The constriction of airway smooth muscle, dilation of blood vessels, and increased secretion from glandular cells are results of parasympathetic system stimulation. Conversely, sympathetic system stimulation leads to relaxation of airway smooth muscle, constriction of blood vessels, and inhibition of glandular secretion. The autonomic nervous system consists of preganglionic and postganglionic neurons in the CNS and postganglionic neurons in the ganglia of specific organs, working together with the CNS and PNS to maintain homeostasis. The lung lacks voluntary motor innervation and pain fibers, which are only present in the pleura.

The parasympathetic innervation of the lung originates from the medulla in the brainstem, specifically the vagus nerve (cranial nerve X). Preganglionic fibers from the vagal nuclei travel through the vagus nerve to ganglia near the airways and blood vessels in the lung. Postganglionic fibers from the ganglia then innervate smooth muscle cells, blood vessels, and bronchial epithelial cells, including goblet cells and submucosal glands. Both preganglionic and postganglionic fibers in the lung contain excitatory (cholinergic) and inhibitory (nonadrenergic) motor neurons. Excitatory motor neurons utilize acetylcholine and substance P as neurotransmitters, while inhibitory motor neurons use dynorphin and vasoactive intestinal peptide. Parasympathetic stimulation via the vagus nerve maintains a slightly constricted smooth muscle tone in the resting lung. It also increases mucus glycoprotein synthesis in bronchial glands, leading to higher mucus viscosity. Parasympathetic innervation is more abundant in larger airways and less extensive in smaller conducting airways.

In contrast, the sympathetic nervous system response is more general compared to the specific and localized response of the parasympathetic nervous system. Mucous glands and blood vessels receive heavy innervation from the sympathetic system, but airway smooth muscle does not. Norepinephrine and dopamine (excluding their influence on the lung) serve as neurotransmitters in adrenergic nerves. Stimulation of sympathetic nerves in mucous glands increases water secretion, disrupting the balanced response between increased water and increased viscosity controlled by sympathetic and parasympathetic pathways. Adrenergic fibers are not present in humans. Additionally, afferent nerve endings are found in the epithelium and smooth muscle cells of the lung.

Respiration is an automatic, rhythmic, and centrally regulated process with voluntary control. The central nervous system, particularly the brainstem, acts as the primary control center for respiration. It is responsible for generating and maintaining a respiratory rhythm, modulating the rhythm through sensory feedback loops and reflexes to adapt to different conditions while minimizing energy expenditure, and activating respiratory muscles for proper gas exchange. Further details on the regulation of respiration can be found.
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