original version of this story Published in Quanta Magazine.
It’s late at night too. You’re alone, wandering down an empty street looking for your parked car when you hear footsteps creeping up behind you. Your heart races and your blood pressure soars. My arms are covered in goosebumps and my palms are sweaty. Your stomach contracts and your muscles coil, ready to sprint or fight.
Now imagine the same scene. However, there is no innate response of the body to external threats. Do you still feel scared?
Such experiences reveal the close integration of brain and body in the creation of the mind, a collage of thoughts, perceptions, emotions, and personalities unique to each of us. The capacity of the brain alone is amazing. The supreme organ gives most people a vivid sensory perception of the world. It stores memories, allows us to learn, speak, and generate emotions and consciousness. However, those who try to save the mind by uploading data to a computer miss the important point that the body is essential to the mind.
How is this important brain-body connection regulated? The answer involves a very unusual vagus nerve. The longest nerve in the body, it runs from the brain throughout the head and trunk, giving commands to and receiving sensations from organs. Many of the mysterious range of functions it regulates, such as mood, learning, sexual arousal, and fear, are automatic and function without conscious control. These complex responses involve a series of brain circuits that connect the brain and body. The vagus nerve is, in one way of thinking, a conduit for the mind.
Nerves are usually named based on the specific function they perform. The optic nerve sends signals from the eye to the brain for vision. The auditory nerve transmits acoustic information for hearing. But the best early anatomists could do for this nerve was to call it the “vagus nerve,” which in Latin means “wandering.” Wandering nerves were evident in the first anatomists, especially the Greek polymath Galen, who lived until about 216 AD. However, understanding its complex anatomy and function required centuries of research. This effort is still ongoing. Research on the vagus nerve is at the forefront of today’s neuroscience.
Currently, the most intensive research is stimulating these nerves with electricity to improve cognition and memory and potentially treat neurological and psychological disorders including migraines, tinnitus, obesity, pain, and drug addiction. It’s about spreading it across a wide range of areas. But can stimulating a single nerve have such a wide range of psychological and cognitive effects? To understand this, we need to understand the vagus nerve itself.
The vagus nerve originates from a cluster of four neurons in the medulla oblongata of the brain, where the brainstem attaches to the spinal cord. Most nerves in our body branch directly from the spinal cord. They are threaded between the vertebrae of the spine in a series of transverse bands and carry information in and out of the brain. But it’s not the vagus nerve. The vagus nerve is one of 13 nerves that exit directly from the brain through a special hole in the skull. From there, branches grow thick and reach almost anywhere on the head or trunk. The vagus nerve also radiates from two major clusters of sentinel neurons, called ganglia, located at strategic locations in the body. For example, large clusters of vagus nerve neurons tend to cling to the carotid arteries in the neck. The nerve fibers follow this network of blood vessels throughout the body, reaching important organs from the heart and lungs to the intestines.
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