A 55-year-old woman taking venlafaxine 225 mg daily for major depressive disorder is prescribed tramadol by an urgent care physician for acute low back pain. Twelve hours later, she presents to the emergency department with agitation, diaphoresis, hyperthermia of 39.2 degrees Celsius, bilateral lower extremity clonus, hyperreflexia, and mydriasis. Her PMHNP recognizes this presentation as serotonin syndrome. Which of the following best describes the pathophysiological mechanism underlying this patient's condition?