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).
 |
| INNERVATION OF TRACHEOBRONCHIAL TREE: SCHEMA |
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.
