EXCITATION AND INHIBITION OF PHRENIC MOTONEURONES BY INFLATION OF THE LUNGS

Abstract

The influence of changes in lung vol. on the discharge of impulses by single phrenic motorneurones was investigated in cats anesthetized with Dial or decerebrated under ether-Inflation of the lungs inhibits the phrenic discharge for a length of time that increases with the vol. of inflation. Large inflations, in addition to this well-known inhibition of inspiration, have an excitant action which is revealed by a brief burst of motor impulses as the lungs expand. This excitant action is of short duration and has a threshold which is relatively independent of the rate of distention. The inspiration-exciting reflex is independent of vascular pressure receptors since it is not reduced during occlusion of the pulmonary artery. Both the inhibition and excitation of inspiration by lung inflation are eliminated by cutting both vagus nerves. It is suggested that these reflexes are due to impulses from 2 distinct sets of pulmonary receptors of different thresholds and different rates of adaptation. Increased activity of phrenic motorneurones is caused by artificial distention of the lungs only when the lungs are inflated to a vol. which exceeds that reached in eupneic respiration. Following a suddenly induced pneumothorax the next inspiration is very much prolonged, but the frequency of discharge follows exactly the same time course as in normal inspiration up to the time when the normal discharge is abruptly curtailed. Inspiration is thus uninfluenced by impulses from pulmonary stretch receptors until its termination is suddenly brought about by the inspiration-inhibiting reflex. The inspiration-exciting reflex initiated by superinflation of the lungs is inactive in normal eupneic breathing, but may serve to increase the depth of any unusually deep inspiration.