the original version of this story appeared in Quanta Magazine.
It’s late at night. You’re alone and wandering down empty streets in search of your parked car when you hear footsteps sneaking up from behind. Your heart pounds, your blood pressure skyrockets. You get goosebumps on your arms and sweat on your palms. Your stomach knots and your muscles tense, ready to run or fight.
Now imagine the same scene, but without any of the body’s innate responses to an external threat. Would you still feel afraid?
Experiences like this reveal the close integration between the brain and the body in the creation of the mind: the collage of thoughts, perceptions, feelings, and personality unique to each of us. The brain’s capabilities alone are amazing. The supreme organ provides most people with vivid sensory perception of the world. It can preserve memories, allow us to learn and speak, generate emotions and awareness. But those who might try to preserve your mind by uploading its data to a computer miss a critical point: the body is essential to the mind.
How is this crucial brain-body connection orchestrated? The answer has to do with the very unusual vagus nerve. The longest nerve in the body, it runs its path from the brain throughout the head and trunk, issuing commands to our organs and receiving sensations from them. Much of the bewildering array of functions it regulates, such as mood, learning, sexual arousal, and fear, are automatic and operate without conscious control. These complex responses involve a constellation of brain circuits that link the brain and body. The vagus nerve is, one way of thinking, the conduit of the mind.
Nerves are often named according to the specific functions they perform. The optic nerves carry signals from the eyes to the brain for vision. The auditory nerves conduct acoustic information for hearing. However, the best the early anatomists could do with this nerve was to call it “vagus,” from the Latin for “wandering.” The vagus nerve was evident to early anatomists, particularly Galen, the Greek scholar who lived until about 216. But it took centuries of study to understand its complex anatomy and function. This effort continues: research on the vagus nerve is at the forefront of neuroscience today.
The most vigorous current research involves stimulating this nerve with electricity to improve cognition and memoryand for a smorgasbord of therapies for neurological and psychological disorders, including migraine, tinnitus, obesity, pain, drug addiction and more. But how could stimulation of a single nerve have such wide-ranging psychological and cognitive benefits? To understand this, we must understand the vagus nerve itself.
The vagus nerve originates from four groups of neurons in the spinal cord of the brain, where the brainstem joins the spinal cord. Most of the nerves in our body branch directly from the spinal cord: they are woven between the vertebrae of our spine in a series of lateral bands to transport information in and out of the brain. But not the lazy one. The vagus nerve is one of 13 nerves that leave the brain directly through special holes in the skull. From there sprout thickets of branches that reach almost all parts of the head and trunk. The vagus also radiates from two main groups of advanced neurons, called ganglia, located at critical points in the body. For example, a large group of vagal neurons attaches like a vine to the carotid artery in the neck. Your nerve fibers follow this network of blood vessels throughout the body to reach vital organs, from the heart and lungs to the intestine.
