INTERACTION BETWEEN POSTURAL AND RESPIRATORY CONTROL OF HUMAN INTERCOSTAL MUSCLES

To study the interaction between postural and respiratory control of i ntercostal muscles, we used electromyography of intercostal muscles of the lateral chest wall in conscious humans. Bipolar fine-wire electro des were placed in external and internal intercostal muscles in the mi daxillary line of four subjects who sat on a bench and breathed throug h a pneumotachograph. They were instructed to hold their breath at end expiration, rotate their thorax to the right or left, and then hold t he rotation while resuming breathing. Holding a rotation induces stead y tonic activity in either internal or external intercostal muscles, d epending on the direction of the rotation. The degree of rotation was varied from one run to the next, resulting in varied levels of tonic p ostural activity. When breathing resumes, internal intercostal muscles have their activity almost completely suppressed with each inspiratio n independently of whether the tonic postural tone is small or large. External intercostal muscles show inspiratory increases in activity su perimposed on the postural tone, which apparently amplifies the effect of respiratory input to their motoneurons.