Enteric nervous system

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Enteric nervous system


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USMLE® Step 1 questions

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High Yield Notes

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Enteric nervous system

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USMLE® Step 1 style questions USMLE

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An investigator is studying the innervation of the gastrointestinal system. She discovers that the gastrointestinal tract has intrinsic innervations, specifically, the enteric nervous system, and extrinsic innervations; specifically, the sympathetic and parasympathetic innervations. Which of the following is most appropriate regarding the enteric nervous system?  


The gastrointestinal tract has intrinsic and extrinsic innervation.

The intrinsic component is the enteric nervous system and the extrinsic component is the sympathetic and parasympathetic innervation which come from the central nervous system.

The enteric nervous system can function independently to control digestive activities, which is why it’s sometimes called the second brain.

So the parasympathetic input basically enhances digestion, and sympathetic input inhibits digestion.

From the esophagus to the anus, the walls of the gastrointestinal tract are lined by the same four layers of tissue.

The outermost layer is either the adventitia, a thick fibrous connective tissue, or the serosa, a slippery serous membrane.

Next is the muscularis externa, a smooth muscle layer, which contracts automatically, without you even having to think about it. If we look closer at this muscle layer, it’s actually composed of an inner circular muscle layer, arranged in circular rings which contract and constrict the tract behind the food, which keeps it from moving backward, while the outer longitudinal muscle layer, arranged along the length of the tract, relaxes and lengthens and therefore pulls things forward. Together, they perform what’s called peristalsis, which is a series of coordinated wave-like muscle contractions that helps squeeze the food bolus in one direction.

In specific places along the tract, like the esophageal sphincter, the circular layer thickens, forming sphincters that keep food from passing from one part of the gastrointestinal tract to the next.

Next is the submucosa, which consists of a dense layer of tissue that contains blood vessels, lymphatics, and nerves.

Finally, there’s the inner lining of the intestine called the mucosa which secretes mucus and digestive enzymes because this is the layer that comes into direct contact with food.

The enteric nervous system is found within the walls of the entire gastrointestinal tract and is made up of two different plexuses.


The enteric nervous system (ENS), also known as the intrinsic nervous system, is a division of the autonomic nervous system that controls the gastrointestinal tract. It is a network of more than 100 million nerve cells (neurons) in the wall of the gut, from the esophagus to the anus. The ENS can operate independently from the brain and spinal cord, meaning that you can still digest food even if you're unconscious.

However, it is affected by the parasympathetic nervous system, which promotes digestion; and the sympathetic nervous system, which inhibits digestion. The ENS helps to regulate digestive functions such as digestion, absorption, and motility (movement of food through the gut). It also helps to control blood flow to and from the digestive organs, and t o activate immune cells in the gut.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Critical numbers of neural crest cells are required in the pathways from the neural tube to the foregut to ensure complete enteric nervous system formation" Development (2008)
  6. "Fundamentals of neurogastroenterology" Gut (1999)

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