Modulation of Sustained Electromyographic Activity by Single Intracortical Microstimuli: Comparison of Two Forelimb Motor Cortical Areas of the Rat

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Abstract
In rats, a rostral and a caudal forelimb motor area (RFA and CFA, respectively) have been distinguished on the basis of intracortical microstimulation effects (see Neafsey et al., 1986, for a review). The goal of the present study was to assess and compare their relative connectional strength with target motor units of the forelimb. This was achieved by averaging modulation responses of sustained electromyographic (EMG) activity triggered by single intracortical microstimuli (S-ICMS) of relatively low intensity (mostly below 35 μA) to minimize both direct and transsynaptic current spread. In chronically prepared and ketamine-sedated rats, this method produced prominent peaks and troughs in the averaged EMG at short latencies with S-ICMS currents as low as 5 μA. S-ICMS at 30–50 μA in CFA sometimes even elicited visible twitches and an EMG burst of the contralateral wrist or digits following each stimulation pulse. Increasing S-ICMS currents to about 1.5 mA revealed a sudden shortening of EMG response latencies, which was most likely induced by current spread to brainstem motor centers. S-ICMS at near-threshold intensity in the majority of effective sites in both CFA and RFA produced modulation responses in more than one group of forelimb muscles, frequently also including muscles of the ipsilateral forelimb. Usually the ipsilateral responses were weaker, as were the suppression effects. Comparison of CFA and RFA revealed similar effects in terms of the number of modulated muscle groups and the response latencies. In contralateral wrist/digit muscles, facilitation responses were elicited at latencies of 9.7 ± 1.8 msec (CFA) and 9.6 ± 1.9 msec (RFA), with the shortest latencies around 6 msec. However, modulations by S-ICMS in RFA had significantly smaller amplitudes, had slower rates of buildup, and required higher thresholds than those obtained from S-ICMS in CFA. It is concluded, on the basis of the S-ICMS method, that both the CFA and the RFA exert a prominent and relatively direct influence on forelimb motoneurons. The present findings, together with calculations of conduction time, suggest that a contingent of corticospinal axons of the rat has oligosynaptic and possibly even monosynaptic connections with forelimb motoneurons. The recruitment of a relatively large number of muscles, including those of the ipsilateral forelimb, by S-ICMS in both areas may be explained by the prominent divergence of corticospinal axons. Further investigations are required to understand the relative positions and roles of the two areas in motor control and their possible homology with primary and nonprimary motor areas of primates.