How does sympathetic and parasympathetic work together to regulate heart rate?

The sympathetic and parasympathetic nervous systems work together to regulate heart rate through a balance of excitatory and inhibitory signals. The sympathetic system increases heart rate and contractility during stress or physical activity by releasing norepinephrine, which stimulates the heart. Conversely, the parasympathetic system, primarily through the vagus nerve, decreases heart rate during rest and relaxation by releasing acetylcholine. This dynamic interplay allows the body to adjust heart rate according to its needs in different situations.