Smooth Muscle Contraction Affects the Diameters of the Trachea, Bronchi, and Bronchioles
There is smooth muscle in the walls of the airways from the trachea to the alveolar ducts. In the trachea, it forms the trachealis muscle, which connects the ends of tracheal car-
FIGURE 45-9 Effect of change in lung volume on airway resistance.The airway is represented in the diagrams on the right side of the figure by the large "circle? to which are attached alveoli, the septa of which link the airway wall to the pleural surface.
As lung volume increases, the alveolar septa become stretched and thus dilate the airway and reduce resistance. FRCf Functional residual capacity; RVf residual volume; TLCf total lung capacity.
FIGURE 45-10 Diagrammatic representation of efferent autonomic innervation of tracheobronchial tree. Smooth muscle β2-adrenergic receptors (β) are activated by circulating catecholamines such as epinephrine (EPI) or, in a few species, by release of norepinephrine (NE) from sympathetic nerves. Muscarinic receptors (M) are activated by acetylcholine (Ach) released from postganglionic parasympathetic nerve terminals.The nonadrenergic inhibitory nervous system that travels in the vagus nerve releases nitric oxide (NO) that activates guanyl cyclase (g-c) in the smooth muscle. Nf Neuron. (From Nadel JA, Barnes PJ, Holtzman MJ: Autonomic factors in hyperreactivity of airway smooth muscle. In Fishman AP, Macklem PT, Mead J, et al, editors: Handbook of physiology, section 3, vol 3, part 2, Bethesda, Md, 1986, American Physiology Society.)
tilages. In the bronchi and the bronchioles» smooth muscle encircles the airways. Smooth muscle actively regulates airway diameter in response to neural and other stimuli.
The parasympathetic nervous system supplies airway smooth muscle through the vagus nerve (Figure 45-10). Activation of this system causes the release of acetylcholine, which binds to muscarinic receptors on airway smooth muscle, leading to muscle contraction. I his narrows the trachea, bronchi, and bronchioles, a phenomenon known as bronchoconstriction. When irritating materials such as dusts are inhaled, tracheobronchial irritant receptors are stimulated, which in turn activates the parasympathetic system, resulting in reflexbronchoconstriction.Airway smooth muscle also contracts in response to many of the inflammatory mediators, particularly histamine and the leukotrienes, that are released from mast cells during an allergic reaction. Some inflammatory mediators act directly on the smooth muscle; others act in a reflexive manner through the parasympathetic system. They are responsible for the airway obstruction that occurs in diseases such as heaves in horses and asthma in cats.
Relaxation of smooth muscle, and therefore dilation of the airways, occurs during activation of β2-adrenergic receptors by circulating catecholamines released from the adrenal medulla. Norepinephrine released from the sympathetic nervous system also causes airway dilation through β2-adrenergic receptors, but to a lesser extent. Another bronchodilator system, the nonadrenergic noncholinergic inhibitory nervous system, exists in some species. The efferent fibers are in the vagus nerve, and neurotransmission involves nitric oxide.