Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 756

Agonists of nucleotide receptors are potent mucin secretagogues. Mucin is released
in response to the G-protein-PLC-PKC pathway. However, only UTP can increase
mucin gene (MUC5AC and MUC5B) transcription using the G-protein-MAPK
axis. Therefore, in differentiated airway epithelial cell cultures, ATP causes mucin
secretion and UTP both mucin synthesis and secretion.
The exocytic priming protein MUnc13-2, or Uncoordinated-13 homolog
Unc13b, is required in the regulated, baseline secretory pathway of Muc5b in Clara
cells [ 1542 ]. On the other hand, MUnc13-4, or Unc13d, a Rab27 effector, triggers
ATP and UTP nucleotide-regulated release of accumulated mucins in secretory
granules.

12.3.1.4 Nervous Control

The lungs and pleura are innervated by the mixed, anterior and posterior, pulmonary
plexi that contain vagal (parasympathetic) and sympathetic nerves. The pulmonary
parasympathetic and sympathetic innervations originate from thoracic plexi. The
pulmonary nerve plexus lies behind each hilum, receiving fibers from both vagi and
thoracic T1 to T4 ganglia of the sympathetic trunk. Each vagus contains sensory
afferents from airways and bronchoconstrictor and secretomotor efferents.
The main pulmonary nerves enter the lungs at their respective hila and then
accompany bronchi and associated pulmonary arteries and veins. Each lobar
bronchus has 4 to 5 nerves around it. Peribronchial nerves emit thick and thin
fibers that enter the bronchial wall [ 1543 ]. Intrabronchial nerves give rise to smaller
branches and bronchial ganglia.
Most airway mucus is produced by submucosal glands in response to neural
signals [ 1544 ]. Various secretory chemical messengers synthesized in the neuroen-
docrine system enable short-range (neurotransmitters and paracrine regulators) and
long-range (circulating hormones) communications.

Lung Innervation

The lung is supplied by motor nerves of the sympathetic and parasympathetic
compartments of the autonomic nervous system and by sensory nerves that originate
primarily from sensory ganglia of the vagus nerve, but also arise from dorsal root
ganglia (Table 12.1 )[ 1544 ].
Pulmonary plexi are located anterior and posterior to the lung roots, close to lung
hila, which are constituted by stem bronchi, pulmonary arteries and veins, bronchial
arteries and veins, and lymphatic vessels with bronchial lymph glands enclosed by
a reflection of the pleura.
Pulmonary plexi innervate smooth muscles of airways and blood vessels, and
glands of the bronchial tree. Inner peribronchial nerve plexi reside in the lamina
propria under the respiratory epithelium. They feed the bronchial smooth muscle
and glands. Large bronchi possess peribronchial and subepithelial nervous plexi
with intraepithelial nerve terminals. In bronchioles, the inner and outer nerve plexi
merge to form a peribronchiolar plexus.