The innervation of the trachea and extrapulmonary bronchi of the mouse

In the mouse, nerves were located throughout the trachea and extrapulmonary bronchi in both the smooth muscle and the connective tissue. However, no nerves were found within the epithelium. In the smooth muscle there were large numbers of nonmyelinated nerves. These were usually 'en passant' elements but varicosities containing small mitochondria and vesicles were also seen; these axons sometimes appeared to be efferent to the muscle. Unilateral cervical vagotomy reduced the numbers of nerves in the muscle of the trachea and ipsilateral primary bronchus, suggesting that they were afferent. The intramuscular nerves were characterized in terms of their complement of cytoplasmic organelles; in particular nerves containing many mitochondria disappeared following vagotomy. Pretreatment of mice with 5-hydroxydopamine to accentuate the electron-opacity of catecholamine-containing granules resulted in 3.5% of the nerves within tracheal muscle showing such granules. The afferent nerves of the smooth muscle may be complex branching structures with many varicosities. The absence of epithelial nerves may be related to the absence of the cough reflex in the mouse.