The electrical activity of intercostal muscles during quiet breathing in rabbits

The electrical activity and the circumstances of contraction of the internal and external intercostal muscles were studied during quiet breathing in non-anesthetized rabbits resting in the sternal semirecumbent position. The activity was recorded via clip electrodes implanted in both muscles in a series of interspaces. The greater part of the expiratory phase was characterized by slight activity in both the internal and external intercostals, preceded and followed by brief periods of inactivity. Inspiration was accompanied by continuous activity in both sets of muscles until the last 20–40 ms of that phase. It was found that both internal and external intercostals were active during inspiration, that their activity was at its greatest during this phase, and that this activity was characterized by a progressive increase in external intercostal activity. There was no evidence of reciprocal innervation of intercostal muscles in this study.