PARASYMPATHETIC INNERVATION OF THE CERVICAL TRACHEALIS MUSCLE IN LIVING DOGS

In 34 anesthetized dogs, the pathways by which parasympathetic fibers were carried to the cervical trachealis muscle were characterized. Nerves were stimulated electrically, and tracheal tension was monitored in segments of the posterior membrane in vivo as described previously. Stimulation of superior laryngeal nerves contracted 34 of 34 cranial cervical segments and 2 of 4 caudal cervical segments. Recurrent laryngeal nerves contributed to innervation of 34 of 34 cranial, and 4 of 4 caudal segments. Stimulation of pararecurrent nerves contracted 11 of 34 cranial and 4 of 4 caudal segments. Mechanical effects of esophageal contraction, induced by stimulating pararecurrent nerves, did not alter tension in tracheal segments. Tracheal contractions induced by stimulation of all 3 pathways simultaneously were significantly less than the sum of contractions produced by stimulating each set individually; this was probably due to anastomoses between terminal neurons, overlap in their anatomic distribution or intercellular nexuses in trachealis muscle. Parasympathetic innervation of the canine trachea is by 3 different neuroanatomic pathways.