If somebody is poisoned by ttx, doctors will give you atropine to resolve the bradycardia. but why does this work?

TTX?: If by ttx, you mean tetrodotoxin, Atropine will not reverse it and can even make it worse. There is no antidote. Anticholinesterase inhibitors (which have the opposite effect as atropine) have had mixed success. Atropine, in general, reveres bradycardia by inhibiting the parasympathetic tone that is normally present and acting on cardiac conduction. Parasympathetics act as "brakes" on the heart.