Which part of the autonomic nervous system affects heart rate and rhythm in arrhythmias?

The correct answer is that both the sympathetic and parasympathetic branches of the autonomic nervous system affect heart rate and rhythm in arrhythmias.

The sympathetic nervous system increases heart rate and enhances cardiac contractility through the release of norepinephrine, primarily affecting the beta-adrenergic receptors in the heart. This activation can lead to a range of arrhythmias since it can increase automaticity and trigger ectopic beats or reentrant circuits, especially under conditions of stress or heightened sympathetic tone.

Conversely, the parasympathetic nervous system, primarily via the vagus nerve, exerts a slowing effect on heart rate by releasing acetylcholine, which acts on the heart's muscarinic receptors. This reduces conduction through the atrioventricular (AV) node and can lead to bradycardic arrhythmias. In some cases, excessive vagal tone can also provoke arrhythmias, emphasizing the role of this branch in rhythm control.

Understanding that both systems contribute to cardiovascular dynamics is key in recognizing their influence, especially in the context of various arrhythmias, where balance or imbalance between these branches can lead to significant clinical manifestations.