Parasympathetic innervation of canine tracheal smooth muscle

To investigate whether efferent parasympathetic fibers to the tracheal smoo th muscle course through the pararecurrent nerve rather than the recurrent or the superior laryngeal nerve, we stimulated all three nerves in anesthet ized dogs. We also recorded the pararecurrent nerve activity response to br onchoconstrictor stimuli and compared it with pressure changes inside a sal ine-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 cmH(2)O for the pararecurrent and the recurrent laryngeal nerve, r espectively. 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 cmH(2)O. Elevation of end-tidal Pco (2) to 50 Torr increased pararecurrent nerve activity by 49 +/- 19% and tra cheal cuff pressure by 8.4 +/- 3.6 cmH(2)O. Further elevation to 60 Torr in creased pararecurrent nerve activity by 101 +/- 33% and tracheal cuff press ure by 11.3 +/- 2.9 cmH(2)O. These results lead us to the conclusion that p arasympathetic efferent fibers reach the smooth muscle of the canine trache a via the pararecurrent nerve.