Responses of a motor-unit population to patterns of activation

The force exerted during a contraction results from the summation of force from multiple motor units. Furthermore, the moment-to-moment force produced by a single motor unit reflects the timing of excitation by a motor neuron. However, the interactions at the population level and determinants of single motor neuron activity cannot be studied experimentally in humans. Therefore, a computational approach was used to address the relations between activation patterns of a model motor-unit population and muscle force and between synaptic input and action potential timing in biologically realistic neuron models. The first study indicated that the presence of correlated activity among a population of motor units created inaccuracies in a commonly used method for assessing single motor unit force in humans. The second study examined how activation properties of the population of motor units influenced the time and frequency structure of the simulated force. These results showed that there are two mechanisms related to the commonality in motor unit activity that are mostly responsible for variability of muscle force. Although the first two studies indicated that similar timing of discharge by motor units is an important determinant of muscle force, little is known about the exact relation between the synaptic input to a population of motor neurons and the correlated discharge by motor units. In the third study, the number of patterns of input and neuron properties that could generate correlated activity between neurons was assessed by an optimization strategy. The results indicated variations in the amount of correlated discharge did not necessitate variations in the level of common input. The fourth study used a deterministic approach to quantify the amount of correlation in discharge evoked by various levels of common input across model neurons with a broad range of properties. These results showed that the properties of the neurons had a strong influence on the degree of output correlation. Taken together, these studies indicate that the relative timing of activation of motor units is the main determinant of force characteristics, and that the correlated activity is reflective of the intrinsic properties of the motor neurons.