Functional organization of cortical motor areas in adult dogs and puppies.

In adult dogs and puppies ranging in age from 1 day up to 3 months the functional organization of the motor cortex has been investigated with 50 or 60 cycle a-c stimulation under Nembutal anesthesia. In adult dogs two separate, somatotopically organized motor areas have been differentiated (i) the "precentral" motor area (MI) which occupies mainly the rostral half of the gyrus sigmoideus posterior, and (ii) the supplementary motor area (MII) located in the lateral two-thirds of the gyrus sigmoideus anterior. In addition, stimulation of the somatosensory area I (SI) in the caudal half of the gyrus sigmoideus posterior elicited somatic movements. Stimulation of MI produced low threshold, relatively simple movements of the contralateral extremities. Movements obtained from SI were similar, but had much higher thresholds. Stimulation of MII yielded high threshold, complex and often bilateral movements. In puppies up to 3 weeks of age cortical stimulation elicited bilateral movements which showed no definite projection pattern. From the 4th week of life a somatotopic organization and contralateral representation of the body began to appear and then gradually to evolve in the direction of the adult pattern. At the end of the 3rd month movements in puppies were essentially similar to those of adult dogs. However some minor differences were still present, suggesting that the process of maturation of cortical motor areas were not yet completed at this age.