Vagal Afferent Nerve Endings in the Trachealis Muscle of the Dog

Abstract

Nerve endings in the canine tracheal muscle were examined by light microscopy after immunohisto-chemical staining with an antibody against neurofilament protein, and by transmission electron microscopy. The endings were found to consist of accumulations of ramified axon terminals. The endings ranged from 100-400 μm in maximal length to 30-80 μm in minimal length. Most of the endings were arranged parallel to the smooth muscle strands, with 80-90% located in the membranous wall and densely distributed in the cranial region of the trachea. The endings were covered with a connective tissue sheath that contained fine elastic fibers. Ultrastructural examination revealed that axon terminals were derived from myelinated axons, and were located in a connective tissue sheath among the smooth muscle cells. Each axon terminal contained large numbers of mitochondria, lysosomes, neurofilaments, glycogen granules and synaptic vesicles. Incomplete coverage by Schwann cells and multiple layers of basal lamina were observed around the axon terminals. Surgical denervation revealed that the endings were of vagal origin. The access pathway to the endings in about one-third of the cranial region of the trachea appeared to be the cranial laryngeal nerve, while in about two-thirds of the caudal region, this appeared to be the tracheal branches derived from the recurrent laryngeal nerve and/or from the vagosympathetic trunk. © 1994, International Society of Histology and Cytology. All rights reserved.