Parasympathetic innervation of canine tracheal smooth muscle

To investigate whether efferent parasympathetic fibers to the tracheal smooth muscle course through the pararecurrent nerve rather than the recurrent or the superior laryngeal nerve, we stimulated all three nerves in anesthetized dogs. We also recorded the pararecurrent nerve activity response to bronchoconstrictor stimuli and compared it with pressure changes inside a saline-filled cuff of an endotracheal tube. Electrical stimulation (30 s, 100 Hz, 0.1 ms, 10 mA) increased tracheal cuff pressure by 21.0 ± 3.2 and 1.3 ± 0.7 cmH₂O for the pararecurrent and the recurrent laryngeal nerve, respectively. Stimulation of the superior laryngeal nerve increased tracheal cuff pressure before, but not after, sectioning of the ramus anastomoticus, which connects it to the pararecurrent nerve. Intravenous administration of sodium cyanide increased pararecurrent nerve activity by 208 ± 51% and tracheal cuff pressure by 14.4 ± 3.5 cmH₂O. Elevation of end-tidal Pco₂ to 50 Torr increased pararecurrent nerve activity by 49 ± 19% and tracheal cuff pressure by 8.4 ± 3.6 cmH₂O. Further elevation to 60 Torr increased pararecurrent nerve activity by 101 ± 33% and tracheal cuff pressure by 11.3 ± 2.9 cmH₂O. These results lead us to the conclusion that parasympathetic efferent fibers reach the smooth muscle of the canine trachea via the pararecurrent nerve.