Stimulation of the parasympathetic system leads to airway smooth muscle constriction, blood vessel dilation, and increased glandular cell secretion, whereas stimulation of the sympathetic system causes relaxation of the airway smooth muscle, constriction of blood vessels, and inhibition of glandular secretion (see
Fig. 26.1 ). The functional unit of the autonomic nervous system is composed of preganglionic and postganglionic neurons in the CNS and postganglionic neurons in the ganglia of the specific organ. As with most organ systems, the CNS and PNS work in concert to maintain homeostasis. There is no voluntary motor innervation in the lung, nor are there pain fibers. Pain fibers are found only in the pleura.
The parasympathetic innervation of the lung originates from the medulla in the brainstem (cranial nerve X, the vagus nerve). Preganglionic fibers from the vagal nuclei descend in the vagus nerve to ganglia adjacent to airways and blood vessels in the lung. Postganglionic fibers from the ganglia then complete the network by innervating smooth muscle cells, blood vessels, and bronchial epithelial cells (including goblet cells and submucosal glands). In the lungs, both preganglionic and postganglionic fibers •Fig. 20.8 Innervation of the Lung. Theautonomicinnervation(motorandsensory)ofthelungandthesomatic(motor)nervesupplytotheintercostalmusclesanddiaphragmaredepicted.
contain excitatory (cholinergic) and inhibitory (nonadrenergic) motor neurons. Acetylcholine and substance P are neurotransmitters of excitatory motor neurons; dynorphin and vasoactive intestinal peptide are neurotransmitters of inhibitory motor neurons. Parasympathetic stimulation through the vagus nerve is responsible for the slightly constricted smooth muscle tone in a normal resting lung. Parasympathetic fibers also innervate the bronchial glands, and these fibers, when stimulated, increase the synthesis of mucus glycoprotein, which increases the viscosity of mucus. Parasympathetic innervation is greatest in the larger airways and most limited in the smaller conducting airways in the periphery.
Whereas the response of the parasympathetic nervous system is very specific and local, the response of the sympathetic nervous system tends to be more general. Mucous glands and blood vessels are heavily innervated by the sympathetic nervous system; however, airway smooth muscle is not. Neurotransmitters of the adrenergic nerves include norepinephrine and dopamine, although dopamine has no influence on the lung. Stimulation of the sympathetic nerves in mucous glands increases water secretion. This disrupts the balanced response of increased water and increased viscosity between the sympathetic and parasympathetic pathways. Adrenergic fibers are absent in humans. In addition to those in the sympathetic and parasympathetic systems, afferent nerve endings are present in the epithelium and in smooth muscle cells in the lung.
Central Control of Respiration
Breathing is an automatic, rhythmic, and centrally regulated process with voluntary control. The CNS, particularly the brainstem, functions as the main control center for respiration (
Fig. 20.9 ). Regulation of respiration requires (1) generation and maintenance of a respiratory rhythm; (2) modulation of this rhythm by sensory feedback loops and reflexes that allow adaptation to various conditions while minimizing energy costs; and (3) recruitment of respiratory muscles that can contract appropriately for gas exchange. Control of respiration is described in greater detail in