Postural and ventilatory functions of intercostal muscles.

During spontaneous breathing, the interchondral muscles present a pattern of activity similar to that of the diaphragm. The external intercostals and most of the internal intercostals generally show electrical discharges not related to ventilatory rhythm. Studies of the electrical responses of these muscles in experimental variations of their length show that the external and internal intercostals are readily activated by this category of reflexes while the diaphragm and the interchondrals are not. Bilateral multisegmental sections of spinal dorsal roots do not affect the respiratory activity of the diaphragm and of the interchondral muscles; on the contrary, all types of activity - spontaneous or reflex - disappear from the intercostals. Electrical stimulation of appropriate points in the bulbar pyramids in decerebrate cats can activate at the same time different intercostals and leg muscles without modifying the rhythmic inspiratory activity of the diaphragm and the interchondrals. In preparations with chronically implanted electrodes, the intercostals muscles are chiefly involved in posture. These results fit very well with our histological findings which disclose a much greater density of muscle spindles in external intercostals than in the diaphragm or in the interchondral muscles.