A 58-year-old man from Delhi presents to the emergency department with acute onset severe headache, palpitations, and profuse sweating for the past 2 hours. His blood pressure is 210/130 mmHg, heart rate 118/min, and respiratory rate 24/min. He appears anxious and tremulous. On examination, he has dilated pupils and piloerection. His past medical history is significant for a pheochromocytoma diagnosed 3 months ago, which he has not yet undergone surgery for. What is the primary autonomic mechanism responsible for his current clinical presentation?