Neural regulation of airway smooth muscle tone.

Airway smooth muscle is innervated by sympathetic and parasympathetic nerves. When activated, airway nerves can markedly constrict bronchi either in vivo or in vitro, or can completely dilate a precontracted airway. The nervous system therefore plays a primary role in regulating airway caliber and its dysfunction is likely to contribute to the pathogenesis of airways diseases. The predominant contractile innervation of airway smooth muscle is parasympathetic and cholinergic in nature, while the primary relaxant innervation of the airways is comprised of noncholinergic (nitric oxide synthase- and vasoactive intestinal peptide-containing) parasympathetic nerves. These parasympathetic nerves are anatomically and physiologically distinct from one another and differentially regulated by reflexes. Sympathetic-adrenergic nerves play little if any role in directly regulating smooth muscle tone in the human airways. Activation of airway afferent nerves (rapidly adapting receptors, C-fibers) can evoke increases in airway smooth muscle parasympathetic nerve activity, or decreases in parasympathetic nerve activity (through activation of slowly adapting receptors). Extrapulmonary afferents can also modulate nerve mediated regulation of airway smooth muscle tone. In guinea pigs and rats, peripheral activation of tachykinin-containing airway afferent nerves evokes bronchospasm via release of substance P and neurokinin A. This effect of airway afferent nerve activation appears to be unique to guinea pigs and rats. The actions and interactions between the components of airway innervation are discussed.