A 19-year-old man volunteers for an autonomic function experiment in his university physiology laboratory. He has no medical history and takes no medications. At baseline, his heart rate is 75/min and blood pressure is 118/76 mm Hg. He receives atropine 0.5 mg intravenously as part of the experimental protocol. Five minutes after administration, his heart rate increases to 105/min and blood pressure is 120/78 mm Hg. He reports no subjective symptoms. The teaching assistant explains that the change in heart rate demonstrates an important principle of autonomic regulation.
Which of the following best explains the mechanism of the observed increase in heart rate?