INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE
The tracheobronchial tree and lungs are innervated by the autonomic nervous system. Three types of pathways are involved: autonomic afferent, parasympathetic efferent, and sympathetic efferent. Each type of ﬁber is discussed here; the neurochemical control of respiration is covered later in the section on physiology (see Plates 2-25 and 2-26).
Autonomic Afferent Fibers
Afferent ﬁbers from stretch receptors in the alveoli and from irritant receptors in the airways travel via the pulmonary plexus (located around the tracheal bifurcation and hila of the lungs) to the vagus nerve. Similarly, ﬁbers from irritant receptors in the trachea and from cough receptors in the larynx reach the central nervous system via the vagus nerve. Chemoreceptors in the carotid and aortic bodies and pressor receptors in the carotid sinus and aortic arch also give rise to afferent autonomic ﬁbers. Whereas the ﬁbers from the carotid sinus and carotid body travel via the glossopharyngeal nerve, those from the aortic body and aortic arch travel via the vagus nerve. Other receptors in the nose and nasal sinuses give rise to afferent ﬁbers that form parts of the trigeminal and glossopharyngeal nerves. In addition, the respiratory centers are controlled to some extent by impulses from the hypothalamus and higher centers as well as from the reticular activating system.
Parasympathetic Efferent Fibers
All parasympathetic preganglionic efferent ﬁbers to the tracheobronchial tree are contained in the vagus nerve, originating chieﬂy from cells in the dorsal vagal nuclei that are closely related to the medullary respiratory centers. The ﬁbers relay with short postganglionic ﬁbers in the vicinity of (or within the walls of) the tracheobronchial tree. This parasympathetic efferent pathway carries motor impulses to the smooth muscle and glands of the tracheobronchial tree. The impulses are cholinergically mediated and produce bronchial smooth muscle contraction, glandular secretion, and vasodilatation.
Sympathetic Efferent Fibers
The preganglionic efferent ﬁbers emerge from the spinal medulla (cord) at levels T1 or T2 to T5 or T6 and pass to the sympathetic trunks via white rami communicantes. Fibers carrying impulses to the larynx and upper trachea ascend in the sympathetic trunk and synapse in the cervical sympathetic ganglia with post- ganglionic ﬁbers to those structures. The remainder synapse in the upper thoracic ganglia of the sympathetic trunks, from where the postganglionic ﬁbers pass to the lower trachea, bronchi, and bronchioles, largely via the pulmonary plexus. The postganglionic nerve endings are adrenergic. Sympathetic stimulation relaxes bronchial and bronchiolar smooth muscle, inhibits glandular secretion, and causes vasoconstriction. Pharmacologic studies indicate that there are two types of adrenergic receptors, α and β. The α receptors are located primarily in smooth muscle and exocrine glands. The β receptors have been differentiated pharmacologically into β1, located in the heart, and β2, located in smooth muscle throughout the body, including bronchial and vascular smooth muscle. Generally, α stimulation is excitatory. β Stimulation may be inhibitory (relaxation of bronchial smooth muscle) or excitatory (increase in both heart rate and force of contraction). β Stimulation also tends to mobilize energy by glycogenolysis and lipolysis.
Certain tissues contain both α and β receptors. The result of stimulation depends on the nature of the stimulating catecholamine and the relative proportion of the two types of receptors. In the lungs, β2 stimulation (there are no β1 receptors there) cause bronchodilatation and possibly decreased secretion of mucus; α- adrenergic stimulation by pharmacologic agents causes bronchoconstriction.