Immediate and Delayed Changes of Rat Motor Cortical Output Representation with New Forelimb Configurations

These experiments examined motor cortical representation patterns after forelimb postural adjustments in rats. The experiments tested the hypothesis that postural adjustments that stretch muscles that are most strongly activated from the primary motor cortex (MI) enlarge their cortical representation. Intracortical electrical stimulation within MI, forelimb muscle activity and movements, and vibrissa movements were used to evaluate the border between the MI forelimb and vibrissa representations before and after forelimb position changes in anesthetized adult rats. The forelimb was originally maintained in retraction (wrist extension and elbow flexion) and then changed to protraction (wrist flexion and elbow extension). Movements and forelimb EMG evoked by electrical stimulation were evaluated during this period (up to 3 hr) through a set of four electrodes implanted in layer V of MI. Changing the forelimb configuration had both immediate and delayed effects on forelimb muscle activity evoked from MI. At some sites, the magnitude of evoked forelimb muscle activity immediately increased with forelimb protraction. At one-quarter of all sites, forelimb muscle activity was evoked where it was not previously detected following an average delay of 22-31 min after forelimb protraction. This change can be interpreted as an expansion of the forelimb area into the vibrissa representation. These data further support the hypothesis that motor cortical representations are flexible and show that sustained changes in somatic sensory input to MI are sufficient to reorganize MI output.