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. While 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.

Postural Representations of the Hand in the Primate Sensorimotor Cortex

“…Because we cannot accurately reconstruct forces and muscle activations from kinematics, we cannot directly test this hypothesis. However, the activity of single neurons in the motor cortex has been shown to drive facilitation and suppression of several muscles (Buys et al, 1986;Griffin et al, 2015;Hudson et al, 2017), consistent with our finding that M1 RFs are large. How forces are encoded in the proprioceptive SCx and how these force signals interact with kinematic ones is largely unknown.…”

Terminal organization of the corticospinal projection from the lateral premotor cortex to the cervical enlargement (C5–T1) in rhesus monkey

“…This would indicate LPMCd CSP proliferation may occur in spinal subsectors mediating proximal limb recovery (e.g., bilaterally in medial lamina VII and lamina VIII). In this regard, a spared LPMCd CSP appears to be in a favorable position to contribute to recovery of postural stabilization and reaching, particularly following compromise of the shoulder/trunk representation of M1 which lies caudal to LPMCd, and directly dorsal to the M1 arm/hand area (Figure a, see area outlined by the dashed red line) (Boudrias et al, ; Darling et al, , ; Hudson, Park, Belhaj‐Saif, & Cheney, ; Kwan, Mackay, Murphy, & Wong, ,b; Weinrich & Wise, ).…”

Recent advances in our understanding of the primate corticospinal system

“…Nevertheless, careful use of single-pulse ICMS by Cheney et al . has been important in determining the complex maps of muscle outputs in primary 27 , 28 and secondary 29 , 30 motor areas. These studies have helped to demonstrate the direct, short-latency influence of M1 over forelimb muscles, compared with slower effects evoked from secondary motor areas.…”