Abstract
Voluntary movements are believed to be advantageously prepared before they are executed, but the neural mechanisms at work have been unclear. For example, there are no overt changes in skeletal muscle activity during movement preparation. Here, using a delayed-reach manual task, we demonstrate a decrease in the firing rate of human muscle afferents (primary spindles) when preparing stretch rather than shortening of the spindle-bearing muscle. This goal-dependent modulation of proprioceptors begun early after target onset but was markedly stronger at the latter parts of the preparatory period. In two additional experiments, whole-arm perturbations during reach preparation revealed a congruent modulation of stretch reflex gains of shoulder and upper arm muscles. Our study shows that movement preparation can involve sensory elements of the peripheral nervous system. We suggest that central preparatory activity can also reflect sensory control, and preparatory tuning of muscle spindle mechanoreceptors is a component of planned reaching movements.
Footnotes
(a) New analyses and figures demonstrating that the preparatory tuning profile of spindle receptors (i.e., markedly stronger suppression late in preparation) is consistently reflected in long-latency stretch reflex responses, regardless of background mechanical loads (lines 235-286; Fig. 7; Fig. 9; Suppl. Fig. 6). (b) Additional discussion of how our paradigm/results contrast with previous paradigms/results (lines 350-394)