Essays on Science and SocietyNeuroscience

Shortcuts and checkpoints on the road to skilled movement

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Science  31 Oct 2014:
Vol. 346, Issue 6209, pp. 554-555
DOI: 10.1126/science.1260778
  • Internal copies and sensory gating in forelimb motor control.

    (A) During limb movement, motor commands elicit motor neuron activation and muscle contraction, generating proprioceptive sensory feedback. Temporal delays in peripheral feedback imply a need for i) a more rapid internal feedback mechanism (copy); and ii) a means to constrain sensory feedback gain (gate). (B) Cervical PNs receive input from descending pathways, including the reticulospinal (RS) tract. Bifurcating PN axons innervate forelimb motor neurons (MN; premotor branch) and the lateral reticular nucleus (LRN; internal copy branch), which projects to the cerebellum (CB). Colored neurons trace a putative cerebellar-motor loop. A conditional viral approach (AAV-FLEX-ChR2-YFP) was used to express ChR2 in cervical V2a interneurons, and ChR2+ PN terminals in the LRN were photostimulated to activate the PN internal copy branch selectively, resulting in a severe disruption of reaching kinematics (blue). (C) Presynaptic inhibitory (GABApre) neurons form axo-axonic contacts onto proprioceptive afferent terminals, in contrast to the far more abundant GABApost neurons, which form direct postsynaptic inhibitory contacts. Diphtheria toxin (DT)–mediated genetic ablation of cervical GABApre neurons uncovered severe limb oscillations during reaching of consistent ∼20 Hz frequency (red). [(B) adapted from (11); (C) adapted from (18)]

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