Respiratory muscle activity during asphyxic apnoea and opisthotonus in the rabbit

The behaviour of submandibular, cervical, thoracic and abdominal respiratory muscles was examined in the pentobarbitone-urethane-anaesthetized rabbit during progressive asphyxia induced by rebreathing. During asphyxic hyperpnoea the external intercostal, interchondral and scalene inspiratory activities augmented until succeeded by the apnoeic period, in which all were inhibited with the diaphragm. Likewise, the genioglossus, sternohyoid and thyrohyoid muscles exhibited inspiratory augmentation during asphyxic hyperpnoea until the onset of apnoeic inhibition. However, late in the apnoea these muscles, together with the sternothyroid, sternomastoid and digastric muscles, generated an augmenting tonic discharge associated with an intense abdominal constriction, and with the extension of the limbs characteristic of opisthotonus. This intense tonic activity, which was never expressed by the diaphragm and thoracic inspiratory muscles, was immediately interrupted or terminated by the subsequent inspiratory efforts of gasping respiration, during which the abdominal muscles were inhibited but all the submandibular, cervical and thoracic inspiratory muscles greatly participated. The mylohyoid muscles presented augmenting expiratory activity during asphyxic hyperpnoea which declined during the apnoea. These muscles, however, did not exhibit the intense tonic discharge expressed by the expiratory abdominal and inspiratory submandibular and cervical muscles in late apnoea and were not active in gasping.