Neural regulation of airway smooth muscle tone

Airway smooth muscle is innervated by sympathetic and parasympathetic nerve s. When activated, airway nerves can markedly constrict bronchi either in v ivo or in vitro, or can completely dilate a precontracted airway. The nervo us system therefore plays a primary role in regulating airway caliber and i ts dysfunction is likely to contribute to the pathogenesis of airways disea ses. The predominant contractile innervation of airway smooth muscle is par asympathetic and cholinergic in nature, while the primary relaxant innervat ion of the airways is comprised of noncholinergic (nitric oxide synthase- a nd vasoactive intestinal peptide-containing) parasympathetic nerves. These parasympathetic nerves are anatomically and physiologically distinct from o ne another and differentially regulated by reflexes. Sympathetic-adrenergic nerves play little if any role in directly regulating smooth muscle tone i n the human airways. Activation of airway afferent nerves (rapidly adapting receptors, C-fibers) can evoke increases in airway smooth muscle parasympa thetic nerve activity, or decreases in parasympathetic nerve activity (thro ugh activation of slowly adapting receptors). Extrapulmonary afferents can also modulate nerve mediated regulation of airway smooth muscle tone. In gu inea pigs and rats, peripheral activation of tachykinin-containing airway a fferent nerves evokes bronchospasm via release of substance P and neurokini n A. This effect of airway afferent nerve activation appears to be unique t o guinea pigs and rats. The actions and interactions between the components of airway innervation are discussed. (C) 2001 Elsevier Science B.V. All ri ghts reserved.