Definition and Overview
The enteric nervous system (ENS) is a unique neural network intrinsic to the gastrointestinal wall, containing approximately 200-600 million (average approximately 500 million) neurons [1]. This is comparable to the number of neurons in the spinal cord, and it can autonomously regulate digestive function without direct commands from the central nervous system. Due to this independence, Gershon designated the ENS as the "second brain" in 1999 [2].
The ENS is distributed throughout the entire gastrointestinal tract from esophagus to anus and utilizes more than 20 types of neurotransmitters [1]. These include serotonin, acetylcholine, nitric oxide, dopamine, substance P, and others. The fact that approximately 95% of the body's serotonin is produced in the gut demonstrates the neurochemical significance of the ENS [3].
While the autonomic nervous system has traditionally been classified into the sympathetic and parasympathetic divisions, the ENS is separately classified as a third division of the autonomic nervous system due to its complexity and independence [1]. The ENS possesses complete reflex circuits comprising sensory neurons, interneurons, and motor neurons, enabling it to independently process sensory input and generate motor output.
Anatomical Structure
The ENS is arranged within the gastrointestinal wall as two major plexuses. Each plexus consists of ganglia (clusters of neuronal cell bodies) connected by nerve fiber bundles [1].
Auerbach Plexus
The Auerbach plexus, also known as the myenteric plexus, is located between the longitudinal muscle layer and the circular muscle layer of the gastrointestinal tract. It is continuously distributed throughout the entire GI tract from esophagus to anus and primarily regulates gastrointestinal motor function [1].
Motor neurons of the Auerbach plexus are divided into excitatory and inhibitory types. Excitatory neurons release acetylcholine and substance P to contract muscles, while inhibitory neurons release nitric oxide and vasoactive intestinal peptide (VIP) to relax muscles. These two types of neurons work cooperatively to produce the contraction-relaxation pattern of peristalsis [1].
Meissner Plexus
The Meissner plexus, also known as the submucosal plexus, is located in the submucosal layer. It is well developed in the small and large intestines and primarily regulates mucosal secretory function and blood flow [1].
Sensory neurons of the Meissner plexus detect chemical composition and mechanical stimuli within the intestinal lumen. Based on this information, they regulate the secretion of digestive enzymes, mucus, and electrolytes and modulate mucosal blood flow. They are responsible for creating the microenvironment necessary for nutrient absorption [1].
Enteric Glial Cells
In addition to neurons, the ENS contains enteric glial cells that outnumber neurons by a factor of 4 to 7 [4]. Enteric glial cells perform functions similar to astrocytes in the central nervous system, including providing trophic support to neurons, regulating neurotransmitter metabolism, and maintaining the intestinal barrier. Accumulating evidence from recent studies indicates that enteric glial cell dysfunction is associated with intestinal inflammation and functional gastrointestinal disorders [4].
Function
Regulation of Peristalsis
The most essential function of the ENS is the regulation of peristalsis. When food stimulates the intestinal wall, sensory neurons detect this stimulus and transmit signals to motor neurons via interneurons. Circular muscles above the food bolus contract while those below relax, propelling contents toward the anus -- this is called the peristaltic reflex [1].
This process is completed entirely by local reflex circuits within the ENS and occurs autonomously without central nervous system involvement. Animal experiments have confirmed that peristalsis is maintained normally even in intestinal segments with all external neural connections severed [1]. In addition to peristalsis, the ENS programs various motor patterns including segmentation and the migrating motor complex of the small intestine.
Secretory Regulation
Centered on the Meissner plexus, the ENS regulates digestive secretion. Secretion of gastric acid, bile, pancreatic juice, intestinal fluid, and mucus is precisely adjusted according to the nutrient composition and acidity of the intestinal lumen [1]. When glucose concentration in the intestinal lumen rises, ENS sensory neurons detect this and trigger a reflex that induces secretion of incretin, a hormone that stimulates insulin release [1].
The ENS also regulates water and electrolyte secretion and absorption in the intestinal mucosa. The mechanism by which cholera toxin causes severe diarrhea involves hyperactivation of the ENS secretory reflex [1].
Mucosal Blood Flow Regulation
Vasomotor neurons of the ENS regulate blood flow in small arteries and arterioles of the submucosal layer. During digestion, they selectively increase blood flow to areas where nutrient absorption is most active [1]. Nitric oxide and VIP serve as the primary vasodilatory neurotransmitters, optimizing oxygen and nutrient delivery to the mucosa through local blood flow regulation.
Immune Regulation
The ENS interacts closely with gut-associated lymphoid tissue (GALT). Approximately 70% of the body's immune cells are located in the intestinal mucosa, and bidirectional communication mediated by neurotransmitters occurs between ENS neurons and immune cells [4]. The ENS also plays a role in defending against pathogen invasion by regulating mucosal barrier permeability and modulating inflammatory responses in the intestinal mucosa.
Gut-Brain Axis
Bidirectional Communication Pathways
The gut-brain axis is a bidirectional communication system between the ENS and the central nervous system. The vagus nerve is the key pathway of this communication, with a significant portion of the ENS's sensory signals being transmitted to the nucleus tractus solitarius in the brainstem via vagal afferent fibers [3].
Top-down signals from the brain reach the ENS via vagal efferent fibers. When the hypothalamic-pituitary-adrenal (HPA) axis is activated under stress, cortisol is released, causing increased intestinal permeability, altered gut motility, and changes in gut microbiota composition [3]. This mechanism explains phenomena such as exam-related stress causing abdominal pain and diarrhea, and the "butterflies in the stomach" sensation during tension.
Gut Microbiota and the ENS
The gut microbiota is a key component of the gut-brain axis. Intestinal bacteria produce neuroactive substances including short-chain fatty acids, tryptophan metabolites, and gamma-aminobutyric acid (GABA), which directly stimulate sensory neurons of the ENS [5].
Germ-free animal studies have confirmed that mice lacking gut bacteria have incomplete ENS development and reduced gastrointestinal motility [5]. Administration of specific probiotics (e.g., Lactobacillus rhamnosus) has been reported to alter GABA receptor expression in the brain via the vagal pathway and reduce anxiety behavior [5]. This effect was absent in animals with vagotomy, confirming that the vagus nerve is an essential communication pathway between gut bacteria and the brain.
Serotonin and Gut-Brain Communication
Approximately 95% of the body's serotonin is synthesized in enterochromaffin cells of the intestinal mucosa [3]. Serotonin released by enterochromaffin cells acts on sensory nerve terminals of the ENS to trigger peristalsis, secretion, and nausea reflexes. Simultaneously, it transmits visceral sensory information such as satiety and nausea to the brain via vagal afferent fibers [3].
Abnormalities in the serotonin signaling system of the intestinal mucosa have been identified in patients with irritable bowel syndrome (IBS), and drugs targeting serotonin receptors (5-HT3 receptor antagonists, 5-HT4 receptor agonists) are used in IBS treatment [3].
ENS Abnormalities and Related Conditions
Irritable Bowel Syndrome
Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits without organic lesions, affecting approximately 11% of the global population [3]. Visceral hypersensitivity of the ENS is one of the key pathophysiological mechanisms. In IBS patients, rectal balloon distension testing elicits pain at lower volumes than in healthy individuals, reflecting lowered thresholds of ENS sensory neurons [3].
Aberrant gut-brain axis communication also contributes to the onset and maintenance of IBS. IBS patients show exaggerated HPA axis responses to stress, and brain imaging studies have confirmed altered activity in visceral sensory processing regions [3]. Based on this evidence, IBS is currently being redefined as a "disorder of gut-brain interaction."
Functional Dyspepsia
Functional dyspepsia is a condition characterized by recurrent epigastric pain, early satiety, and postprandial bloating without organic cause, with a worldwide prevalence of approximately 10-30% [3]. Abnormalities in sensory and motor function of the gastric ENS are involved, with impaired gastric accommodation and delayed gastric emptying being the primary mechanisms. Gastric accommodation is a reflex in which the proximal stomach relaxes after meals to receive food, mediated by inhibitory motor neurons of the ENS [1].
Parkinson's Disease and the ENS
Alpha-synuclein aggregates, the pathological hallmark of Parkinson's disease, are also found in the ENS. According to Braak's hypothesis, alpha-synuclein pathology may originate in the ENS and propagate in an ascending manner to the brainstem via the vagus nerve [6]. Supporting this, epidemiological studies have shown that patients who underwent vagotomy had an approximately 40% reduced risk of developing Parkinson's disease [6].
Constipation is observed 10-20 years before the onset of motor symptoms in more than 80% of Parkinson's disease patients, suggesting early ENS involvement [4]. Alpha-synuclein deposition has been confirmed in colonic mucosal biopsies even in pre-motor stage patients, and ENS pathology is being investigated as an early diagnostic biomarker for Parkinson's disease.
Congenital Megacolon
Hirschsprung disease is the prototypical condition characterized by congenital absence of ENS neurons. During development, neural crest cells fail to migrate to the distal colon, resulting in aganglionosis of the affected segment [1]. The aganglionic segment cannot relax, causing functional bowel obstruction, occurring in approximately 1 in 5,000 neonates [4]. Surgical resection of the aganglionic segment is the standard treatment.
Diabetic Gastrointestinal Disorders
Chronic hyperglycemia causes neurodegeneration in the ENS. At least one gastrointestinal symptom is reported in approximately 75% of diabetic patients, with gastroparesis being the most representative [4]. Oxidative stress caused by hyperglycemia damages ENS neurons and enteric glial cells, with loss of inhibitory motor neurons being particularly prominent [4].
Testing and Evaluation
A standardized single test for directly evaluating the ENS has not yet been established, but indirect assessment is possible through the following methods.
- Autonomic function testing: Comprehensive evaluation of autonomic function including the vagus nerve through heart rate variability (HRV) analysis, deep breathing test, and Valsalva maneuver. Reduced high-frequency (HF) component of HRV reflects decreased vagal tone, which may be associated with gut-brain axis dysfunction [3].
- Gastrointestinal motility testing: Gastric emptying scintigraphy quantitatively measures gastric emptying rate to diagnose gastroparesis. Colonic transit study uses radiopaque markers to evaluate colonic transit time.
- Anorectal manometry: Measures pressure in the rectum and anus to evaluate defecatory function. In Hirschsprung disease, absence of the rectoanal inhibitory reflex is a characteristic finding [1].
- Intestinal mucosal biopsy: Performed for research purposes to evaluate morphological changes in ENS neurons and alpha-synuclein deposition [6].
- Quantitative EEG (QEEG): An adjunctive tool for evaluating the central nervous system aspect of the gut-brain axis, which can be used to identify brain functional changes associated with autonomic dysfunction.
Lifestyle Management
The following lifestyle guidelines are recommended for maintaining ENS health and optimizing gut-brain axis balance.
- Regular meals: Eating at consistent times regularizes the migrating motor complex rhythm of the ENS. Eating hastily or irregularly can cause gastrointestinal motility disorders [1].
- Dietary fiber intake: Consuming 25-30 g of dietary fiber daily is essential for maintaining gut motility and preserving gut microbiota diversity. Dietary fiber is converted to short-chain fatty acids by gut bacteria, providing energy to ENS neurons [5].
- Fermented food consumption: Fermented foods such as kimchi, yogurt, and miso contribute to gut microbiota balance by supplementing beneficial bacteria. Clinical studies have reported that probiotics can alleviate anxiety and stress responses through the gut-brain axis [5].
- Slow diaphragmatic breathing: Breathing at 6 breaths per minute (4-second inhalation, 6-second exhalation) activates the vagus nerve and improves gut function. Studies have shown significant reduction in abdominal pain and constipation symptoms in IBS patients after 4 weeks of breathing training [3].
- Regular aerobic exercise: Walking, swimming, cycling, etc. for 30 minutes or more, 3-5 times per week promotes gut motility and increases vagal tone. Research has also shown that regular exercise increases gut microbiota diversity [5].
- Stress management: Chronic stress increases intestinal permeability and disrupts ENS function through HPA axis hyperactivation. Meditation, yoga, and relaxation training can help restore gut-brain axis function [3].
- Adequate sleep: Regular sleep of 7-8 hours is essential for autonomic balance restoration. Sleep deprivation increases intestinal permeability and alters gut microbiota composition.
If symptoms persist or worsen, or if chronic digestive symptoms remain unexplained by routine gastrointestinal testing, specialist evaluation including autonomic function assessment should be pursued.