What effect does the vagus nerve have on heart rate?

The vagus nerve plays a crucial role in regulating heart rate, primarily through its parasympathetic control over the heart. When the vagus nerve is stimulated, it releases neurotransmitters such as acetylcholine, which act on the heart's pacemaker cells (located in the sinoatrial node) to decrease their firing rate. This leads to a reduction in heart rate, making the heart beat more slowly.

This parasympathetic action is counterbalanced by the sympathetic nervous system, which increases heart rate during stress or physical activity. In normal physiological conditions, the vagus nerve helps maintain a baseline heart rate by exerting a slowing effect. This is particularly evident during relaxation or deep breathing, where vagal tone increases, leading to a notable decrease in heart rate.

Understanding the role of the vagus nerve is essential in the context of bradycardia, where an excessively slow heart rate may occur due to enhanced vagal activity. Thus, the effect of the vagus nerve on heart rate is primarily a decrease, making the option that states it decreases heart rate the most accurate response.