Ultrastructural study of guinea pig tracheal smooth muscle and its innervation

The ultrastructure of guinea pig tracheal smooth muscle and its innervation was studied by transmission electron microscopy. Smooth muscle cells were arranged into bundles separated by collagen and connective tissue cells. Gap junctions were found only rarely between adjacent smooth muscle cells. Bundles of axons (predominantly unmyelinated) were seen on the periphery of the muscle bundles. Some penetrated deep into the bundles and a few made close neuromuscular contacts (less than 20 nm). Such a spatial arrangement of nerves and smooth muscle suggests a potential for nervous control and limited myogenic activity. Axon profiles containing small agranular vesicles characteristic of cholinergic nerves and small granular vesicles characteristic of adrenergic nerves were seen in close association with airway smooth muscle and with one another. Some large granular vesicles were associated with varicosities of both nerve types. Quantitative studies demonstrated a preponderance of cholinergic over adrenergic nerve profiles in both upper and lower segments. Varicosities, characteristic of adrenergic nerves, were observed more frequently in upper compared with lower segments; however, observed differences were not significant. The morphological findings are consistent with the control functions exerted by cholinergic and adrenergic nerves but provide no basis for control by nonadrenergic inhibitory nerves.