Behavioural brain research
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The mammalian motor system contains multiple interconnected supraspinal networks, but little is known about their relative roles in producing different movements and behaviors, particularly given their apparently fused activity in primates. We tested whether the task context, as well as using a phylogenetically older mammal, rats, could distinguish the separate contributions of these networks. We obtained simultaneous multi-single neuron recordings from the forelimb motor cortex and magnocellular red nucleus as rats performed two contextually different, but kinematically similar, forelimb reach-like tasks: highly learned, skilled reaching for food through a narrow slot, a task requiring extensive training, versus the swing phases of treadmill locomotion. ⋯ In the mRN, the majority of task-modulated neurons peaked in their firing rate in the middle of the reach when the rat was preparing to project the arm through the slot, whereas large subgroups of M1 neurons displayed elevated firing rates during the initial and terminal phases of the reach. These results suggest that motor-behavioral context can alter the degree of overlapping activity in different supraspinal sensorimotor networks. Moreover, results for the skilled reaching task in rats may have highlighted a distinct processing role of the rubral complex: adapting natural muscle synergies across joints and limbs to novel task demands, in concert with cortically based learning.
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Unilateral forelimb sensorimotor cortex lesions in adult rats produce a compensatory hyper-reliance on the forelimb ipsilateral to the lesion and temporally related glial and neural plasticity in the contralateral homotopic cortex. Recently, we found that these lesions enhance acquisition of a motor skills task with the ipsilateral, non-impaired, forelimb in comparison to shams. This effect might be related to a denervation-induced facilitation of neuroplastic changes in the motor cortex opposite the lesion and/or to the lesion-induced hyper-reliance on the non-impaired forelimb. ⋯ Forced-use improved reaching performance relative to controls, but this effect was less enduring than the improvements produced by transections alone. The addition of forced-use to transections did not further enhance performance. These findings suggest that denervation-induced changes are likely to be a major contributor to the enhanced learning observed after unilateral sensorimotor cortex lesions.