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Abstract:
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The existence of monosynaptic connections between primary motor cortex neurons and motoneurons is well established (Lemon and Porter, 1993). These Corticomotor (CM) neurons and the signals they carry are integral to understanding mechanisms underlying movement execution. Such a direct synaptic connection is assumed to drive motoneuron activity and ultimately muscle activity during skilled movements. If this is the case, the activity of a CM cell would be expected to covary closely with that of its target muscles during voluntary movement. To test this hypothesis, the presence of postspike facilitation (PSpF) in spike-triggered averages of electromyographic (EMG) activity was used to identify cortical neurons with excitatory synaptic linkages to motoneurons. Cell-target muscle covariation was quantified by plotting the average firing rate of each CM cell against the EMG activity of each of its facilitated target muscles. The Spearman’s Correlation Coefficient, obtained from the resulting scatter plot, was used to quantify the covariation of CM cell and target muscle activity. The majority of cell-target muscle pairs covaried positively during the task (83.5%). The strength of covariation between the activity of CM cell-target muscle pairs was related to 1) the magnitude of PSpF (r = .26, P < .01) and 2) mean CM cell firing rate (r = .22, P < .01). Our data establishes the existence of correlation between CM cell and target muscle activity and for the first time demonstrates that the strength of CM cell-target muscle correlation varies as a function of the strength of the CM synaptic linkage. |
