The longest nerve in the human body

Imagine a person walking alone on a road late at night and hearing footsteps behind them. The person’s heart would race, blood pressure would spike, goose bumps would appear on the arms, palms would sweat, the stomach would tighten, and the muscles would contract, ready to sprint or fight. Such experiences demonstrate the close connection between the brain and the body. How is this connection organized?

The answer has to do with the vagus nerve. The longest nerve in the body, it runs from the brain through the head and torso, giving commands to the organs and receiving sensations from them. Most of the functions it regulates, such as mood, learning, and fear, are automatic and operate without conscious control.

Ancient anatomists knew about the vagus nerve, notably Galen, the Greek scientist who died around 216. But it took centuries of research for scientists to understand its complex anatomy and function. The effort continues today.

The vagus nerve originates from four clusters of nerve cells in the medulla oblongata, where the brain stem connects to the spinal cord. Most nerves in the human body branch directly from the spinal cord, but the vagus nerve does not. It is one of 13 nerves that leave the brain directly through special holes in the skull. From there, it branches out to almost every part of the head and torso.

The vagus nerve also branches out from two large clusters of nerve cells called ganglia, located at key points in the body. For example, there is a large cluster of vagus nerve cells that attach like vines to the carotid artery in the neck. The nerve fibers travel through the blood vessels throughout the body to reach vital organs such as the heart, lungs, and intestines.

Every complex system in the body needs the equivalent of a brake and an accelerator pedal to maintain control. The vagus nerve plays such a role in many of our innate responses. Signals between the brain stem and the body travel up and down the nerve to contract the intestines during digestion, direct the immune system to ward off bacterial threats, regulate heart rate, and raise and lower blood pressure. The vagus nerve also squeezes the bronchial tubes to bring air into the lungs, triggers the gag reflex, and causes coughing.

The main function of the vagus nerve is to reduce the body’s response. For example, after a panic attack, the body’s strong response needs to stop to restore normal heart rate, breathing, blood flow… The influence of the vagus nerve is so strong that it can stop a racing heart. Conversely, this nerve can also stimulate the body’s response by “releasing the brakes” to speed up.

The vagus nerve’s “braking” effect is the biological basis for new therapies that stimulate the nerve to quell seizures, relieve anxiety disorders, calm inflammation, prevent migraines, and many other potential treatments. Unlike deep brain stimulation, which treats some of the same conditions, vagus nerve stimulation can be performed without neurosurgery.

For example, nerve fibers can be stimulated with mild electrical pulses by inserting electrodes into the chest or simply clipping them to the ear. This technique has been used to treat epilepsy and depression for decades. In 2021, the US Food and Drug Administration (FDA) also approved the use of this technique to treat movement problems caused by stroke.

Without the vast reach and powerful impact of the longest nerve, the vital, tight connection between the brain and the body would be broken, and many core emotions and experiences – fear, joy, how to react quickly to a threat and regain calm afterward – would be disrupted. So while it doesn’t need to be a panacea, the vagus nerve is still remarkable.

Thu Thao (Theo Smithsonian)