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 submucosal or Meissner’s plexus is found in the submucosa and the myenteric or Auerbach’s plexus, is found within the muscularis externa, between the longitudinal muscle layer and the circular muscle layer.