Irritable bowel syndrome (IBS) is characterized by altered bowel habits, such as constipation, diarrhea, or both, accompanied by abdominal pain or discomfort. Visceral (gut) hypersensitivity is believed to be a key factor in the cause of IBS pain and may also contribute to symptoms such as stool urgency and bloating. The cause of visceral hypersensitivity, however, is currently unknown.
The complex interactions between the gut and the central nervous system are essential to normal homeostasis and the functions of the gastrointestinal tract. This delicately balanced system allows bidirectional messages between the brain and the gut known as the brain-gut axis (BGA). The BGA is responsible for movements, reflexes, and sensory perceptions in the gut. In IBS patients, gut sensations and the bidirectional neurotransmission may be altered, manifesting clinically as pain, altered gut motility, and even psychological dysfunction in the form of anxiety and depression. Alterations in the bidirectional signaling between the brain and the gut may play a significant role in the pathophysiology of IBS.
The vagus nerve is the longest cranial nerve in the body. It passes through the neck and thorax to the abdomen and has the widest distribution in the body. It regulates the digestive system and also affects the cardiovascular, endocrine, immune, and respiratory systems. In addition, the vagus nerve innervates the gut, kidneys, liver, lungs, and spleen and is the conduit for the BGA and connects all the major organs (excluding the adrenal glands and thyroid). Not surprisingly, the proper functioning of the vagus nerve is crucial for both physical and mental well-being. Although it isn’t the only nerve in the parasympathetic nervous system, it’s by far the most important because it has the most far-reaching effects.
Vagus nerve dysfunction (when the vagus nerve is either overactive or underactive) can result in the manifestation of a wide range of symptoms. For example, an underactive vagus nerve can lead to delayed gastric emptying (gastroparesis). That’s because peristalsis (the involuntary constriction and relaxation of the muscles of the intestine) is under the control of the vagus nerve, and any damage to it will impede gut motility. When the vagus nerve is hyperactive, it can result in an abnormally low heart rate (bradycardia), temporary loss of consciousness (syncope), and a variety of other symptoms.
Healthy vagus nerve activity promotes relaxation, lowers heart rate, decreases anxiety, and reduces depression. Higher vagal tone (health and vibrancy) is associated with better mood and greater resilience to stress. Conversely, vagus nerve dysfunction or a low vagal tone is associated with increased heart rate, elevated blood pressure, heightened stress response, cognitive impairment, and poor digestion. The vagus nerve plays a role in almost every disease that involves hyperarousal and therefore, not surprisingly, has been implicated in IBS.
Because the vagus nerve interfaces with the parasympathetic nervous system, its health and activity influence gastrointestinal peristalsis, satiety from food, neural health, inflammation, and a variety of other vital body functions. Pain is one of the most common symptoms of vagus nerve dysfunction (as well as a common symptom of IBS). Vagus nerve damage or dysfunction can inhibit swallowing and impair the normal gag reflex. Because it controls some of the muscles in the throat, damage to the nerve can also alter a person’s voice.
The connection between anxiety and the vagus nerve is well established. When functioning properly, the vagus nerve opposes the sympathetic fight-or-flight response. When it is dysfunctional, however, the sympathetic response is left unopposed, leading to anxiety, depression, hyperarousal, increased heart rate, and insomnia.
Vagus nerve stimulation (VNS) counterbalances the stress response of the sympathetic nervous system and normalizes hypothalamic-pituitary-adrenal (HPA) axis dysregulation, decreasing heart rate and promoting healthy digestion and relaxation. It also has an antidepressant effect and helps keep inflammation in check.
VNS refers to any technique that stimulates the vagus nerve, including manual or electrical stimulation. Various forms of paced breathing can also influence brain electrical activity, which might be mediated by VNS arising from the diaphragm. This type of cardio-respiratory stimulation of the vagus nerve helps explain some of the positive emotional and cognitive benefits of aerobic exercise, deep breathing, and yoga.
In conventional VNS, a device is surgically implanted under the skin on the chest. When the device is activated, it sends electrical signals along the vagus nerve to the brain stem, which then sends signals to certain areas of the brain. Although vagus nerve stimulation can be invasive, new noninvasive methods, which don’t require surgical implantation, are now available in the United States for episodic cluster headaches (a rare but extremely painful primary headache disorder characterized by recurrent unilateral attacks). Noninvasive VNS devices have also been approved for use in Europe to treat depression, epilepsy, and pain. Although these have not yet been approved for use in the United States, they are currently being studied.
In a follow-up article, I will cover noninvasive methods you can try at home to stimulate the vagus nerve to improve vagal tone, so stay tuned.
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