Increased Beta-Band Input to Spinal Motor Neurons is Not Confined to the Responding Muscle During a Go/No-Go Task

Abstract

During movement preparation and inhibition, the sensorimotor cortex shows alpha- and beta-band oscillations that are also expressed in the discharge of spinal motor neurons. Whether this oscillatory activity is confined to the motor neurons innervating the muscles involved in the prepared action is unknown. Here we combined electroencephalography with recordings of spinal motor neurons innervating the tibialis anterior. Participants held a submaximal dorsiflexion while performing a Go/No-Go task in which the instructed response was either a ballistic dorsiflexion or handgrip, making the tibialis anterior the responding or a non-responding muscle, respectively. Alpha- and beta-band power increased after the imperative cue in both cortical activity and motor neuron output, including when the tibialis anterior was not involved in the response. Because power modulations in motor neuron output can be confounded by the concurrent change in discharge rate, we assessed the oscillatory activity shared across motor neurons by calculating coherence between disjoint subsets, which is largely insensitive to discharge rate. We then used simulation-based inference to identify synaptic input modulations compatible with our observations. These analyses indicated that beta-band common input increased whether or not the tibialis anterior was involved in the response, whereas alpha-band input increased only when it was the responding muscle. The concurrent decrease in discharge rate and torque was attributed to reduced net excitatory drive, whereas alpha- and beta-band inputs accounted for the increase in synchronisation. These results demonstrate that cue-related oscillatory inputs reach spinal motor neurons more broadly than classically assumed, and shape motor neuron synchronisation.