Ultrastructural studies on the neuromuscular control of human tracheal and bronchial muscle

This study presents an ultrastructural analysis of neural and myogenic control of smooth muscle in human trachea, in small (approximately fourth to seventh order) bronchi. A moderate frequency of gap junctions between smooth muscle cells and a sparse innervation was observed in trachea. In contrast, small junctions without identifiable gaps were identified in bronchi along with a dense, sometimes close innervation of muscle. Most profiles of nerve varicosities in both types of airways were of two types: (1) those containing mostly small agranular and few or no large granular vesicles; and (2) nerves (probably adrenergic) with some small granular vesicles along with other vesicles present in bronchi. In the bronchi some mast cells were found near nerve profiles and in close proximity to smooth muscle. These morphological findings suggest that tracheal muscle may be organized for more neural and less myogenic control of activity. Interactions between mast cells, nerves and muscles in bronchi could influence airway function. These findings imply that a single type of airway muscle may not adequately represent the structural and functional properties of human airway.