Stimulus-triggered averaging (StTA) of forelimb muscle electromyographic (EMG) activity was used to investigate individual forelimb muscle representation within the primary motor cortex (M1) of rhesus macaques with the objective of determining the extent of intra-areal somatopic organization. Two monkeys were trained to perform a reach-to-grasp task requiring multi-joint coordination of the forelimb. EMG activity was simultaneously recorded from 24 forelimb muscles including 5 shoulder, 7 elbow, 5 wrist, 5 digit, and 2 intrinsic hand muscles. Microstimulation (15 µA at 15 Hz) was delivered throughout the movement task and individual stimuli were used as triggers for generating StTAs of EMG activity. StTAs were used to map the cortical representations of individual forelimb muscles. As reported previously (Park et al., 2001), cortical maps revealed a central core of distal muscle (wrist, digit and intrinsic hand) representation surrounded by a "horseshoe" shaped proximal (shoulder and elbow) muscle representation. In the current study, we found that shoulder and elbow flexor muscles were predominantly represented in the lateral branch of the "horseshoe" whereas extensors were predominantly represented in the medial branch. Distal muscles were represented within the core distal forelimb representation and showed extensive overlap. Center of gravity maps for individual muscle representations confirmed the preferential localization of proximal muscle flexors laterally and extensors medially in the forelimb representation. For the first time, we also show maps of inhibitory output from motor cortex, which follow many of the same organizational features as the maps of excitatory output.
- motor cortex
- Copyright © 2015, Journal of Neurophysiology