Respiratory Action of the Intercostal Muscles

Andre De Troyer, Peter A. Kirkwood, Theodore A. Wilson

Expansion of the rib cage and abdominal wall are prominent features of the inspiratory phase of the breathing cycle. The expansion of the abdominal wall is produced by the action of the diaphragm. Thus, as the diaphragm is activated, its muscle fibers shorten and its dome (which corresponds essentially to the central tendon) moves in the caudal direction, pushing the abdominal viscera caudally and displacing the abdominal wall outward. When the diaphragm in anesthetized dogs, rabbits, cats, and horses is activated selectively by electrical stimulation of the phrenic nerves, however, the tension it exerts at the points of attachment on the lower ribs and the rise in abdominal pressure commonly cause expansion of the most caudal portion of the rib cage, but the fall in pleural pressure causes contraction of a large fraction of the rib cage (23, 69, 108, 183).

Measurements of thoracoabdominal motion during phrenic nerve pacing in humans with traumatic transection of the upper cervical cord (24, 196) and during spontaneous breathing in subjects with traumatic transection of the lower cervical cord (76, 163, 200) have shown that the human diaphragm acting alone similarly produces an expansion of the caudal portion of the rib cage but an inward displacement of the cranial half of the rib cage. Therefore, the normal expansion of the rib cage during inspiration must be primarily produced by the intercostal muscles, although in humans the scalenes may also be involved (31, 38, 76, 85, 174). However, the intercostal muscles are diverse and widely distributed throughout the rib cage, and until recently, the respiratory functions of the individual muscles have been poorly understood.