Nerves

The airways are innervated by afferent and efferent nerves that respectively serve sensory and effector functions in the central nervous system regulation of airway function (Table 2, Fig. 3) [20]. Slowly adapting receptors (or pulmonary stretch receptors) are located in the smooth muscle of the central airways (trachea to larger bronchi), respond to airway stretch, and are thought to be involved in the reflex control of ventilatory drive. Rapidly adapting receptors (or irritant receptors) ramify within the epithelium of the central airways and are sensitive to chemical or irritant stimuli (e.g., inflammatory mediators), mechanical stimuli, and interstitial edema. Activation of these receptors results in an increase in the rate or depth of breathing and in bronchoconstriction mediated through a central nervous system reflex in efferent cholinergic nerve activity. Inhalation of foreign substances, such as particulates, can activate these receptors to elicit reflex bronchoconstriction. Afferent C-fibers are tachykinin-containing nerves that ramify within the epithelium and between smooth muscle cells [21]. Chemical (e.g., inflammatory mediators), particulate, and mechanical stimuli activate

Table 2 Innervation of the Airways

Nerve type

Putative function

Afferent Slowly adapting receptor (pulmonary stretch receptor)

Breuer-Hering reflex (inhibition of inspiration; prolongation of expiration)

Rapidly adapting receptor

Responds to airway irritants, mechanical stimuli, interstitial edema

C-fiber

Responds to airway irritants, mechanical stimuli

Neuroepithelial body

Responds to hypoxia

Efferent

Adrenergic Cholinergic

Vasoconstriction

Bronchoconstriction, mucus secretion Bronchodilation, mucus secretion

Nonadrenergic noncholinergic inhibitory afferent C-fibers to cause rapid, shallow breathing or apnea and to evoke central reflex bronchoconstriction through increased efferent cholinergic nerve activity [20,22]. Under conditions of cholinoceptor blockade, central reflex bronchodila-tion through activation of efferent nonadrenergic noncholinergic nerves may be observed [23]. Stimulation of afferent C-fibers can result in the release of tachykinins at the site of stimulation and alter airway function independent of the central nervous system, e.g., by inducing mucosal edema [24]. These nerves are thought to be important sensory modalities for conveying retrosternal discomfort induced by inhaled irritants. Neuroepithelial bodies are located in the epithelium of the central airways and are intimately associated with the endings of nerves, which are primarily afferent in nature [20,24]. Each neuroepithelial body comprises groups of neuroendocrine cells that contain biogenic amines, such as serotonin, and peptides, such as calcitonin gene-related peptide (cGRP) [25]. Hypoxia induces the release of these biologically active substances, which can then activate the sensory nerve endings to elicit a central reflex or act locally on adjacent tissues, such as blood vessels or airway smooth muscle [20,25]. Cholinergic nerves are carried to the airways in the vagus nerve and innervate airway smooth muscle and submucosal glands. The neurotransmitter acetyl-choline, released from cholinergic nerves, promotes bronchoconstriction [26] and mucus secretion [27,28]. Nonadrenergic noncholinergic inhibitory nerves, also carried in the vagus nerve, are the sole bronchodilator innervation of airway smooth muscle [29]. These nerves may also inhibit airway mucus centrat nervous system

Figure 3 Role of afferent and efferent nerves in altering airway function. Stimulation of afferent (or sensory) nerves, such as afferent C-fibers, rapidly adapting receptors or slowly adapting receptors, results in an increase in electrical impulse traffic to the central nervous system. Depending on the afferent nerve activated, processing and integration in the central nervous system may result in an increase in the activity of (a) efferent motor nerves governing muscles that regulate breathing (i.e., affect rate and depth of ventilation) or (b) efferent autonomic nerves, such as cholinergic and nonadrenergic noncholinergic inhibitory nerves, that modify mucus secretion or airway caliber through changes in smooth muscle tone. Afferent C-fibers may also serve an efferent function insofar as impulses can spread throughout the C-fiber network from the site of C-fiber stimulation to result in the release of tachykinins (such as substance P and neutokinin A). These released substances may then act on blood vessels to increase permeability or on smooth muscle to increase vascular permeability and elicit bronchoconstriction, respectively.

Figure 3 Role of afferent and efferent nerves in altering airway function. Stimulation of afferent (or sensory) nerves, such as afferent C-fibers, rapidly adapting receptors or slowly adapting receptors, results in an increase in electrical impulse traffic to the central nervous system. Depending on the afferent nerve activated, processing and integration in the central nervous system may result in an increase in the activity of (a) efferent motor nerves governing muscles that regulate breathing (i.e., affect rate and depth of ventilation) or (b) efferent autonomic nerves, such as cholinergic and nonadrenergic noncholinergic inhibitory nerves, that modify mucus secretion or airway caliber through changes in smooth muscle tone. Afferent C-fibers may also serve an efferent function insofar as impulses can spread throughout the C-fiber network from the site of C-fiber stimulation to result in the release of tachykinins (such as substance P and neutokinin A). These released substances may then act on blood vessels to increase permeability or on smooth muscle to increase vascular permeability and elicit bronchoconstriction, respectively.

secretion [30]. Adrenergic nerves do not innervate human airway smooth muscle [31] and have little effect on mucus secretion in human airways [28,31].

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