Lung inflation: effects on heart rate, respiration, and vagal afferent activity in seals

In the anesthetized harbor seal, Phoca vitulina, the Hering-Breuer inflation reflex was weak and comparable to that in humans. Single inflations of the lungs from a syringe during the expiratory phase of normal breathing caused temporary inhibition of breathing and an immediate tachycardia dependent on the integrity of the cervical vagosympathetic nerves. A similar cardiac response occurred when the lungs were artificially inflated during an experimental dive and under conditions in which apnea and bradycardia were reflexly induced by a combination of stimulation of the carotid body chemoreceptors and of the trigeminal or laryngeal input. Recordings from single vagal afferent nerve fibers innervating presumptive pulmonary stretch receptors showed a close relationship between the increase in impulse frequency and increase in lung volume or transpulmonary pressure. It appears that in diving the decrease in pulmonary stretch receptor activity during apnea, combined with cessation of central inspiratory neuronal drive, is an important integrative mechanism that helps development of the reflex bradycardia of trigeminal, carotid, chemoreceptor, and baroreceptor origin.