Linear transmission of cortical oscillations to the neural drive to muscles is mediated by common projections to populations of motoneurons in humans

Abstract

Non-technical summary Since the human central nervous system controls muscle contraction through inputs to spinal motoneurons, oscillations recorded on the primary motor cortex during voluntary movements are correlated with the electrical activity produced on the surface of the muscles. We show through theoretical derivations and experimental recordings that cortical input is transmitted partly in a linear way to the population of motoneurons. The results demonstrate the effective spread of the cortical projections to the motoneuron pool to allow an efficient control of the muscle force output.Oscillations in the primary motor cortex are transmitted through the corticospinal tract to the motoneuron pool. This transmission has been previously investigated using coherence analysis between concurrent recordings of EEG and surface EMG signals. In this study we used a mathematical derivation and motor unit recordings in vivo to investigate the origin of linear transmission of cortical input to the motoneuron spike trains (neural drive to muscle). The theoretical derivation showed that a common input spread to a relatively small number of motoneurons is partly transmitted in a linear way, overcoming the interference signal generated by the non-linearity of the individual motoneurons. We further calculated the corticomuscular coherence between experimental EEG signals and the cumulative spike trains of motor units in the abductor digiti minimi muscle of seven healthy men. The experimental results indicated that, on average, only four to five motor units were sufficient to reach the same level of coherence as estimated from the surface EMG. The results demonstrate that linearity in the transmission of the cortical input to motoneurons through the corticospinal tract is achieved because (i) this input is largely common to all motoneurons, and (ii) its frequency content requires only a small fraction of active motoneurons to be accurately sampled. In this way, the central nervous system can directly transmit oscillations to the control signals to muscles for practically the entire range of functionally relevant forces.