The vagus nerve, which runs from the brainstem to the organs of the chest and abdomen, sends and receives messages to control the functioning of the digestive system, heart, lungs, and some glands. The longest nerve in the cranium, it has multiple branches that influence many organs. Sensory receptors relay vital information through this nerve from the organs and the ear to the brain. Nerve impulses from the brain control muscle contractions in the digestive organs, lungs, and heart as part of the autonomic nervous system. This nerve is also called cranial nerve X, as it is the tenth of the 12 pairs of cranial nerves.
Many aspects of digestion are controlled by the vagus nerve. It is first stimulated by the smell, sight, and taste of the food as it enters the mouth. This sets in motion the release of stomach acids and digestive juices. Branches of the nerve influence swallowing, while others control peristalsis, the contraction of muscles in the digestive system that move food along. Excretion of wastes is influenced by the branches of the nerve that reach the colon and kidneys.
Heart rate and blood pressure are influenced by this nerve. As part of the parasympathetic nervous system, its effects are to lower both heart rate and blood pressure. In the lungs, the vagus nerve acts to constrict bronchi by causing the smooth muscles to tighten. A branch of the nerve controls the muscles that move the vocal folds inside the larynx, and damage to it can cause hoarseness or other voice changes.
Vagal nerve stimulation (VNS) is a treatment that has been approved in some countries for epileptic seizures. An implant sends electrical pulses to the brain through this nerve. Epilepsy patients who feel an impending seizure can activate the system in an attempt to stop it from happening. Although only a small percentage of patients experience no further seizures, vagal nerve stimulation decreases seizure frequency in approximately two-thirds of people who use it. Research indicates that the treatment may be useful in patients suffering from depression who don’t respond to medication, as well as in treating heart failure.
During times of extreme stress, the vagus nerve can be overactivated. This causes heart rate and blood pressure to drop enough that sufficient blood supply to the brain is interrupted. The result is vasovagal syncope — or fainting — caused by the sudden stimulation of the nerve. An affected person will feel hot, nauseated, and lightheaded before losing consciousness. Many things besides stress can trigger vasovagal syncope, including remaining stationary for long periods of time, seeing blood or medical procedures, or standing up too quickly.