Characteristic behavior of the respiratory muscles, esophagus, and external anal and urethral sphincters during straining, retching, and vomiting in the decerebrate dog.

To ascertain differences in the brainstem pattern generators for straining and retching, discharges of 230 respiratory single motor units from the intercostal nerves and discharges of the nerves to accessory respiratory muscles, the esophagus, and external anal and urethral sphincters were observed during straining, retching, and vomiting (fictive expulsion) in decerebrate and paralyzed dogs. Straining and retching were identified with coactivation of the phrenic nerve and the nerve to the rectus abdominis, which was elicited by distension of the rectum and stomach, respectively. Synchronous discharges with each retch and straining were always exhibited by the phrenic nerve, the nerves innervating the abdominal part of the rectus abdominis and the external anal and urethral sphincters. In contrast, different behaviours were constantly observed in four nerves. 1) The nerves to the serratus dorsalis cranialis was inhibited during straining, but activated synchronously with each retch and vomiting. 2) Vagal pharyngeal branches innervating the upper esophagus and branches of the recurrent nerve innervating the lower part of the cervical esophagus fired synchronously with straining. The esophageal nerves, on the other hand, did not fire with each retch, but did fire strongly between retches. 3) The nerve to the sacrocaudalis dorsalis lateralis discharged concomitantly with straining but did not with retches. Straining and retching behaviors exhibited by the nerves innervating other thoracic accessory inspiratory muscles (the intercartilagineus, rectus thoracis, scalenus) varied from dog to dog. The nerves innervating the thoracic parts of the rectus abdominis and obliquus externus abdominis always discharged synchronously with straining, and discharged with retches in the majority of dogs, but did not in a minority of dogs. The expiratory units from the internal intercostal nerves showed intense discharges synchronously with coactivation in straining (96%), retching (65%), and vomiting (100%). In contrast, none of the inspiratory units from the external intercostal nerves exhibited such intense discharges with straining and retching. These results suggest that the functional difference in straining and retching mainly depends on the differences in the behavior of the serratus dorsalis cranialis and esophagus. The forms of this dependence were discussed.