What is the main function of the vagus nerve within the autonomic nervous system?

The vagus nerve plays a crucial role in the autonomic nervous system, particularly impacting both the heart and the digestive system. It is a key component of the parasympathetic division, which is responsible for promoting a "rest and digest" state in the body. The main functions of the vagus nerve include regulating heart rate, where it exerts a calming effect, and stimulating digestive tract movements, which facilitate processes such as peristalsis and enzyme secretion. By managing these functions, the vagus nerve helps maintain homeostasis and promotes overall bodily functions related to rest and digestion, making this the correct choice.

Other options focus on functions outside the vagus nerve's primary roles. The control of voluntary muscle movements pertains to the somatic nervous system, while the transmission of sensory information from the head is mainly associated with cranial nerves other than the vagus. Additionally, the facilitation of facial expressions is primarily managed by different cranial nerves, specifically the facial nerve.