Spatial distribution of external and internal intercostal activity in dogs

1, The observation that the external and internal interosseous intercostal muscles in the dog show marl;ed regional differences in mechanical advantag e has prompted us to re-examine the topographic distribution of electrical activity among these muscles during spontaneous breathing. 2. Inspiratory activity was recorded only from the areas of the external in tercostals with an inspiratory mechanical advantage, and expiratory activit y was recorded only from the areas of the internal intercostals with an exp iratory mechanical advantage, The expiratory discharges previously recorded from the caudal external intercostals and the inspiratory discharges recor ded from the rostral internal intercostals were probably due to cross-conta mination. 3. Activity in each muscle area was also quantified relative to the activit y measured during tetanic, supramaximal nerve stimulation (maximal activity ). External intercostal inspiratory activity was consistently greater in th e areas with a greater inspiratory advantage (i.e. the dorsal aspect of the rostral segments) than in the areas with a smaller inspiratory advantage, and internal intercostal expiratory activity was invariably greatest in the areas with the greatest expiratory advantage (i.e. the dorsal aspect of th e caudal segments). 4. This topographic distribution of neural drive confers to the external in tercostal muscles an inspiratory action on the lung during breathing and to the internal interosseous intercostals an expiratory action.