Stochastic properties of motoneuron activity and the effect of muscular length

Abstract

The activity of single motor units contributing to small tonic isometric contractions in human muscle at different muscular lengths was analyzed. The form of motor unit firing patterns shows that the interspike intervals compose independent sequences with about a 10% coefficient of variation and have a gamma distribution. The variability and the distribution shape curves show that as the mean interval decreases the variance also decreases and the interval density function becomes more symmetric. More significant is the fact that the form of the firing pattern remains unchanged when a motor unit has the same mean interval but with the muscle at different lengths. Comparison of these facts with experimental data from neuron models and cat motoneurons indicates that in the human the only relevant input-output relationship in motoneurons is that the net excitation adjusts the firing rate.