The precision of skilled forelimb movement has long been presumed to rely on rapid feedback corrections triggered by internally-directed copies of outgoing motor commands – but the functional relevance of inferred internal copy circuits has remained unclear. One class of spinal interneurons implicated in the control of mammalian forelimb movement, cervical propriospinal neurons (PNs), has the potential to convey an internal copy of pre-motor signals through dual innervation of forelimb-innervating motor neurons and pre-cerebellar neurons of the lateral reticular nucleus. We have examined whether the PN internal copy pathway functions in the control of goal-directed reaching. In mice, PNs include a genetically-accessible subpopulation of cervical V2a interneurons, and their targeted ablation perturbs reaching while leaving intact other elements of forelimb movement. Moreover, optogenetic activation of the PN internal copy branch recruits a rapid cerebellar feedback loop that modulates forelimb motor neuron activity and severely disrupts reaching kinematics. Our findings implicate V2a PNs as the focus of an internal copy pathway assigned to the rapid updating of motor output during reaching behavior.

The impressive precision of mammalian limb movements relies on internal feedback pathways that convey information about ongoing motor output to cerebellar circuits. The spino-cerebellar tracts (SCT) in the cervical, thoracic and lumbar spinal cord have long been considered canonical neural substrates for the conveyance of internal feedback signals. Here we consider the distinct features of an indirect spino-cerebellar route, via the brainstem lateral reticular nucleus (LRN), and the implications of this pre-cerebellar “detour” for the execution and evolution of limb motor control. Both direct and indirect spino-cerebellar pathways signal spinal interneuronal activity to the cerebellum during movements, but evidence suggests that direct SCT neurons are mainly modulated by rhythmic activity, whereas the LRN also receives information from systems active during postural adjustment, reaching and grasping. Thus, while direct and indirect spino-cerebellar circuits can both be regarded as internal copy pathways, it seems likely that the direct system is principally dedicated to rhythmic motor acts like locomotion, while the indirect system also provides a means of pre-cerebellar integration relevant to the execution and coordination of dexterous limb movements.

Propofol is an intravenous anesthetic with neuroprotective effects against cerebral ischemia or hypoxia injury. However, the underlying mechanisms remain obscure. Recent years emerging evidence has demonstrated that metallothionein-3 (MT-3), a growth inhibitory factor that exists mainly in the central nervous system, exhibited neuroprotective effect in vivo. Here, we used a model of hypoxia/re-oxygenation (H/R) injury to examine the hippocampal neuroprotective effect of propofol, and explored the role of MT-3 in this action. H/R resulted in reduced cell viability and increased cell death in hippocampal neuron culture, as indicated by MTT assay and lactate dehydrogenase (LDH) release assay, respectively. Pretreatment of propofol at different concentrations (50, 150, and 250 μmol/L) reversed H/R-induced neurotoxicity and increased MT-3 mRNA and protein expressions. Moreover, propofol failed to exert neuroprotective effect when MT-3 was silenced by the transfection with the specific siRNA, suggesting that MT-3 was the crucial mediator for propofol's neuroprotective effect against H/R. In conclusion, our findings showed that propofol is neuroprotective in H/R model on hippocampal neuron cells and that it may act by up-regulation of MT-3.

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