Neuromodulation enables transient flexible control of motoneurons

A motoneuron pool is often regarded as a rigid controller because the largely shared synaptic input across motoneurons leads to strongly correlated activity. However, brief deviations from this correlated behavior have been observed even in some constrained tasks, raising the question of whether these results reflect limitations of the rigid view of motoneuron pool control. Here we show that they do not. We developed a biophysical model of a motoneuron pool receiving shared excitatory and inhibitory synaptic inputs that also included the motoneuron-specific effects of neuromodulation; model parameters were tuned based on large-scale motoneuron recordings in humans. Simulations showed that the intrinsic differences in how motoneurons respond to neuromodulation are both necessary and sufficient to transiently decorrelate pairs of motoneurons receiving a shared synaptic input. Crucially, such transient decorrelation is only observed when motoneurons have different sensitivity to neuromodulation, consistent with experimental observations during volitional control in humans. Our model also explains how participants can improve their ability to transiently decorrelate the activity of motoneurons innervating the same muscle by leveraging refined behavioral strategies that exploit the differential response of motoneurons to neuromodulation, rather than through physiological changes. These results identify that heterogeneous sensitivity to neuromodulation enables brief flexibility in the otherwise rigid control of motoneurons enforced by a shared synaptic input, and show how practice allows participants to exploit latent flexibility within otherwise rigid constraints.