Control of airway caliber.

Tracheobronchial smooth muscle tone may be affected by 4 nervous mechanisms: (1) Vagal cholinergic parasympathetic nerves, which are the main agents for resting tone and most reflex bronchoconstrictions. Their activity is blocked by atropinic drugs. (2) Sympathetic adrenergic dilator nerves, which may act mainly on beta-adrenoceptors in the pulmonary bronchi; alternatively, they may inhibit ganglionic transmission in the vagal constrictor pathway. (3) Vagal nonadrenergic dilator nerves (NAIS). The neurotransmitter at these nerves is probably vasoactive intestinal polypeptide (VIP), although purines could be involved. The role of this system in physiologic and pathologic conditions has not been established. (4) Local axon constrictor reflexes in afferent nerves. These respond to mucosal irritation and cause local smooth muscle contraction by release of substance P. Their importance has not yet been assessed. The motor innervation of the airways is activated reflexly by many stimuli, some of which cause constriction and others dilation. Most of the reflexes are blocked by atropine, which suggests that the cholinergic constrictor pathway is dominant. Other responses include changes in laryngeal caliber and secretion of mucus. Aspirations into the airways will lead to bronchoconstriction, laryngospasm, and secretion of mucus, as well as to respiratory and cardiovascular reflexes. The balance, effectiveness, and development of these responses requires much further study.