Inspiratory muscle activity during rebreathing in intact and vagotomized rabbits

The relation between relative peak 'integrated' diaphragmatic (Adi) and cranial (u) or caudal (1) inspiratory intercostal activity (Aic) was assessed in intact and vagotomized rabbits during rebreathing. During unloaded rebreathing Aic increased markedly more than Adi, and Aicu more than Aicl, independently of intact vagi. Vagal signals facilitated inspiratory intercostals, since vagotomy or vagal blocks increased Adi more than Aic without changing diaphragm activity time course, and delta Aic/delta Adi in the same range of Adi values was greater before than after vagotomy. Reflexes from the chest wall were not involved since phrenic block after vagotomy modified rib cage motion without changing Aic, independently of chemical drive. Possibly chemical and vagal dependent changes in fusimotor activity contribution to VT, as the latter increases. Obstruction at FRC shifted postvagotomy Aic vs. Adi relationship upwards, suggesting reflex facilitation of intercostals by chest wall receptors. The strength of this reflex was largely independent of intact vagi, since pre- and postvagotomy Aic vs. Adi relations coincided, and of chemical drive, since postvagotomy delta Aic/delta Adi for obstructed and unimpeded rebreathing were similar.