found that the data on rate lim iting in human subjects greatly constrained the possible organizations of characterized synaptic input systems. Only when the synaptic organ ization included a gradual ''crossover'' between two specific types of input systems could the human data be accurately reproduced. Low inpu t/output levels relied on a system organized like the monosynaptic la input, which produces greater effective synaptic currents in low- than in high-threshold motor units. Above a sharply defined crossover leve l, all further increases in output were produced by a system organized like the oligosynaptic rubrospinal input, which generates the opposit e pattern.