The motor cortex of the rat: cytoarchitecture and microstimulation mapping.

The first motor (MI) cortex of the rat was identified as the region from which movements could be evoked by the lowest intensity of electrical stimulation. The location of this region was correlated with cytoarchitecture in the frontal and parietal cortex. Two frontal areas can be discerned in Nissl-stained sections: (1) the medial agranular field, marked by a pale-staining layer III and a compact layer II, and (2) the lateral agranular field, which has more homogeneous superficial layers and a broad layer V containing large, densely staining cells. Both of these regions project to the spinal cord and can therefore be included in the somatic sensorimotor cortex. MI in the rat coincides with the lateral agranular field but also overlaps with part of the adjacent granular cortex of the first somatic sensory (SI) representation. We conclude that the rat MI cortex can be identified by microstimulation techniques and by cytoarchitecture in the rat.