The Enteric Nervous System

The Gut Brain

The digestive system has its own nervous systemThe organ system that controls body functions using electrical and chemical signals..  Seriously.  Some textbooks are now starting to include the gut-brain. They consider the digestive system’s nervous system as its own branch of the nervous system.  The gut-brain IS its own brain.  There is a lot of sensory info. It’s not sent to the brain. Instead, it is sent to other cellsThe basic structural and functional units of life. located in the area.  For example, chemoreceptors in the stomach connect to muscularis externaA smooth muscle layer of the digestive tract responsible for peristalsis and segmentation. smooth muscle fibers also in the stomach.  This is what is called a short reflex.  The brain is not involved.  There are also pacemaker cells in certain parts of the digestive tract.  Much like the heart, these cells do their own thing. However, they can choose to pay attention to signals coming from the brain.  When the brain is involved, we call it a long reflex. 

Myenteric Plexus

The myenteric nerve plexus is located between the two layers of smooth muscle found in the muscularis externa. It is a network of nerves controlling motility. This nerve plexus contains autonomic motor neuronsNeurons that carry outgoing information from the CNS to effector organs., as the contractions are not consciously controllable. Another nerve plexus, the submucosal plexusA network of nerves in the submucosa that regulates glandular secretions and blood flow in the diges, is located in the submucosa layer. It contains both sensory and motor neuronsThe functional cells of the nervous system that transmit signals., sending information to the brain (e.g., about pHA measure of hydrogen ion concentration in a solution. levels) and controlling the glands of the submucosa and mucosaThe innermost lining of the digestive tract that contains mucus-secreting cells for protection and a. Together, these plexuses regulate muscle contractions and glandular secretions within the digestive system.

The myenteric plexus, also known as Auerbach’s plexus, is sandwiched between the two layers of the muscularis externa.  It’s location allows it to innervateTo invade a tissue with nervous tissue is to innervate it. the two layers of smooth muscle of the muscularis externa.  This is a prime location for controlling peristalsis Rhythmic contractions of smooth muscle that move food through the digestive tract. and segmentation.  The myenteric plexus is never inactive.  It is constantly sending action potentials to maintain a low level of contraction.  This way, the muscle is ready whenever it is needed.  The myenteric plexus does not operate in isolation but interacts with extrinsic inputs from the autonomic nervous systemThe part of the peripheral nervous system that controls involuntary functions such as heart rate, di. Parasympathetic fibers, like those from the vagus nerve, generally promote motility and digestion. Sympathetic fibers inhibit motility and serve as a protective mechanism under stress or danger.

Gastroparesis is characterized by delayed gastric emptying. It is often caused by damage to the myenteric plexus. This damage results in impaired gastric motility. This condition is commonly seen in diabetes and can lead to nausea, bloating, and early satiety.

Irritable Bowel Syndrome (IBS) is a dysfunction in the coordination of motility. It is regulated by the myenteric plexus. This coordination issue leads to symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). like abdominal cramping. People with IBS may experience altered bowel movements, such as diarrhea, constipation, or alternating patterns, and bloating.

Submucosal Plexus

The submucosal plexus, also known as Meissner’s plexus, controlsVariables that remain constant to ensure a fair test. the secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine. of digestive enzymesProteins that speed up chemical reactions in the body.. It also manages fluid release from various glands (such as Brunner’s glands in the duodenumThe first section of the small intestine where most digestion occurs; it receives chyme, bile, and p). It responds to local stimuliChanges in the environment that are detected by sensory receptors., like the presence of food. It also adjusts secretion levels based on neural inputs for digestion and absorption. It helps regulate local blood flow to the gastrointestinal mucosa by influencing the smooth muscle tone of the arteriolesSmall arteries that regulate blood flow into capillaries through vasoconstriction and vasodilation. The myenteric plexus controls more motor functions such as peristalsis. The submucosal plexus contributes to more localized movements, like villous contractions. These movements are important for the mixing and absorption of chymeA semi-fluid mixture of partially digested food and digestive juices that moves from the stomach int. It helps coordinate fine motility patterns that enhance nutrient absorption at the mucosal surface.

Inputs and Outputs

There are sensory inputs to the brain. There are also motor outputs. This is like any system in your body controlling any type of muscle or gland. But it’s quite complicated with the GI tract. There are baroreceptors that sense stretching in the GI tract as food enters different segmentsThe subdivisions of lung lobes, supplied by tertiary bronchi.. ChemoreceptorsSensory receptors that detect chemical stimuli, such as odors or blood pH. are present throughout your GI tract to sense pH changes. As food is digested, it generally becomes more acidicA solution with a pH below 7, having a higher concentration of H⁺ ions.. There are chemoreceptors in your digestive tract that look for end products of digestion. They look for amino acids—have we broken down all of the proteinsLarge molecules made of amino acids with various functions in the body.? They look for fatty acids—have we broken down all of our fats? There are chemoreceptors that sense osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments.. Have we released tons of ionsCharged atoms or molecules. into our little slurry or chyme? Does it have a high presence of ions? All of these things are just a few of the many sensory inputs that are involved in digestive control. 

Outputs only control two things: smooth muscle via the myenteric plexus or glands via the submucosal plexus.  When parasympathetic fibers from the vagus nerve secrete acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction., we active the digestive system in our rest and digest phase.  Parasympathetic fibers from the myenteric plexus secrete acetylcholine. This action causes the smooth muscle of the muscularis externa to contract. It contracts in peristalsis and segmentation.

Central Gastric Reflexes

The digestive system also involves reflexesAutomatic responses to stimuli. like the gastroenteric reflex, which stimulates stomach motility and small intestine secretion. Another reflex, vomiting, occurs when the brain detects irritation and relaxes the sphincters, allowing food to flow backward. The gastroileal reflex, often humorously called “making room for more,” moves food from the small to the large intestine. It sometimes stimulates bowel movements.