How the body prepares to move: Independent control of muscle
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A number of brain areas change their activity before we execute a planned voluntary movement. A new study identifies a novel function of this preparatory neural activity, highlighting another mechanism the nervous system can use to achieve its goals. The study demonstrates a preparatory change in the sensitivity of muscle spindle receptors and the motor reflexes they enable.

Here, using an instructed-delay manual task, researchers demonstrate a decrease in human muscle afferent activity (primary spindles) when preparing to reach targets in directions associated with a stretch of the spindle-bearing muscle.

This goal-dependent modulation of proprioceptors began early after target onset but was markedly stronger at the latter parts of the preparatory period. Moreover, whole-arm perturbations during reach preparation revealed modulation of stretch reflex gains (shoulder and upper arm muscles) that reflected the observed changes in spindle activity.

The investigators suggest that one function of central preparatory activity is to tune muscle stiffness according to task goals via the independent control of muscle spindle sensors.

Science Advances
Source: https://advances.sciencemag.org/content/7/9/eabe0401
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