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.
Each plexus is a weblike formation of neurons that expands throughout the gastrointestinal tract. This web is composed of ganglia which are collections of neurons, and for each ganglion, there are multiple interneurons that connects that ganglia to other ganglia.
Some interneurons even travel from the submucosal plexus to the myenteric plexus to connect the ganglia of the two plexus! You can think of ganglia like dense cities connected by highways.
Now when we eat a hamburger, it gets chewed up and enters the gastrointestinal tract. As it works its way through, the food distends the walls of the gastrointestinal tract, which stimulates mechanoreceptors in the muscularis externa and the mucosa.
The food also increases the levels of protein, fat, sugar in the gastrointestinal tract which stimulates chemoreceptors in the mucosa.
Both mechanoreceptors and chemoreceptors send afferent information through axons that synapse with ganglia in the submucosal plexus.
The information then travels through the plexus from ganglion to ganglion via interneurons.
The submucosal ganglia then send signals through efferent fibers that stimulate various glands and enteroendocrine cells that helps with digestion and absorption. These ganglia also cause blood vessels to vasodilate and that brings increases blood flow to the gastrointestinal tract.
The ganglia in the myenteric plexus are in charge of gastric motility, and they send signals through efferent fibers to the smooth muscles within the circular and longitudinal layers. This causes an increase in the number and strength of peristaltic contractions. Overall, this is referred to as a short reflex - because both the afferent and efferent components are mediated by the enteric nervous system.
Now, the central nervous system can also receive afferent information from, and send efferent information to the gastrointestinal tract via sympathetic and parasympathetic systems, and this is called the long reflex.
When the body is in “rest and digest” mode, the parasympathetic nervous system is active and it enhances digestion.