Our gut has its own nervous system consisting of 100 million neurons, and it is known as the enteric nervous system (ENS). ENS consists of the myenteric plexus and submucosal plexus. It is mostly developed in humans among other vertebrates, functioning under intrinsic and extrinsic control through the sympathetic nervous system and the parasympathetic nervous system. Sympathetic control is through the thoracolumbar outflow (T5 to L2/3) and parasympathetic control is through the vagus nerve.
The vagus nerve is a mixed nerve containing both motor and sensory fibers; it is the 10th cranial nerve, responsible for the functions of most of the viscera of the thorax and the abdomen. One of it’s most important functions includes the innervation of the gastrointestinal tract from the throat up to the proximal 2/3 of the transverse colon. The vagus has central connections to the tractus solitarius, dorsal nucleus of the vagus, and the nucleus ambiguous. Fibers from the dorsal nucleus of the vagus form the primary connection with the gut through the parasympathetic nervous system. Preganglionic parasympathetic nerve fibers from here go directly to the gut and relay on the intramural ganglionic cell bodies, which form the postsynaptic parasympathetic nerves.
The vagus nerve contributes to the taste sensation from the posterior oropharyngeal wall and takes this information to the tactus solitarius. Through general visceral efferent fibers (GVE), it also innervates the myenteric nerve plexus which is responsible for the motility (peristalsis) of the gut by contraction and relaxation of muscularis externa. It also controls the submucosal plexus responsible for the secretory function of the submucosa, dilation of blood vessels in the submucosa, and contraction of smooth muscle fibers in the muscularis interna to release various substances from glands in the submucosa. The GVE fibers are centrally connected to the dorsal nucleus of the vagus, situated in the medulla.
Another function of the vagus related to the gut is through its visceral sensory connections. These connections receive sensations from the stretch receptors of the gut through general visceral afferent fibers of the vagus and play a key role in the initiation of the extrinsic control of the gut. These stretch receptors are also involved in the vasovagal reflex and enterogastric reflex.
The vagus and a healthy gut
After learning about the function of the vagus related to the gastrointestinal tract, it is obvious that it is necessary to maintain our gut health. The vagus is an important nerve for our digestive functions, including the secretory, peristaltic, sphincter functions, and even absorptive functions. The vagus contributes to the swallowing process of food and helps us push the bolus through by the contraction of the muscles and a decrease in the sphincter tone.
Activation of the vagus nerve causes an increase of secretions, including HCl secretion in the stomach, pancreatic juices from the pancreas, bile from the gallbladder, and promoting the absorption of the digested food by increasing the blood flow to the submucosa of intestines.
Practical implications of the vagus nerve
The vagus nerve is involved in certain reflexes, including the emetic reflex, gag reflex, swallowing reflex, and vasovagal reflex. The swallowing reflex is the one we experience daily during deglutition, in which the bolus is pushed into the back of the mouth, and then travels through the esophagus. In this regard, the dysfunction of this reflex can lead to dysphagia.
The gag reflex is induced by touching the posterior oropharyngeal wall with an object, which suddenly induces a gag movement. It is involuntary and sometimes causes vomit. The sensory component of the gag reflex is made by glossopharyngeal nerve(9th cranial nerve), and the motor component is composed of the vagus nerve. This reflex can be used to check the functions of both the 9th and 10th cranial nerves in the clinical practice.
The emetic reflex is also known as the vomiting reflex. In this case, there’s an expulsion of the the chyme from the proximal duodenum and the gastric contents out of the body through the mouth. It is sometimes facilitated by the gag reflex, but there are many other triggers. The vomiting reflex activates in response to many factors, one of which is an overstimulated vagus nerve. This overstimulation can be due to certain food, toxins, certain chemicals, and drugs. One of the important over-stimulator drugs includes chemotherapeutic agents, which is why vomiting is one of the major problems faced by people undergoing chemotherapy.
Dysfunctional vagus and gut health
Lesions of the vagus are more common than any other cranial nerve because of its long route through different parts of the body from the cranial cavity through the neck, thorax, upper abdomen, and all the way through the proximal transverse colon. When there’s an excessive function of the vagus, it leads to excessive peristalsis, excessive secretions, and progressive problems that include peptic ulcer disease and diarrhea.
Parietal cells in the stomach have receptors stimulated by the vagus to produce Hydrochloride acid, which causes stomach ulcers in excess. When there’s also a decreased pyloric sphincter tone, this acid can also cause ulcers in the duodenum. People who use non-steroidal anti-inflammatory drugs are also prone to get peptic ulcers because the mucosal protective function of the stomach is lost, and the parietal cells keep on secreting hydrochloride acid.
Another problem related to the vagus nerve is that a decreased function can lead to reduced gut motility, and the patient ends up with constipation. The vagus is responsible for the motility of the esophagus and inadequate function can lead to a condition known as dysphagia, which means not being able to swallow properly. Gastro-oesophageal reflux disease is another problem that may be related to the vagus, in which there’s decrease tone of the lower oesophageal sphincter, which leads to the reflux of gastric contents into the mouth, or more dangerously into the trachea.
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