The Anatomy of the Enteric Nervous System

The enteric nervous system (ENS) is located in the gastrointestinal tract. It is a system of sensory neurons, motor neurons, and interneurons that extends from the esophagus to the rectum. It is a part of the autonomic nervous system (ANS), which also includes the sympathetic nervous system and the parasympathetic nervous system.

The ANS is responsible for the involuntary processes in the body. The ENS is the part of the ANS that regulates the digestive functions of the gastrointestinal tract. It receives internal input from the gastrointestinal tract and external input from the brain and other parts of the ANS to regulate digestion.

Anatomy

Even though the ENS is located in the digestive tract, it is sometimes called the "second brain." Broadly speaking, "enteric" refers to the intestine or the small intestine.

The ENS works with the help of the central nervous system (CNS), but it also can carry out some of its functions in the digestive process without communicating with the brain.

Structure

The ENS is a large, mesh-like network. It contains somewhere between 200 and 600 million neurons. There are three different types of neurons in the ENS: efferent (motor), afferent (sensory), and interneurons.

Efferent nerves carry messages from the CNS to other organs and are responsible for peristalsis, the wave-like movement of the smooth muscles in the digestive tract that controls the propulsion of food along its length.

Afferent nerves react to what a person eats and drinks and carry their messages away from the digestive tract and toward the CNS.

Location

In adults, the ENS can be about 30 feet long because it extends all the way through the abdomen from the esophagus to the rectum. The neurons of the ENS are grouped together in thousands of clusters called ganglia that are largely contained in two main networks: the myenteric plexus and submucosal plexus.

The myenteric plexus contains primarily efferent neurons and surrounds the digestive organs between the lower part of the esophagus and the anus. The submucosal ganglia contain primarily sensory neurons and form plexuses located inside the small and large intestines.

Anatomical Variations

There are several different types of diseases and conditions that affect the ENS. There are congenital disorders (conditions one is born with) that affect parts of the ENS and can cause difficulties with digestion.

There is a wide range in the severity of these conditions, which can be anything from sporadically symptomatic to those that are progressive in nature and lead to severe disability. The ENS is also subject to disease and injury. It can be affected by gastrointestinal disorders, by inflammation, or by other conditions such as diabetes.

Function

The ENS is responsible for various aspects of the digestive process in the esophagus and the intestines. It is sophisticated in its response to the changing needs of digestion, such as with diet alterations or if a virus or bacteria affects the gastrointestinal tract.

It connects the brain and the digestive system through its afferent and efferent nerves, which pass messages back and forth between the CNS and the ENS. The ENS controls secretions, blood flow, hormone release, and motility (the movement of food through the digestive tract), all of which are part of the digestive process.

Cells called interstitial cells of Cajal are located in the ENS between muscle layers in the digestive tract. These cells coordinate peristalsis, or the slow contractions of the smooth muscle in the digestive tract that moves food and waste along.

The ENS also regulates the response to the food and drink that are taken in. For instance, the ENS can respond with vomiting and diarrhea in case of food poisoning by bacteria or viruses.

The microbiota, which is the makeup of the bacteria, viruses, and fungi found in the digestive tract, may also have an effect on the ENS. Changes to the microbiota may influence how the ENS behaves to regulate digestion.

However, the CNS also controls various parts of the digestive system and its function, including stomach secretions and voluntary bowel movements.

Associated Conditions

The ENS has control over a significant portion of the digestive system. For that reason, injury or disruption to the neurons there can affect digestion. Several different conditions have been identified as being caused in whole or in part due to a problem with the ENS.

Hirschsprung’s Disease

Hirschsprung’s disease is a rare congenital condition in which some of the ENS ganglia don't develop in the last part of the large intestine. This leaves the bowel unable to move food through part of the intestine and complete a bowel movement. The most common treatment is to remove the part of the bowel that is involved.

Esophageal Achalasia

Esophageal achalasia is a rare autoimmune condition of the esophagus. The nerves of the myenteric ganglia are attacked by the body's immune system and become less able to function. This results in the inability to swallow food easily.

There is a wide range in the symptoms and the seriousness of this disease. Symptoms include the inability to swallow, coughing, heartburn, regurgitation, chest pain, and vomiting.

Treatment can include balloon dilation (which is effective in the majority of cases), medications (such as nitrates or calcium channel blockers), and surgery to cut the fibers of the lower esophageal sphincter.

Gastroparesis

Gastroparesis means "paralyzed stomach." This describes this disease's main sign—that food doesn't empty from the stomach in a timely way. The cause of gastroparesis is unknown in almost half of cases, but it can also be associated with diabetes. There is some research that another cause could be a result of inflammation.

It is thought that changes in the neurons of the ENS (whether from disruption or inflammation is still under investigation) may lead to the development of gastroparesis. Treatment can include managing diabetes (in those cases for which it is a contributing factor), diet changes, medication, tube feeding, and electrical stimulation.

Chronic Idiopathic Intestinal Pseudo-Obstruction (CIIP)

CIIP is a rare disorder where the bowel behaves as though it is obstructed, but there is no mechanical reason found for the obstruction. This condition can be present at birth, or it can progress over time. There are several different forms of CIIP. The cause is not always identified. In some cases, CIIP may be caused by damage to the ENS.

There is no one particular treatment for all individuals who live with CIIP. Instead, management is often focused on alleviating symptoms of the disease, such as nausea, vomiting, pain, abdominal distention, and constipation.

Treatment can include managing any underlying disease and medications such as antibiotics, antiemetics, prokinetics, and tricyclic antidepressants. Partial or total parenteral nutrition may also be needed. Surgery to decompress the bowel, bowel resection, or (very rarely) intestinal transplants may also be used when needed.

Rehabilitation

The ENS is vital to the proper function of the gastrointestinal tract. When there is insult or injury to it, the digestive process suffers. The ability to affect the ENS in a positive way to treat digestive disorders is an ongoing area of research.

Stem cell therapy, in particular, is being studied to rehabilitate the ENS when it becomes damaged. There are also various medications that act on the ENS to alter how it functions by causing it to, for instance, release different levels of hormones or digestive secretions.