Amongst the recent therapies used to enhance brain function or to improve the outcome of a neurological disorder, the stimulation of the vagus nerve appears to be a safe and pretty effective method. It has been around for a few decades as an invasive procedure (meaning it takes a surgical implant to be inserted) mainly in order to treat seizures, but it is only recently that we discover that it can also have other benefits for other disorders as well. Over the years and after many experiments, it appears that:
- Vagus nerve stimulation is useful for many other conditions such as TBI, Alzheimer’s disease, migraine, PTSD, anxiety, or depression.
- It is possible to stimulate the vagus nerve using non-surgical methods, which is much safer and can be easily implemented at home.
The vagus nerve is one of the 12 cranial nerve, and like every other cranial nerves, it originates in the brain and distributes to the periphery. Longer than any other cranial nerves, the vagus nerve has multiple centers (called nuclei) located deep in the brain, that act as relay stations between the brain and the end organs. Some nerve fibers go to different part of the face, and other nerve fibers run down the side of the throat near the carotid arteries to the thoracic and abdominal regions. Besides its length, what makes the vagus nerve unique compared to other cranial nerves, is the broad variety of functions that it carries:
-Some fibers go to the throat (soft palate, phraynx and larynx) to activate muscles of mastication, vocalization and for taste perception
-Some fibers provide sensation of the ear
-Some fibers go to the heart, lungs, viscera and digestive tract
-Some fibers go to different organs such as esophagus, stomach, gallbladder, pancreas, and different glands in the body
Another specificity of the vagus nerve is that it is a 2-direction pathway, conveying information from the brain to those organs and vice-versa. That detail is extremely important as we going to see later, because it means that both the end organs and the brain can be influenced.
Let’s have a look into some specific vagal nerve fibers, particularly the ones going to the visceral organs and the taste fibers in the back of the tongue. Those fibers will merge together to form the inferior ganglion and then will reach within the brain a central nucleus, called the nucleus tract solitarius (or NTS). The NTS is extremely important because it has been showed to project neurons to different parts of the brain (Bailey and Bremer 1938; Dell and Olson 1951; Maclean 1990). Some fibers go to the orbitofrontal cortex, responsible for social and emotional behavior, motivation and decision making. Those fibers are also tightly connected with the amygdala, our fear center, and the anterior rhinal sulcus, area where temporal epilepsy lies in. This anatomical connection might explain why stimulating the NTS through the vagus nerve can help with seizures and PTSD.
Function of the vagus nerve
The vagus nerve’s major function is to convey parasympathetic (“rest-or-digest” state) information to the body, in opposition with the sympathetic nervous system, the “fight-or-flight” response. When your brain feels that you are in a situation where you basically can relax, it sends signals to the rest of the body through the vagus nerve, which will have different effect depending on the targeted organs and the type of neutrotransmitter involved:
– Slowing down of heart rate
– Decreasing blood pressure
– Increased peristalsis, i.e. motility of the muscles of the viscera and small intestine to promote digestion
– Secretion of digestive enzymes from the glandular system to further promote digestion
Imagine when you are seated down, completely relaxed, doing mediation for instance. Your mind is clear and you feel at peace. When you are in that state you typically promote the parasympathetic nervous system.
And that’s not all. The vagus nerve plays also a key role in the relationship between the nervous, endocrine, and immune systems, which is called the neuroendorine-immune axis. As we saw earlier, that axis can dysregulate after a brain injury.
In the 19th century, doctors observed that massaging the carotid would suppress seizures in some patients. Medical investigations allowed to identify the vagus nerve being responsible to attenuate the seizures by being gently pressed, causing a light stimulation. Things leading to another like it usually happens in medical research, scientists started to stimulate the vagus by placing electrodes directly in the animal brains first to see how effective that would be. And effective it was. Noticeable reduction of seizures were found. Later on, techniques of stimulation prorgressed and were developed in humans, where implants (either on the brain or along the vagus nerve) were surgically inserted. Not only it worked for seizures reduction but it also improved outcome in headaches, heart diseases and mood disorders such as depression, anxiety, bipolar disorders. Furthermore cognitive improvement were noted in patients suffering with Alzheimer’s disease.
Because of its far reach to the small intestine, the vagus nerve plays an important role in the gut-brain axis. It is thought that people suffering gastrointestinal disorders following a brain injury is in fact due to abnormal firing of the vagus nerve to the intestine. The opposite can happens too, and people suffering from inflammation in the gut, whether it is due to dysbiosis or presence of pathogens also often suffers with neurological findings such as headaches or brain fog. In a scientific study, rats were injected with lipopolysaccharide (LPS, an endotoxin that is found in some bacteriae) in their viscera, causing production of inflammatory cytokine the brain, particularly in the hippocampus, the hippothalamus and the pituitary gland, which are the critical areas responsible for hormones release and immune response. But once the vagus nerve was cut off, those cytokines disappeared, showing that the cytokines went through the vagus nerve to signal the brain that something was wrong down in the gut.
In short, the vagus nerve is the main conveyer of the parasympathetic nervous system. When stimulated, it allows to reduces many symptoms and is capable to trigger brain inflammation from an inflammatory signal in the gut. As you might suspect by now, we are better off having a vagus nerve that works well, and if we can enhance its effectiveness that would be even better. Let’s now see how is it possible to do that.