The organization of intrinsic connections in rat motor cortex was studied by combining microstimulation and tract-tracing techniques. Maps of forelimb and vibrissal movements were constructed from the distribution of cortical sites from which movements were evoked in response to intracortical microstimulation. Then, a single injection of a fluorescent dextran was placed into either a vibrissal or a wrist representation zone, or into a region bordering these zones, resulting in anterograde labeling of long intrinsic, horizontal axons. Following injection into the vibrissal area, axons were largely restricted to the whisker representation zone and to the border region with the forelimb representation. Injections into a wrist zone labeled projections largely restricted to the forelimb area and to the border with the vibrissal area. Injections into a border region labeled dense projections throughout most of the forelimb and vibrissal areas. These findings indicate that intrinsic axon collaterals in the motor cortex form specific and extensive connections among representation zones related to movements of the same body part. These connections may be involved in the coordination of activity in different representation zones for the execution of complex movement patterns. The projection of axon collaterals into border regions may be the anatomical substrate for the rapid reorganization of motor cortical maps that occurs following various experimental manipulations.

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