ABSTRACT
The respiratory muscles of the chest wall may be divided into the diaphragm and the muscles of the rib cage. The contraction of these muscles during quiet breathing follows the pattern of neural input from respiratory motoneurons. In early inspiration, some respiratory motoneurons reach their firing threshold, and then, additional motoneurons are recruited in response to activity from descending inspiratory bulbospinal neurons (lscoe et al., 1976). Respiratory muscle contraction and the resultant muscle shortening then follow an augmenting pattern (Fig. 1). Respiratory muscle shortening moves the chest wall, which subsequently expands the lung. Lung expansion creates a pressure gradient between the upper airway and the lung; hence, air flows into the lung. Although the muscles of the chest wall are involved in many other functions, they are required to contract continually in this manner to support breathing.
