Contribution of the rib cage inspiratory muscles to breathing in baboons

We have measured the electromyograms of the rib cage inspiratory muscles, including the neck muscles, in five lightly anesthetized baboons breathing at rest in the supine and head-up postures. When supine, the animals did not have any activity in the scalene (three heads) or sternomastoid muscles. In contrast, a phasic inspiratory electrical activity was invariably recorded from the parasternal intercostals, external intercostals, and levator costae. Measurements of the changes in length of the parasternal intercostals indicated that these muslces also shortened during inspiration, and they further showed that this inspiratory shortening was eliminated after selective muscle denervation. Similar observations were made in the head-up posture, although the inspiratory shortening of the parasternal intercostals was smaller in this posture. These observations thus indicate that: (1) the inspiratory expansion of the rib cage in baboons results entirely from the actions of the inspiratory intercostals muscles and mostly from the action of the parasternal intercostals: and (2) the load imposed on these muscles is greater in the head-up posture, presumably because of the action of gravity on the chest wall.