Bronchomotor tone is the net product of moment-to-moment interactions between the autonomic nervous system, circulating humoral influences, and endogenous production of locally secreted mediators. Some degree of airway smooth muscle contraction exists in all individuals. In normal persons, it appears to result largely from parasympathetic innervation to the airway. Endogenous bronchomotor tone theoretically is antagonized physiologically by inhibitory parasympathetic reflexes and by circulating epinephrine. Direct innervation to human airways by sympathetic nerves is insignificant. The homeostatic role of sympathetic secretion in regulating broncho-motor tone, however, remains to be defined. Recent investigations indicate that endogenous secretion of catecholamine does not increase during bronchoconstriction, unless it is associated with severe hypoxemia or hypotension.
Nonadrenergic inhibitory innervation (NAI) is capable of antagonizing exogenously induced changes in bronchomotor tone. However, the NAI system also is not activated by changes in airway caliber, and the homeostatic significance of this system for airway innervation also remains undefined. In asthmatic individuals, bronchomotor tone is increased substantially. Because parasympatholytic agents largely are ineffective in treating asthma, it appears that this increase is not related substantially to increased parasympathetic activity. Scores of other contractile influences have been suggested to account for the increase in bronchomotor tone in asthmatic individuals. The role of beta-adrenergic “deficiency” seems questionable, since normal individuals show no increase in airway reactivity even under conditions of nearly complete beta-adrenoceptor blockade. natural breast enhancement
Recent studies have elucidated mechanisms for modulation of bronchomotor tone by the overlying airway epithelium and the underlying serosa from which neurohumoral and fixed and circulating blood elements are transported. The epithelium appears to secrete mediator(s) that cause tonic inhibition of bronchomotor tone. Removal of the epithelial layer augments airway contractile responses in dogs to acetylcholine, histamine, and serotonin in vitro. However, these experiments have been performed under conditions greatly removed from the physiologic state, and there is a need to establish the inhibitory role of bronchial epithelium in situ.
Tags: airway smooth, bronchoconstriction, hyperreactivity