The human body has 12 pairs of cranial nerves connecting the major body systems directly to the brain. Each pair of nerves serves a specific function. In regards to the function of the vagus nerve, its purpose is to innervate the organs of the body, known collectively as the viscera. Specifically, the function of the vagus nerve is to relay signals between the brain and various body systems to regulate heart rate, speech, sweating, blood pressure, digestion, glucose production, and certain aspects of breathing. Not only is the nerve responsible for innervating organs, but it also controls certain skeletal muscles, such as the larynx.

Cranial nerve X, as the vagus nerve is also known, serves as the communication pathway between the brain and the viscera. Both output from the brain and input from various organ systems travel along the various never fibers that make up the vagus nerve. Communicating the status of various organs is the primary function of the vagus nerve. Up to 90 percent of the nerve fibers associated with cranial nerve X are sensory or afferent nerves that relay information back to the brain.

Another name for the vagus nerve is the pneumogastric nerve, due to its responsibility for innervating the stomach and the lungs. In addition, the autonomic or involuntary function of the lungs is controlled by nerve fibers and branches associated with the vagus nerve. The sensory and motor functions of the stomach are also controlled. When eating, for example, the sensation of being full is transmitted to the brain by the vagus nerve. Digestive enzymes are produced and released according to input from the pneumogastric nerve.

Parasympathetic fibers, sensory tissues in body areas between the neck to the lower trunk, oppose sympathetic fibers to help regulate various body functions. For example, cranial nerve X supplies parasympathetic fibers for the heart, in addition to other organs. Should the need arise, the vagus nerve secretes neurotransmitters responsible for lowering the heart rate or reducing blood pressure through these fibers. Likewise, similar fibers act to open the larynx during breathing and move various mouth muscles to enable speech. Operation of these parasympathetic fibers is another function of the vagus nerve.

Numerous other systems rely on the proper function of the vagus nerve in order to communicate necessities to the brain. Innervation of the outer ear canal is the responsibility of cranial nerve X. Additionally, glucose production is decreased when the liver sends certain insulin and enzyme information to the brain along vagi fibers. Other functions include transmitting taste sensations, controlling the muscles responsible for swallowing and coughing, and triggering sweat glands to help regulate body temperature.