Certain efferent cortical fiber pathways of the Mongolian gerbil (Meriones unguiculatus)

Three areas of responsive cortex were demonstrated by electrical stimulation in the gerbil. Low voltage stimulation of frontal areas yielded a sequential pattern of discrete contralateral movements. Proceeding from a rostroventral to a caudodorsal position facial movements were obtained followed by upper extremity, trunk and lower extremity movements. This area includes primary motor cortex (area 4) in the gerbil. A comparable rostrocaudal motor pattern was obtained by stimulation above the rhinal fissure (insular cortex) and also from certain parietal areas. Although the sequential motor pattern was obvious in these additional areas of excitable cortex, the movements were more generalized and slightly higher voltages were required to obtain satisfactory results. A series of destructive lesions were made in each of these cortial areas. Subsequent degeneration studies, using reduced silver techniques, revealed that frontal and parietal cortex gave rise to corticospinal fibers. In addition all three areas gave rise to fibers which coursed to certain extrapyramidal nuclei of the basal ganglia, ventral thalamus, midbrain tegmentum, and medullary reticular formation.