BILATERAL MOTOR OUTPUTS FROM THE RETICULOSPINAL SYSTEM TO THE UPPER LIMBS DURING REACHING IN THE MONKEY.

Abstract

Purpose/Hypothesis: This study describes the motor outputs and neural activity of the reticulospinal system in the monkey during reaching. The hypotheses are 1) that single sites in the reticular formation will produce bilateral motor outputs to the upper limbs and 2) that neural activity in the reticulospinal system will code for the preparation as well as the execution of upper limb movement during voluntary reaching. Number of Subjects: 2 young adult male Macaca fascicularis monkeys were the subjects. Materials/Methods: Subjects were trained to reach with the R or L arm to a target on the R or L side, depending on instructions given during a waiting period before each trial. An electrode positioning system was then surgically implanted, allowing daily access to the brainstem with tungsten microelectrodes in the awake, behaving animal. Single-pulse microstimula-tion at 10 Hz was applied to areas throughout the reticulospinal system in the brainstem as EMG was recorded throughout the shoulder girdle and upper limbs (24 muscles). Extracellular action potentials were also recorded from cells in this region. Neural activity patterns in relation to preparatory and movement-related periods of the task were analyzed as were averaged muscle responses to microstimulation. Results: Microstimulation of the reticulospinal system tended to inhibit extensors and facilitate flexors ipsilateral to the stimulus and produce the opposite effects contralaterally. Effects were strongest and most frequently elicited in the extensors and in the proximal muscles, especially the upper trapezius. Bilateral effects were the norm, though some sites did produce strictly ipsi or contralateral effects in the muscles studied. About 25% of the 196 neurons analyzed had activity related to preparation, 25% had activity related to preparation and to movement, and 50% had only movement-related activity. About 30% of the neurons had activity that was related to the arm used for reaching, and 30% (non-exclusive) had activity related to the target contacted. Only about 10% had activity that was unique to a certain hand-target combination. Conclusions: Reticulospinal neurons have motor output effects and neural activity patterns during movement that demonstrate a role in the control of voluntary reaching. Effects were observed in the shoulder girdle, shoulder, elbow, and even at the wrist. Characterization of the reticulospinal system as strictly for postural control or for control of locomotion may be oversimplified. This major descending system also contributes to the execution of discrete, skilled reaching movements. Clinical Relevance: The motor patterns emanating from the reticulospinal system are reminiscent of the limb movements associated with the ATNR and with synergies associated with recovery from stroke. The present findings lend strength to the long-held concept that the reticulospinal system may be an alternative pathway for voluntary motor control after stroke.