A 32-year-old woman from rural Maharashtra presents to the emergency department with a 3-day history of progressive muscle rigidity, trismus, and risus sardonicus. She sustained a puncture wound to her left foot from a rusty nail 10 days ago while working in her garden. On examination, she has opisthotonus and generalized muscle rigidity with preserved consciousness. Blood pressure is 140/90 mmHg and heart rate is 110 bpm. What is the pathophysiological mechanism of the muscle rigidity in this patient?