Role of the Vagus Nerves in Bronchoconstriction in Humans

Abstract

Studies of animals have shown the parasympathetic nervous system to influence airway caliber and to mediate reflex bronchoconstriction provoked by chemical or mechanical irritation of the respiratory tract. The involvement of this autonomic system in the bronchial hyperreactivity of humans with asthma has been assessed by comparing bronchomotor responsiveness before and after giving agents intended to block parasympathetic efferent activity. The results of these studies suggest that the degree of involvement of parasympathetic mechanisms in bronchial responses is variable among subjects and that other, nonparasympathetic mechanisms must also be involved. The situation appears to be much more complicated than depicted in Figure 1, for studies with distilled water aerosol suggest that simple stimulation of afferent nerves in the respiratory tract need not produce bronchoconstriction. While it is conceivable that different populations of afferent receptors mediate reflex cough and bronchoconstriction and that these populations differ in location or in their sensitivity to lidocaine and cromolyn, it seems likely that the activation of parasympathetic efferent pathways is secondarily provoked by nonspecific stimuli, possibly by the action of chemical mediators on afferent nerves, ganglia, or efferent nerve endings in the airways.