Extreme stimulation of which nerve could lead to significant changes in heart rate?

The vagus nerve plays a crucial role in regulating heart rate as it is part of the parasympathetic nervous system, which is responsible for the body's rest-and-digest functions. When the vagus nerve is stimulated, it releases acetylcholine, which binds to receptors in the heart and decreases heart rate by inhibiting the pacemaker activity of the sinoatrial node. This mechanism is essential for maintaining homeostasis, particularly in situations where the heart rate needs to be lowered, such as during periods of relaxation or after eating.

In contrast, the other nerves listed do not have a direct influence on heart rate to the same extent. The accessory nerve is primarily involved in controlling certain neck and shoulder muscles, the phrenic nerve controls the diaphragm and regulates breathing, and spinal nerves generally provide motor and sensory functions to various parts of the body but do not specifically target the heart in the way that the vagus nerve does. Therefore, extreme stimulation of the vagus nerve can significantly alter heart rate, making it the correct choice.