Cranial Nerve Anatomy: the Vagus Nerve

Cranial nerves extend from the underside of the brain. There are 12 pairs. Each one has a roman numeral to identify it. Cranial nerves provide direct connection to the brain for special sensory organs such as muscles in the head, neck, and shoulders. They also innervate (supply with nerves) the heart and gastrointestinal tract. The cranial nerves, gastrointestinal tract, and their effect on digestion are the focus of this article.

 

There are a few cranial nerves that help with chewing, swallowing, and digestion. The oculomotor, trochlear, and abducens nerves (III, IV, VI) help with chewing. The glossopharyngeal nerve (IX) helps with swallowing. The hypoglossal nerve (XII) moves the tongue for swallowing. The major cranial nerve that plays a role in digestion is the vagus nerve (X). The vagus nerve travels from the base of the brain through the neck and torso. It carries information from organs to the brain. It also delivers parasympathetic signals for digestion to the organs.

 

As you can see, the vagus nerve seems to be a digestive catch-all. It provides the body with a sense of fullness. It functions to relax the stomach. This prepares the stomach for food. It also empties the stomach contents into the small intestines. The stomach contractions are also signaled by the vagus nerve. In conjunction with the parasympathetic branch of the autonomic nervous system, (PNS —> ANS —> Parasympathetic nervous system), the vagus nerve helps both contract and relax the muscles to facilitate emptying the stomach into the small intestines.

 

FUNDOPLICATION SURGERIES

 

Fundoplication surgeries are done to strengthen the valve between the esophagus and the stomach.  They are often performed under severe reflux conditions.  The upper portion of the stomach, called the fundus, is wrapped around the base of the esophagus and sewn into place.  This strengthens the connection.  As the stomach contracts to reflux or expel food, the stomach contractions squeeze the base of the esophagus.  This keeps food and acid in the stomach.  The surgery is often done laparoscopically.  However, this surgery comes dangerously close to critical cranial nerves facilitating the gastrointestinal tract.

 

The Vagus nerve (cranial nerve X) runs very close to the esophagus. During fundoplication surgeries, it is possible to damage this nerve. While that sounds absolutely horrible, it isn’t an all-or-nothing deal. There are varying degrees to which it can be affected. Damage certainly does not always mean severing the nerve. To make matters worse, it is not always possible to diagnose damage. Damage cannot always be seen.

 

SYMPTOMS OF VAGUS NERVE DAMAGE

 

  1. Gastroparesis is an extremely common symptom. This is where the stomach does not empty properly, allowing food to accumulate in the stomach. Gastroparesis must be managed with diet and medications. Over time, the body can learn to adapt to a new lifestyle that accommodates gastroparesis. Gastroparesis has no cure because of it’s autonomic-pathological causes.

 

  1. Nausea and vomiting (potentially from the early feeling of fullness from gastroparesis) are very common vagus nerve damage side effects. 

 

  1. Diarrhea is another common side effect. The diarrhea may improve over time.

 

  1. Gas bloating can also occur. Excess gas in the stomach can also irritate the nerve. With gastroparesis, food remains in the stomach for too long. As food is digested, air is released. As this air remains trapped in the stomach with the food, bloating can occur. This decreased gastric emptying can be caused by the vagus nerve, but then the built-up air can aggravate the nerve. It becomes a cyclical pattern that is hard to break.

 

  1. Reflux is also a symptom. Because of the stomach’s inability to hold larger amounts of food (from the backing up of gastroparesis), the stomach content may reflux back into the esophagus.

 

Since the vagus nerve relays messages back and forth to the stomach, as well as controlling the muscles that break down food and allow it to pass into the small intestines, it is easy to see how the slightest damage can cause large problems. There are many other symptoms that can occur. Since it is close in proximity to the esophagus, it is plain to see how easily it can be damaged during routine procedures. Gastric and esophageal (as any) surgeries create scar tissue when they heal. Scar tissue is much lighter in color, thicker, and harder to dissect. If repeat surgeries need to be done, the nerves are even harder to see. The more delicate tissues are then considerably more vulnerable to damage. The risks increase with each surgery.

 

MEDICATIONS (CHOLINERGIC AND ANTICHOLINERGIC)

 

Acetylcholine is a neurotransmitter (chemicals that transmit signals) that helps aid the movement of smooth muscles in the stomach, increasing gastric motility and digestion. Higher acetylcholine levels increase parasympathetic functions such as peristalsis (gastric contractions) and facilitate gastric emptying. There are both Cholinergic and Anticholinergic medications………

 

 

  • Cholinergic medications increase the amount of acetylcholine that can attach to cells. This increases the Parasympathetic nerve function, which facilitate stomach contractions and the digestion of food. So, this means that cholinergic medications increase smooth muscle in the stomach and increase stomach motility. When erythromycin is used in low doses to increase stomach contractions, it is working through a cholinergic pathway.  Cholinergic medications include bethanechol (Urecholine) and scopolamine (Hyoscine).

 

 

 

  • Anticholinergic medications block acetycholine in the CNS and PNS. They inhibit Parasympathetic nerve function by blocking acetylcholine from attaching to cells. As you’ll remember from above, Parasympathetic nerves aid the involuntary movement of smooth muscles in the gastrointestinal tract. Anticholinergic medications decrease the amount of acetylcholine. So, this means that anticholinergic medications decrease smooth muscle movement in the stomach and decrease stomach motility. These meds help ease nausea and relieve diarrhea. They are also able to ease pylorospams (exaggerated and inefficient contractions of the pylorus- the valve that empties the stomach into the small intestines).  Anticholinergic medications include diphenhydramine (Benadryl), and dicyclomine (Bentyl).

 

 

Cholinergic medications increase parasympathetic activity and increase gastric emptying while anticholinergic medications decrease parasympathetic activity and decrease gastric emptying, but also decrease nausea.

 

Nausea medications and gastric motility medications may work in opposite ways from one another. In most cases, one medication needs to take precedence over the other.  It is very important to review all of your medications with your doctors to make sure you are on a correct balance. I often find that different specialists prescribe medications without realizing the others may be canceling the effects. When it comes to medications of the Autonomic Nervous system (Sympathetic and Parasympathetic), it can be extremely difficult to keep things straight.

 

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Kathryn Rohr is a freelance writer from Canadian Pharmacy World specializes in digestive and nutritional health. She believes accurate and valuable information should be easy to find!

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