Motor Organization in the Frog’s Spinal Cord

Abstract

We activated the spinal cord by microstimulation and recorded the forces elicited at the ankle. By moving the leg throughtout the workspace and repeating this measurement we obtained an estimate of a time-varying force resulting from the stimulation. In lateral neuropil regions, stimulation elicited force fields that consistently convereged to single equilibria. Stimulation of lateral neuropil also consistently elicited a balanced activation of muscles. Equilibria were found in flexion or extension of the leg depending on the site in the lateral area of the lumbar cord that was stimulated. In contrast, force fields elicited from stimulation of motor muclei were often divergent or parallel. While afference seems to play a role in the structuring of the force fields, most such fields could be accounted for largely by coactivation of muscles. We found partial deafferentation did not abolish the convergence of the force fields. Using a simple model, we explored the hypothesis that motoneuron spatial frequency in the cord is alone sufficient to explain the results of microstimulation. We concluded that other mechanisms must also be invoked to explain the experimental results.