Do corticomotoneuronal cells predict target muscle EMG activity?

Data from two rhesus macaques were used to investigate the pattern of cortical cell activation during reach-to-grasp movements in relation to the corresponding activation pattern of the cell's facilitated target muscles. The presence of postspike facilitation (PSpF) in spike-triggered averages (SpTAs) of electromyographic (EMG) activity was used to identify cortical neurons with excitatory synaptic linkages with motoneurons. EMG activity from 22 to 24 muscles of the forelimb was recorded together with the activity of M1 cortical neurons. The extent of covariation was characterized by 1) identifying the task segment containing the cell and target muscle activity peaks, 2) quantifying the timing and overlap between corticomotoneuronal (CM) cell and EMG peaks, and 3) applying Pearson correlation analysis to plots of CM cell firing rate versus EMG activity of the cell's facilitated muscles. At least one firing rate peak, for nearly all (95%) CM cells tested, matched a corresponding peak in the EMG activity of the cell's target muscles. Although some individual CM cells had very strong correlations with target muscles, overall, substantial disparities were common. We also investigated correlations for ensembles of CM cells sharing the same target muscle. The ensemble population activity of even a small number of CM cells influencing the same target muscle produced a relatively good match (r >/= 0.8) to target muscle EMG activity. Our results provide evidence in support of the notion that corticomotoneuronal output from primary motor cortex encodes movement in a framework of muscle-based parameters, specifically muscle-activation patterns as reflected in EMG activity.