It was shown recently that functional activation across brain motor areas during locomotion and foot movements are similar but differ substantially from activation related to upper extremity movement (Miyai [2001]: Neuroimage 14:1186-1192). The activation pattern may be a function of the behavioral context of the movement rather than of its mechanical properties. We compare motor system activation patterns associated with isolated single-joint movement of corresponding joints in arm and leg carried out in equal frequency and range. Eleven healthy volunteers underwent BOLD-weighted fMRI while performing repetitive elbow or knee extension/flexion. To relate elbow and knee activation to the well-described patterns of finger movement, serial finger-to-thumb opposition was assessed in addition. After identifying task-related voxels using statistical parametric mapping, activation was measured in five regions of interest (ROI; primary motor [M1] and somatosensory cortex [S1], premotor cortex, supplementary motor area [SMA] divided into preSMA and SMA-proper, and cerebellum). Differences in the degree of activation across ROIs were found between elbow and knee movement. SMA-proper activation was prominent for knee, but almost absent for elbow movement (P < 0.05); finger movement produced small but constant SMA-proper activation. Ipsilateral M1 activation was detected during knee and finger movement, but was absent for the elbow task (P < 0.05). Knee movement showed less lateralization in M1 and S1 than other tasks (P < 0.05). The data demonstrate that central motor structures contribute differently to isolated elbow and knee movement. Activation during knee movement shows similarities to gait-related activation patterns.
It is assumed that somatosensory input is required for motor learning and recovery from focal brain injury. In rodents and other mammals, corticocortical projections between somatosensory and motor cortices are modified by patterned input. Whether and how motor cortex function is modulated by somatosensory input to support motor learning is largely unknown. Recent human evidence suggests that input changes motor excitability. Using transcranial magnetic stimulation (TMS), this study tested whether motor cortex excitability is affected by patterned somatosensory stimulation in rodents. Motor potentials evoked in gastrocnemius muscles in response to TMS (MEP(TMS)) and to cervical electrical stimulation (MEP(CES)) were recorded bilaterally. Initially, the first negative peak of the MEP(TMS) was identified as a cortical component because it disappeared after decortication in three animals. Subsequently, we studied the effects of 2 h of electrical stimulation of one sciatic nerve on the cortical component of the MEP(TMS), i.e., on motor cortex excitability. After stimulation, its amplitude increased by 117 +/- 45% ( P<0.01) in the stimulated limb. A significantly smaller effect was found in the unstimulated limb ( P<0.02) and no effect was observed in unstimulated control animals. The subcortically evoked MEP(CES) were not affected by stimulation. It is concluded that somatosensory input increases motor excitability in rat. This increase outlasts the stimulation period and is mediated by supraspinal structures, likely motor cortex. Modulation of motor cortex excitability by somatosensory input may play a role in motor learning and recovery from lesion.
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