Different Contributions of Primary Motor Cortex, Reticular Formation and Spinal Cord to Fractionated Muscle Activation

Coordinated movement requires patterned activation of muscles. In this study we examined differences in selective activation of primate upper limb muscles by cortical and sub-cortical regions. Five macaque monkeys were trained to perform a reach and grasp task, and electromyogram (EMG) was recorded from 10-24 muscles while weak single-pulse stimuli were delivered through microelectrodes inserted in the motor cortex (M1), reticular formation (RF) or cervical spinal cord (SC). Stimulus intensity was adjusted to a level just above threshold. Stimulus-evoked effects were assessed from averages of rectified EMG. M1, RF and SC activated 1.5±0.9, 1.9±0.8 and 2.5±1.6 muscles per site (mean±SD); only M1 and SC differed significantly. In between recording sessions, natural muscle activity in the home cage was recorded using a miniature data logger. A novel analysis assessed how well natural activity could be reconstructed by stimulus-evoked responses. This provided two measures: normalized vector length L, reflecting how closely aligned were natural and stimulus-evoked activity, and normalized residual R, measuring the fraction of natural activity not reachable using stimulus-evoked patterns. Average values for M1, RF and SC were L=119.1±9.6, 105.9±6.2 and 109.3±8.4 and R=50.3±4.9, 56.4±3.5 and 51.5±4.8 respectively. RF was significantly different from M1 and SC on both measures. RF is thus able to generate an approximation to the motor output with less activation than required by M1 and SC, but M1 and SC are more precise in reaching the exact activation pattern required. Cortical, brainstem and spinal centers likely play distinct roles as they cooperate to generate voluntary movements.

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