ABSTRACT
RESPIRATORY MUSCLES AND NERVES Muscles of the Ventilatory Pump The diaphragm is the primary muscle of inspiration. The diaphragm functions well only when active in harmony with other respiratory muscles and when the mechanical charac teristics of the respiratory system are well matched to the capacity of the diaphragm (and other muscles of respiration) to perform work. The parasternal, scalene, and external inter costal muscles aid the diaphragm during inspiration at rest. During exercise, the pattern of intercostal muscle activation changes to enhance inspiratory forces; additional accessory muscles of the upper chest and neck, such as the stemocleidomastoids, may become active during inspiration. The diaphragm is innervated by the phrenic nerves, which originate from cervical nerves C3-C5, travel over the anterior scalene muscles, and dive into the thoracic cavity adjacent to the subclavian arteries and veins. Within the thoracic cavity, the nerves run along the lateral aspects of the pericardium until they ramify over the surface of the diaphragm. The intercostal muscles are innervated segmentally by intercos tal nerves. The accessory muscles are innervated by a variety of nerves originating from the eleventh cranial nerve or cervical or intercostal nerves.
