The Vagus Nerve: Your Body's Communication Superhighway
The vagus nerve serves as the body's superhighway, carrying information between the brain and the internal organs and controlling the body's response in times of rest and relaxation. The large nerve originates in the brain and branches out in multiple directions to the neck and torso, where it's responsible for actions such as carrying sensory information from the skin of the ear, controlling the muscles that you use to swallow and speak and influencing your immune system.
The vagus is the 10th of 12 cranial nerves that extend directly from the brain, according to the Encyclopedia Britannica. Although we refer to the vagus nerve as singular, it's actually a pair of nerves that emerge from the left and right side of the medulla oblongata portion of the brain stem. The nerve gets its name from the Latin word for wandering, according to Merriam-Webster, which is appropriate, as the vagus nerve is the largest and most widely branching cranial nerve.
By wandering and branching throughout the body, the vagus nerve provides the primary control for the nervous system's parasympathetic division: the rest-and-digest counterpoint to the sympathetic nervous system's fight-or-flight response. When the body is not under stress, the vagus nerve sends commands that slow heart and breathing rates and increase digestion. In times of stress, control shifts to the sympathetic system, which produces the opposite effect.
The vagus nerve also carries sensory signals from internal organs back to the brain, enabling the brain to keep track of the organs' actions.
The brain-gut axis
Large divisions of the vagus nerve extend to the digestive system. About 10% to 20% of the vagus nerve cells that connect with the digestive system send commands from the brain to control muscles that move food through the gut, according to the textbook "Nerves and Nerve Injuries Volume 1" (Academic Press, 2015). The movement of those muscles is then controlled by a separate nervous system embedded within the walls of the digestive system.
Related: 11 Surprising Facts About the Digestive System
The remaining 80% to 90% of the neurons carry sensory information from the stomach and intestines to the brain. This communication line between the brain and the gastrointestinal tract is called the brain-gut axis, and it keeps the brain informed about the status of muscle contraction, the speed of food passage through the gut and feelings of hunger or satiety. A 2017 study published in the Journal of Internal Medicine found that the vagus nerve is so closely entwined with the digestive system that stimulation of the nerve can improve irritable bowel syndrome.
In recent decades, many researchers have found that this brain-gut axis has another counterpart — the bacteria that live inside the intestines. This microbiome communicates with the brain through the vagus nerve, affecting not just food intake but also mood and inflammation response, according to a 2014 review published in the journal Advances in Experimental Medicine and Biology. Much of the existing research involves experiments with mice and rats rather than humans. Nonetheless, the results are striking and show that changes in the microbiome may cause changes in the brain.
