Last year we showed that there is a spinal circuit that processes sensory feedback from the muscles of the wrist and elbow, and then generates stretch reflexes in elbow muscles tuned to how far the hand had been moved from a static position https://tinyurl.com/yfzmy9pj (2/11)
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The tuning of these reflexes is an efficient way to re-establish the hand’s position in space. This was surprising bc many believe the output of spinal circuits – stretch reflexes – only attempt to regulate or re-establish the angular position of individual joints (3/11)
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That result opened the door for us to address other questions. In this work we asked whether the efficient spinally-mediated stretch reflexes that help maintain the postural position of the hand can also support the efficient control of reaching (4/11)
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We had participants reach to a target that required them only to extend their elbow and on a subset of trials we applied mechanical perturbations the moment they began their movement. These perturbations flexed the elbow, and simultaneously flexed or extended the wrist (5/11)
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We found that the perturbations that moved the hand furthest from the target did so with the least amount of elbow flexion and perturbations that moved the hand towards the target did so with a large amount of elbow flexion (6/11)pic.twitter.com/myIwhp34Nf
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We also found that the spinal reflexes evoked in the triceps muscle were not proportional to how much the elbow was flexed – and thus how much the triceps was stretched - but instead were efficiently scaled to the hand’s distance from the reaching target. (7/11)pic.twitter.com/qsBExF76NQ
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In the next exp participants made the same reaching movement and we applied the same mechanical perturbations, but we changed their arm orientation, which diametrically altered how the rotation of the wrist moved the hand towards the target. (8/11)pic.twitter.com/0RP6UDHuIe
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We found that changing the arm's orientation diametrically altered how spinal reflexes in the triceps were evoked, and in such a way that were again efficiently scaled to the hand's distance from the reaching target (9/11)pic.twitter.com/534H9SrbB8
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This means there is a spinal circuit that, 1) integrates info from the wrist and the elbow, 2) knows how this joint info and the arm’s orientation maps onto the hand’s movement in space, 3) produces appropriate elbow muscle activity that moves the hand to the target (10/11)
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Our findings demonstrate that spinal circuits can help efficiently control the hand during dynamic reaching actions, and show that efficient and flexible motor control is not exclusively dependent on processing that occurs within supraspinal regions of the nervous system (11/11)
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