[Activity of the respiratory muscles during natural defecation: a study on experimental animals]

Activity of the respiratory muscles during natural defecation was studied in two anesthetized and two decerebrate dogs. In anesthetized dogs, excitation of the abdominal muscles and an increase in gastric pressure were observed during defecation. However, pleural pressure was little influenced by such increase in abdominal pressure, maintaining the same rhythmic changes as observed during spontaneous respiration. The rhythmic changes in pleural pressure were associated with rhythmic activity of the diaphragm. When gastric pressure increased during defecation, the diaphragmatic activity also increased during both the inspiratory and expiratory phases. In a decerebrate dog, airflow and airway pressure changed similarly to during defecation. The diaphragm was continuously active, with superimposed rhythmic augmentation. In a paralyzed and artificially ventilated dog with open-chest, the phrenic nerve similarly developed discharges. We conclude that the non-respiratory activity and rhythmic augmentation of phrenic nerve discharge during defecation is pre-programmed in the command for defecation. The activity of phrenic motoneurons may be further modulated by changes in thoracic and abdominal pressure. These mechanisms may act together to coordinate respiration and defecation.