Innervation of intrapulmonary airway smooth muscle of the dog, monkey and baboon

Intrapulmonary airways of the dog, monkey and baboon were examined with the electron microscope. Differentiation of adrenergic and nonadrenergic nerves was facilitated by the use of 5-hydroxydopamine which was infused into the pulmonary arteries (500 microgram/min for 30-45 min). In all species, nerves composed mainly of unmyelinated axons lay external to the muscularis and in the muscularis between bundles of smooth muscle cells. Nerves composed of varicose unmyelinated axons that ran parallel with the smooth muscle bundles contributed fibers that surrounded and entered these bundles. Most of the varicosities associated with airway smooth muscle were cholinergic, and longitudinal sections of the muscle bundles revealed elongate profiles of these varicosities. Most cholinergic varicosities in dog, monkey and baboon airway smooth muscle had no special morphologic relationships to the surrounding smooth muscle cells. Other cholinergic varicosities in the primates lay in depressions of the sarcolemmae. Some of these varicosities were apposed to the sarcolemmae and formed neuromuscular clefts devoid of electron-dense material. There were some adrenergic varicosities near bronchial smooth muscle cells of all species studied. Another type of nerve varicosity, which was present in all species contained many large dense-core vesicles 90-120 nm in diameter and some small, agranular vesicles 40-60 nm in diameter.