title: Representation of individual forelimb muscles in primary motor cortex.
abstract: 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 somatotopic organization. Two monkeys were trained to perform a reach-to-grasp task requiring multijoint 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 present 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. 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.NEW & NOTEWORTHYWhile the orderly representation of major body parts along the precentral gyrus has been known for decades, questions have been raised about the possible existence of additional more detailed aspects of somatotopy. In this study, we have investigated this question with respect to muscles of the arm and show consistent features of within-arm (intra-areal) somatotopic organization. For the first time we also show maps of how inhibitory output from motor cortex is organized.
authors.affiliation: Department of Physical Medicine and Rehabilitation, University of Kansas Medical Center, Kansas City, Kansas.
authors.forename: Heather M
authors.initials: HM
authors.lastname: Hudson
authors.affiliation: Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas; and.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Park
authors.affiliation: Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas; and.
authors.forename: Abderraouf
authors.initials: A
authors.lastname: Belhaj-Saïf
authors.affiliation: Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas; and pcheney@kumc.edu.
authors.forename: Paul D
authors.initials: PD
authors.lastname: Cheney
mesh_terms: D000818:Animals; D001931:Brain Mapping; D005552:Forelimb / Q000294:innervation* / Q000502:physiology; D018737:Hand Strength; D008253:Macaca mulatta; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology
keywords: EMG; ICMS; forelimb; mapping motor cortex