The Role of the Motor Cortex in the Control of Axial and Proximal Muscles in Learning

Abstract

The involvement of the motor cortex in controlling the muscles of the shoulder and scapula during formation of a new motor coordination of the head and forelimb was studied in dogs. Dogs were trained to flex the forelimb to operate a lever to raise a bowl containing food and hold it up during feeding with the head tilted towards the feeder. At the early stage of training, raising of the limb occurred with anticipatory upwards displacement of the head and, on lowering the head to the feeder, lowering of the elevated limb; this is the natural coordination of head and limb movements. The new coordination needed to obtain food – maintaining the elevated limb in a posture with the head lowered – could be achieved only as a result of learning and was critically dependent on the integrity of the motor cortex. In the natural coordination, limb elevation consistently involved the main shoulder flexors, i.e., the deltoid and teres major muscles, and inconsistently involved teres minor, infraspinatus, supraspinatus, and trapezius. In this latter group, muscles often operated in antiphase to the main shoulder flexors, i.e., were active on standing and stopped being active before limb elevation. Learned limb elevation in the posture with the lowered head involved all the muscles listed, some rearranging their initial activity to the opposite. Lesioning of the greater part of the forelimb representation in the motor cortex in trained dogs led to recovery of the natural coordination of head and limb movements and the initial muscular pattern during limb elevation. Thus, it was only with involvement of the motor cortex that the initial pattern of the activity of the phylogenetically ancient axial and proximal musculature underwent rearrangement and started to operate in a new way.