The effects of different types of synaptic input on the steady-state input-output relations of the mammalian motoneuron pool were investigated by the use of computer simulations. The properties of the simulated motor units and their synaptic inputs were based as closely as possible on the experimental data from studies in the cat hindlimb. 2. Three basic types of synaptic input systems were simulated: postsynaptic, presynaptic, and neuromodulatory. The effects of these inputs on three aspects of the system input-output structure were studied: gain, precision, and motor-unit type utilization. 3. The gain analyses were based on a simulation of the steady-state homonymous Ia input. The gain of this steady-state Ia "reflex" was found to be determined largely by the slope of the pool input-output function. Precision was evaluated in two ways, from the amplitudes of the quantal steps due to motor-unit recruitment and from the sensitivity of the input-output function to noise. The pattern of motor-unit type utilization allowed indirect assessment of fatigue resistance: the larger the percentage of force generated by FF units, the lower the fatigue resistance. 4. A uniformly distributed input (i.e., one that generates equal input in all motoneurons) generates outputs that are solely determined by the intrinsic properties of the motor units. Thus the gain, precision, and motor-unit type patterns generated by a uniform input were used as the basis with which the effects of all other input systems were compared. 5. Postsynaptic excitatory inputs with nonuniform distributions within the pool did influence gain. The greatest effect was the increase mediated by the rubrospinal excitatory input (27% increase at 30% of maximal force). However, this input also greatly decreased both fatigue resistance and precision, due to increased activation of FF units at low force levels. In contrast, the Ia input slightly decreased gain (12% decrease at 30% of maximum force) while slightly increasing fatigue resistance and precision. 6. The simulated neuromodulatory input was based on the monoaminergic reticulospinal effect on motoneurons. Gain was generally increased by the monoaminergic input. However, the magnitude of the increase strongly depended on whether the monoaminergic effects were largest on S units (giving a 20% increase at 30% of maximum force), equal on all types (52%), or largest on FF units (102%). Presynaptic inhibition reduced gain with no effect whatsoever on fatigue resistance or precision. 7. Therefore Ia reflex gain was modifiable by all three types of input: postsynaptic, presynaptic, and neuromodulatory.(ABSTRACT TRUNCATED AT 400 WORDS)
