Combined corticospinal and reticulospinal effects on upper limb muscles

• The motor cortex and reticulospinal system were stimulated conjointly, or one at a time, in the awake behaving monkey.
• EMG activity was recorded from proximal and distal muscles, flexors and extensors, in both upper limbs.
• Patterns of coordinated motor output from these two major parts of the motor system were analyzed.
• Simple neural circuits explained the most commonly observed patterns of responses.

Both the reticulospinal and corticospinal systems are known to control recruitment of upper limb muscles, yet no known studies have attempted to assess their combined effects in the same experiment in the awake, behaving primate. The purpose of this study is to present an approach for the analysis of the cooperative control from these two motor systems. Muscle responses to electrical stimulation in the reticulospinal system and corticospinal system alone or in combination were studied. The responses were categorized based on simple neural circuits that could explain the interactions of these systems. Five such circuits were identified that could explain 86% of the observed patterns of combined recruitment during stimulation. Improved understanding of the cooperation between these motor systems could provide insight for development of better rehabilitation approaches for stroke patients and others with movement disorders.