Unity of costal and crural diaphragmatic activity in respiration.

In chronically implanted rats, we examined the respiratory EMG activity of the two parts of the diaphragm, costal and crural, during sleep and wakefulness. Their activity was compared and contrasted with that of the EMG activity of the cricothyroid muscle. Whether in wakefulness, while grooming and drinking, or in nonrapid eye movement (non-REM) sleep, and independent of the gas mixture breathed (4 to 5% CO2 or 10% O2 in nitrogen), the two parts of the diaphragm paused during REM apnea episodes whereas the cricothyroid muscle ceased its activity or exhibited sustained activity. We conclude that the diaphragm, mainly an inspiratory muscle, acts as a single functional unit when under the respiratory control system. The cricothyroid muscle functions as an inspiratory and/or expiratory muscle, also under the respiratory control systems. Both muscles in the rat come under other neural control mechanisms governing nonrespiratory functions, e.g., swallowing, defecation, and coughing, but not vomiting.