To really understand the gut-brain connection, one of the critical structures that needs to be examined and understood is the vagus nerve. The vagus nerve is what is known as a cranial nerve, that means its originate begins in the brain itself, rather than the spinal cord. There are 12 cranial nerves and the vagus nerve is the 10th cranial nerve.
As a plastic surgeon, I had to learn quite a bit about the cranial nerves because they basically include the innervation to all the structures in the face, mouth and throat that a plastic surgeon would typically operate on (such as in facial paralysis or cleft lip surgery). You have to know where they are, what they do and their specific anatomy. The vagus nerve is no different. It innervates many of the vital structures within the cranium, face and neck, and as I have said, they are vitally important for a plastic surgeon to know a lot about.
The vagus nerve is interesting because it leaves the cranium and innervates not only many structure in the head and neck but the vagus nerve also innervates virtually the all the viscera of the the chest and the abdomen. The root of the word vagus comes from the Latin word which means vagrant, and the vagus nerve is indeed is a vagrant nerve. It strays very far from the rest of its 11 cranial nerve brothers and sisters.
Before we get into the vagus nerve itself let’s talk about the basics of nerves. Nerves are either classified as somatic nerves, which deal with voluntary actions such as movement of skeletal muscles as well as things like senation, or they can be visceral, or autonomic nerves, components, which control the function of the autonomic nervous system, or that part of the nervous system not under our direct control. This includes the sympathetic and and the parasympathetic nervous systems which you may heard about, which control things like your heartbeat, which unlinke your biceps, for instance, are not under your direct control.
In the cranial nerves, many nerves can have both elements the somatic and visceral nervous system, and so, as a result, these nerves can get very complex as they can embody basically every aspect of nervous functioning that is known in the body. The vagus nerve is no different. It innervates skeletal muscle that is voluntary, it provides sensory inputs to the brain, and it also regulates the thoracic and the abdominal viscera.
Now, we are finally going to get a quick overview of what the vagus nerve does.
The vagus nerve has four divisions: cranial, cervical, thoracic and abdominal. I will talk about each specifically.
- Cranial. The vagus nerve arises from a series of rootlets in the medulla and leaves the skull through a structure called the jugular foramen in company with cranial nerve IX and XI which are also known as the glossopharyngeal nerve and the spinal accessory nerve, respectively. They give out branches in the cranial section of the vagus nerve which innervate the dura matter or the thick fibrous casing of the brain and provide a nerve branch to the ear called the auricular branch (so when you are nibbling on your partners earlobe on Valentine’s Day, don’t forget it’s the vagus nerve you are stimulating!)
- The cervical portion of the vagus nerve is defined when the vagus nerve exits the brain through the jugular foramen and swings around the carotid sheath (blood vessels in the neck) to continue to the root of the neck. In this area, the vagus nerve provides branches to the pharyngeal plexus. This is important for a plastic surgeon to know about because of cleft palate conditions. There are also branches of the vagus nerve that provides cervical cardiac branches (part of the parasympathetic nervous system that regular your heartbeat). The superior laryngeal nerve is also derived from the vagus nerve as well as the recurrent laryngeal nerve. These nerves help you speak. This nerve can be injured in thyroid surgery or throat surgery or conditions such as thyroid cancer or lung cancer that is in the superior portion of the lung. This is what happened to the famous singer and actress Julie Andrews, who you all know from the Sound of Music and Mary Poppins.
- The thoracic portion of the cranial nerve includes thoracic, cardiac branches, pulmonary branches and branches to the esophageal plexus. This is where the visceral fibers start to be denoted because the abdominal viscera which includes the heart and the abdominal organs from the esophagus to the colon and rectum are all innervated by the vagus nerve via the visceral or autonomic nervous system. This means they are not under your conscious control but are regulated through a series of feedback loops which are not under voluntary control, as I mentioned. You can’t control your heartbeat, as we’ve said, and you can’t control when your stomach is “acting up” in most cases either with things like jitters and butterflies before you give a speech, for instance. Of note, these autonomic fibers are not simply motor branches because visceral branches contain what is known as afferent branches or sensory branches. When you have this weird pain in your stomach or weird pain in your gut or you feel things in your gut (eg butterflies), that is the afferent visceral nervous system acting up via the vagus nerve.
- The abdominal section of the vagus nerve. Here is where we get into the “meat and bones” of the vagus nerve as it relates to the essentials of the gut and the brain. The abdominal branches of the vagus nerve include the gastric branches, hepatic branches, pyloric (stomach) branches, adrenal branches and intestinal branches. They go to the left to the hepatic (liver) flexure and they end in about the area of the transverse colon. Here is where things obviously get very interesting, and have been getting interesting recently, in regards to the gut and the brain. Interestingly enough, when acid was thought to cause ulcers, surgical treatment included surgical division (or cutting) of the vagus nerve. This procedure was called vagatomy. These were popular procedures before the advent of H2 blockers (things like Pepcid, which I am sure you have heard of). However, as we learned more about ulcers, we learned that they were caused by bacteria (H Pylori). This was then treated with antibiotics. But as we learned and are still learning, conditions like small intestinal bacterial overgrowth (SIBO) affects more people than commonly thought, and may in fact, be the underlying cause, whose treatment is diet (ie removing grains, taking probiotics, and removing other toxins from the diet, etc). Moreover, as we learn more and more about the gut-brain via the vagus nerve, we are learning that it may be the vital link to understanding things like depression and anxiety. Many patients who have abdominal problems either through autoimmune disease like Crohn’s disease and celiac disease to inflammatory bowel syndrome and irritable bowel syndrome often are plagued with depression. They can also have skin problems like acne as well. It is also important to know that small intestinal bacterial overgrowth may be a critical factor in treating these patients. In fact, I suspect that we will have to think of treatments that include surgery (as we did initially) that tend to only treat the symptoms as it seems the the actual root cause which needs to address healing your gut through diet and lifestyle.
I hope you enjoyed this blog post, it was one of my longer ones and most complex ones. So, if you have any questions, please send them to me.