An EMG analysis on regulation of the diaphragm and abdominal muscles

Neural mechanisms regulating activities of diaphragm and abdominal muscles were investigated on anesthetized, tracheostomized and spontaneously breathing dogs. EMGs of costal diaphragm and external oblique (EO)abdominal muscle were recorded by fine-wire electrodes. The electrodes for diaphragm-EMG were implanted without opening abdominal cavity using laparoscopy. During quiet breathing the mean firing frequency of diaphragm-EMG was 7.6 +/- 0.7 (S.D.) Hz. The discharge spikes of diaphragm-EMG observed in early-inspiration or in post-inspiratory phase, were low in amplitudes, consistent with previous reports that phrenic motoneurons were comprised of two populations. During quiet breathing spikes were not observed in EO-EMG in a half of the animals, and were tonically-firing in the remaining dogs. Hypercapnia increased firing frequency of diaphragm-EMG, but did not recruit spikes of high amplitude. On the other hand, hypercapnia recruited EO-spikes of higher amplitude, and increased firing frequency. When spikes fired tonically in EO-EMG, hypercapnia suppressed these spikes during inspiration. Airway occlusion gradually recruited EO units of higher amplitude. Mechanical stimulation of upper airway suppressed tonic-EO activity during inspiration. In conclusion, we demonstrated that 1) abdominal wall is not related with control mechanism of diaphragm; 2) the control mechanism of dog's EO differs from cat's internal intercostal muscles; and 3) the tonic component of EO is affected by respiratory commands from higher neuronal architecture.