A 48-year-old female with major depressive disorder and chronic lower back pain presents to the emergency department with agitation, diaphoresis, bilateral lower extremity clonus, mydriasis, hyperthermia of 39.1°C, and diarrhea. Her medication list includes sertraline 150 mg daily and tramadol 50 mg every 6 hours as needed, which was started five days ago by an urgent care provider for an acute pain exacerbation. Her vital signs show heart rate 118 bpm and blood pressure 162/98 mmHg. Which of the following best explains the pharmacologic mechanism underlying this patient's presentation?