A 58-year-old woman with no significant past medical history is undergoing general anesthesia for elective hysterectomy. During the induction phase, she receives intravenous propofol followed by succinylcholine. The anesthesiologist observes that the patient progresses through stage II (excitement stage) with visible muscle fasciculations, tachycardia, and a transient rise in blood pressure. Which physiological mechanism is primarily responsible for the cardiovascular changes observed during this stage?