A short primer on your longest nerve
The word “vague” has a Latin root, which means “wandering”.
If we climb a little higher up the tree, we arrive in 16th century France, where the word is used to signify emptiness or vacancy. In English, we use this word to mean “uncertain” but the same word in French means “wave”.
Like a wave in the ocean, coaxed from the water by the wind and forever aimlessly rolling over itself.
I imagine the first anatomists as cosmonauts, discovering the longest nerve in the body, tracing its tortuous path from the bottom of the brain and down into the gut and naming it in Latin: nervus vagus. The wandering nerve.
The intrepid traveller in the uncharted universe of human biology.
These are the thoughts that run uncensored through my brain as it sloshes against the inside of my skull, which has just made uncomfortable contact with the linoleum beneath the nurse’s station where four large vials of my blood sit on a plastic tray.
I don’t characterize myself as squeamish. My compulsion to understand how the brain works has required me to cut apart a fair number of them over the years, and I’ve been enamoured of all things gross since I was a small child. But like so many of our inexplicable, irrational behaviours, the tendency to hit the floor at the sight of blood is reflexive, an evolutionary short circuit that bypasses your rational mind and heads straight for your autonomic nervous system.
We can find similar examples across so many different species that it seems to be a conserved evolutionary strategy to avoid predation; a secret third option when neither fight nor flight are available.
We see it in the extreme in the opossum, who can convincingly reproduce all of the physiological hallmarks of death when a predator is near. Mice, deer, and even fish, will instantaneously freeze when they detect hints of danger, like shadows looming overhead.
Most animals are simply very convincing actors. They retain awareness while they appear to be dead. Humans, on the other hand, completely lose consciousness — thanks to the vagus nerve, and for reasons that still aren’t entirely understood.
One major factor, though, is our posture.
Human brains are demanding machines. They require high volumes of blood flow relative to the brains of other animals, not to mention the added burden of bipedalism: a consequence of walking upright is that these humongous resource consuming supercomputers in our heads are located far enough above our hearts that it requires a lot of cardiac force to keep things running smoothly.
Because of this, even a slight drop in blood pressure will disrupt the flow of glucose and oxygen to the brain enough to shut the lights off temporarily.
This is one of the first things that happens when the vagus nerve is activated — heart rate drops, blood pressure drops, and pretty shortly thereafter, so do you.
Our bodies are equipped with an impressive variety of sensors and effectors that are constantly working to maintain balance. If they’re doing their job, you won’t even notice them.
The vagus nerve orchestrates a lot of this cross-referencing between your internal organs and your brain, by relaying signals through deep-brain structures like the medulla and the hypothalamus. To do so, it requires finely tuned internal “monitors” at all the sites to which it ventures and reports back.
In the case of blood pressure, these monitors are receptors, sitting at the major junctions and crossroads of blood flow as it travels to and from the heart.These receptors are activated whenever they’re stretched — which happens if there’s an increase in your blood pressure. And once they’re engaged, they signal through the vagus to the medulla, a structure at the very bottom of the brain stem that interfaces directly with the spinal cord and controls mission-critical processes like respiration and digestion.
From here, the signal is relayed on through the central nervous system to dampen the activity of the sympathetic nervous system — the branch of your autonomic nervous system that’s associated with fight-or-flight.
This loss of consciousness, byproduct of a blood-pressure dip, can be explained physiologically by our greedy brains and upright posture — but the evolutionary logic for this trick is slightly less straightforward. This is how we work now, but how did we get here?
One idea is that very early in our history, if we found ourselves entangled in an unwinnable violent struggle, the almost immediate shut-off of all of our vital functions might even the odds; either by fooling our would-be attackers into thinking they’d won or at least losing interest, or to limit the amount of blood that was allowed to leave our gravely injured bodies.
Sometimes when I’m especially mystified by human behaviour, I try to remind myself that hundreds of thousands of years of collective effort have honed and expanded our menu of emotional experience and motivation to such an overwhelming degree that the scientific disciplines we invented to understand them can’t even keep up.
But in the beginning, the options were pretty simple: you survive, and you make more people who hopefully will also survive, or you don’t. You’re eaten by something larger and faster than you. You succumb to an injury or a preventable disease.
Your friends are tired of your unfortunate nocturnal respiratory problems, so they kill you with sharp rocks.
Living as we do in a world of well-fortified shelter from the elements, vaccines, and CPAP machines, we now have before us thousands of subtler and more complicated things to worry about.
Our brains, though, are a mess of electrochemical cross-talk and hard-wired instinct. Those same reflex circuits that engaged to protect us from immediate death before we figured out fire and agriculture, can now be engaged by any strong emotion. In medicine this is referred to as “neurogenic syncope”, which, roughly translated, means a sudden loss of consciousness due to anything that can’t be readily medically diagnosed.
Which isn’t to say it’s any less ingenious as a protective mechanism. In moments of extreme stress or grief or otherwise incomprehensible trauma, having this built-in emergency shut-off mechanism can help us conserve our physiological and psychological resources when no useful alternatives exist. And as our behavioural repertoire expands, new and surprising roles for the vagus nerve and the circuits it controls are also coming to light.
Of late, there have been scores of claims that the calming effects of singing, deep breathing, and yoga all stem from vagus nerve activation. However, it’s difficult to find well-controlled studies in animal models to substantiate these theories. This is a basic limitation of using animal models to study uniquely human behaviours — although it’s entertaining to imagine dressing dozens of mice in tiny yoga pants and then interviewing them individually about their stress levels.
Many studies do, however, reliably point to a direct relationship between the vagus nerve and the immune system. The branch of the vagus that transmits information to the brain has receptors for chemicals produced in response to infection, so it can sense immune responses quickly and report back to your brain.
In the opposite direction, from brain to body, vagus activation can actually suppress the production of these chemicals. That reduces local inflammation, promoting speedy recovery from illness. At the same time, the vagus is telling other parts of your body to settle in for rough weather, adjusting your appetite, the quality of your sleep, and your mood to essentially power down all your vital systems while they’re being repaired.
Inflammatory markers are also highly correlated with depression: they’re found in higher levels in patients with major depressive disorder who are otherwise physically healthy, and they’re thought to interfere with your brain’s ability to use serotonin efficiently.
So it’s been suggested that vagal activation might be an effective treatment for depression, because of its ability to limit the production of these inflammatory factors.
It’s interesting to look critically at homeostasis.
It feels almost counter-intuitive; in science, we take for granted that what we’re interested in is change, whether that means advancing toward some new technology or drug or simply understanding the fundamental processes of development, disease, and decay.
Homeostasis is the study of how things stay the same. And that requires precise, delicate, and continuous effort.
The vagus nerve is sometimes referred to as “the great wandering protector,” travelling to the farthest reaches of your body and passing messages back to your brain, letting you know when to eat, when to breathe, when it’s time to panic and run away, and when it’s safe to relax again. It’s constantly active, constantly sensing change and adjusting the way you behave, the hormones you produce, and how quickly your heart beats in response to your ever-changing internal and external states.
On a larger scale, though, maintaining balance is still the primary goal.
Evolutionary biologists have summed up and expanded this concept in the Red Queen hypothesis, suggesting that adaptive change and diversity in biology is a strategic manoeuvrer against the even more rapid evolution of viruses. We change our DNA just enough so that the last virus that decimated our population can’t quite figure out how to do it again a second time — until it does, and then it’s up to us to adapt or die.
In this way, the vagus nerve is also like the Red Queen of Alice in Wonderland, constantly working to keep us in a stalemate with the elements.
In physiology, as well as evolution, it takes a whole lot of running just to stay in one place.