Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Postural equilibrium is known to be controlled by sensorimotor reflexes and automatic control loops but also depends on high-level body representation in space, probably implicating the right temporoparietal cortex. Indeed, short-term prism adaptation to a 10 degrees rightward visual shift has been shown to reduce predominant postural imbalance in patients with right hemisphere damage, as it did for neglect symptoms. These effects are likely to be explained by a high level effect of prism adaptation on body and space representation, rather than by a sensorimotor effect. ⋯ Indeed, only adaptation to a leftward visual shift induced significant rightward postural bias in normal subjects. The rightward postural lateral displacement was negatively correlated with the visual vertical. Both transfer and direction specific effect of visuo-manual adaptation to prisms on postural control suggest that effects of adaptation act more on high-level postural control linked to body representation in space or at least reveal close interaction between sensorimotor plasticity and body representation.
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It is well known that sensorimotor adaptation will transfer from the practiced to the unpracticed arm, which has been taken as evidence that adaptation is located in the brain before the divergence point for left and right arm control. We now explore whether adaptation will transfer between different movement categories as well. ⋯ This benefit of pointing persisted when the use of cognitive strategies was minimized by a concurrent, attention-demanding task, but it was lost when pointing amplitudes were very small. We conclude that adaptation is located in the brain before the divergence point for different movement categories, and that movements with a large ballistic component facilitate adaptation transfer.
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The complex structure of the neck musculoskeletal system poses challenges to understanding central nervous system (CNS) control strategies. Examining muscle activation patterns in relation to musculoskeletal geometry and three-dimensional mechanics may reveal organizing principles. We analyzed the spatial tuning of neck muscle electromyographic (EMG) activity while subjects generated moments in three dimensions. ⋯ The focus of muscle tuning did not change with moment magnitude, demonstrating that co-contraction did not increase with load. Axial rotation was found to have a strong influence on neck muscle spatial tuning. The uniform results among subjects indicate that the CNS has consistent strategies for selecting neck muscle activations to generate moments in specific directions; however, these strategies depend on three-dimensional mechanics in a complex manner.
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One subtype of ATP-gated ion channel, the P2X(3) receptor, is expressed primarily on peripheral sensory neurons. While it is known that P2X(3) receptors can participate in certain forms of nociceptive signaling, their involvement in neuropathic pain transmission is not known. We have examined the expression and function of P2X(3) receptors in a rat spinal nerve ligation model of neuropathic pain. ⋯ Interestingly, P2X(3) immunoreactivity and P2X(3)-like responses were also detected in a subset of larger diameter (50 micro m) neurons and the number and amplitude of these responses were unchanged after spinal nerve ligation. These results suggest that, while there appears to be a decrease in fast desensitizing P2X(3) receptors following L5/L6 nerve ligation injury, certain subsets of small and large DRG neurons maintain normal P2X(3) receptor expression and function. These remaining receptors may provide a P2X(3) receptor-mediated component to neuropathic pain.
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In normal subjects, focal repetitive transcranial magnetic stimulation (rTMS) of the hand motor area evokes muscle potentials (MEPs) from muscles in the hand (target muscles) and the arm (non-target muscles). In this study we investigated the mechanisms underlying the spread of MEPs induced by focal rTMS in non-target muscles. rTMS was delivered with a Magstim stimulator and a figure-of-eight coil placed over the first dorsal interosseus (FDI) motor area of the left hemisphere. Trains of 10 stimuli were given at a suprathreshold intensity (120% of motor threshold) and at frequencies of 5, 10 and 20 Hz at rest. ⋯ Using a control MEP size matched with that observed after a conditioning contralateral rTMS, we found that paired-pulse inhibition remained unchanged. Yet a single TMS conditioning pulse sufficiently strong to evoke a MEP in the contralateral FDI and biceps muscles simultaneously (as rTMS did) left paired-pulse inhibition unchanged. We conclude that the spread of EMG activity to non-target muscles depends on cortical mechanisms, mainly including changes in the excitability of the interneurones mediating intracortical inhibition.