Action potentials in parasympathetic and sympathetic efferent fibres to the trachea and lungs of dogs and cats

Abstract

Action potentials were recorded from 74 single and 29 small multifiber nerve strands efferent to the trachea and lungs of cats and dogs. From the pathway (vagal or sympathetic), spontaneous activity, conduction velocity and responses to various interventions the efferent fibers were classified in the following way. Group I, vagal. These had a mean conduction velocity of 9.7 m/sec, and had a respiratory but seldom a cardiac rhythm. Their discharge was inhibited during hypertension caused by injections of adrenaline and during inflation of the lungs, but was increased during tracheal occlusion, stimulation of peripheral chemoreceptors and irritation of the larynx. The fibers are thought to be constrictor to the airways. Group n, sympathetic. These had a mean conduction velocity of 0.85 m/sec and usually had inspiratory and cardiac rhythms. Their discharge usually responded qualitatively as that of group I fibers to the various interventions, but with clear quantitative differences. They are divided into 3 subgroups on the basis of their responses to injections of adrenaline and to asphyxial stimuli. Group HI, vagal and sympathetic These had a mean conduction velocity of 9.0 m/sec, very slow discharge rates and often an expiratory and cardiac modulation. They were activated during hypertension due to adrenaline and often by tracheal occlusion, chemoreceptor stimulation, laryngeal irritation and lung inflation. Their motor action is unknown. Group IV, vagal and sympathetic. These had conspicuous cardiac rhythms resembling those of vascular baroreceptors, but their discharge could not be correlated with arterial blood pressure. Their mean conduction velocity was 6.6 m/sec. Some were active after combined vagotomy and sympathectomy. Together with some unclassified fibers, those of group IV are thought to be aberrant afferent nerves or collateral afferent branches, and possibly to subserve local reflexes. The results are discussed in relation to nervous control of effector tissues in the airways and autonomic nervous control generally.