Nervous System of the Digestive System

Enteric Nervous System

The enteric nervous system (ENS) is a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system.

Learning Objectives

Describe the structure and function of the enteric nervous system (ENS)

Key Takeaways

Key Points

  • The enteric nervous system (ENS), which is embedded in the lining of the gastrointestinal system, can operate independently of the brain and the spinal cord.
  • The ENS consists of two plexuses, the submucosal and the myenteric. The myenteric plexus increases the tone of the gut and the velocity and intensity of contractions. The submucosal plexus is involved with local conditions and controls local secretion, absorption, and muscle movements.
  • While described as a second brain, the enteric nervous system normally communicates with the central nervous system (CNS) through the parasympathetic (via the vagus nerve ) and sympathetic (via the prevertebral ganglia) nervous systems, but can still function when the vagus nerve is severed.
  • The ENS includes efferent neurons, afferent neurons, and interneurons, all of which make the ENS capable of carrying reflexes and acting as an integrating center in the absence of CNS input.
  • The ENS contains support cells, which are similar to the astroglia of the brain, and a diffusion barrier around the capillaries surrounding the ganglia, which is similar to the blood –brain barrier of cerebral blood vessels.

Key Terms

  • enteric nervous system: A subdivision of the peripheral nervous system that directly controls the gastrointestinal system.

Examples

The second brain of the enteric nervous system is the reason we get butterflies in our stomach or need to use the restroom more frequently when we are nervous and/or under stress.

The gastrointestinal (GI) system has its own nervous system, the enteric nervous system (ENS). Neurogastroenterology is the study of the enteric nervous system, a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system. The ENS is capable of autonomous functions such as the coordination of reflexes.

Although it receives considerable innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord. The ENS consists of some 100 million neurons, one-thousandth of the number of neurons in the brain, and about one-tenth the number of neurons in the spinal cord. The enteric nervous system is embedded in the lining of the gastrointestinal system.

Ganglia of the ENS

The neurons of the ENS are collected into two types of ganglia:

  1. The myenteric (Auerbach’s) plexus, located between the inner and outer layers of the muscularis externa
  2. The submucosal (Meissner’s) plexus, located in the submucosa

The Myenteric Plexus

The myenteric plexus is mainly organized as a longitudinal chains of neurons. When stimulated, this plexus increases the tone of the gut as well as the velocity and intensity of its contractions. This plexus is concerned with motility throughout the whole gut. Inhibition of the myenteric system helps to relax the sphincters —the muscular rings that control the flow of digested food or food waste.

The Submucosal Plexus

The submucosal plexus is more involved with local conditions and controls local secretion and absorption, as well as local muscle movements. The mucosa and epithelial tissue associated with the submucosal plexus have sensory nerve endings that feed signals to both layers of the enteric plexus. These tissues also send information back to the sympathetic pre-vertebral ganglia, the spinal cord, and the brain stem.

This is an illustration of neural control of the gut wall by the autonomic nervous system and the enteric nervous system. A sensory neuron is shown to stimulate the nerves in the submucosal and myenteric plexuses, which are connected to nerves in the sympathetic and parasympathetic nervous systems. The sensory neuron is also shown signal the ganglia and central nervous system.

Neural control of the gut: An illustration of neural control of the gut wall by the autonomic nervous system and the enteric nervous system.

Function and Structure of the ENS

The enteric nervous system has been described as a second brain. There are several reasons for this. For instance, the enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However, vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function.

In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input. For instance, the sensory neurons report mechanical and chemical conditions, while the motor neurons control peristalsis and the churning of intestinal contents through the intestinal muscles. Other neurons control the secretion of enzymes.

The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in the CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body’s serotonin is in the gut, as well as about 50% of the body’s dopamine, which is currently being studied to further our understanding of its utility in the brain.

The enteric nervous system has the capacity to alter its response depending on factors such as bulk and nutrient composition. In addition, the ENS contains support cells that are similar to the astroglia of the brain, as well as a diffusion barrier around the capillaries that surround the ganglia, which is similar to the blood–brain barrier of the cerebral blood vessels.

Regulation of ENS Function

The parasympathetic nervous system is able to stimulate the enteric nerves in order to increase enteric function. The parasympathetic enteric neurons function in defecation and provide a rich nerve supply to the sigmoid colon, the rectum, and the anus.

Conversely, stimulation of the enteric nerves by the sympathetic nervous system will inhibit enteric function and capabilities. Neurotransmitter secretion and direct inhibition of the enteric plexuses cause this stall in function. If the gut tract is irritated or distended, afferent nerves will send signals to the medulla of the brain for further processing.

Gastrointestinal Reflex Pathways

The digestive system functions via a system of long reflexes, short reflexes, and extrinsic reflexes from gastrointestinal (GI) peptides that work together.

Learning Objectives

Differentiate among the gastrointestinal reflex pathways

Key Takeaways

Key Points

  • Long reflexes to the digestive system involve a sensory neuron that sends external or internal digestive information to the brain. This type of reflex includes reactions to food, emotion, or danger.
  • Short reflexes to the digestive system provide shortcuts for the enteric nervous system (ENS) to act quickly and effectively, and form a sort of digestive brain. It reacts to digestive movement and chemical changes.
  • The enterogastric reflex is stimulated by the senses. This reflex releases acid in the duodenum or in the stomach, and suppresses the release of digestive proteins.
  • The gastrocolic reflex increases movement in the gastrointestinal tract, and reacts to stretches in the stomach walls as well as in the colon. It is responsible for the urge to defecate, the movement of digested material in the small intestine, and it makes room for more food within the stomach.
  • The gastroileal reflex works with the gastrocolic reflex to stimulate the urge to defecate. It does so by opening the ileocecal valve and moving the digested contents from the ileum of the small intestine into the colon for compaction.
  • GI peptides act on a variety of tissues including the brain, the digestive accessory organs, and the GI tract.

Key Terms

  • gastrocolic reflex: One of the three extrinsic physiological reflexes that control the motility or peristalsis of the gastrointestinal tract; it involves an increase in the motility of the colon, creates the urge to defecate along with the gastroileal reflex, and helps make room for food in the stomach.
  • enterogastric reflex: One of the three extrinsic reflexes of the gastrointestinal tract that is stimulated by the presence of acid levels in the duodenum or in the stomach. It releases acids and controls the release of stomach proteins such as gastrin.
  • gastroileal reflex: One of the three extrinsic reflexes of the gastrointestinal tract that works with the gastrocolic reflex to stimulate the urge to defecate. This reflex is stimulated by the opening of the ileocecal valve and moves the digested contents from the ileum of the small intestine into the colon for compaction.

Examples

The gastrocolic reflex can cause irritable bowel syndrome. This can lead to abdominal pain, diarrhea, or constipation.

Food in the Digestive System

The digestive system has a complex system of food movement and secretion regulation, which are vital for its proper function. Movement and secretion are regulated by long reflexes from the central nervous system (CNS), short reflexes from the enteric nervous system (ENS), and reflexes from the gastrointestinal system (GI) peptides that work in harmony with each other.

In addition, there are three overarching reflexes that control the movement, digestion, and defecation of food and food waste:

  1. The enterogastric reflex
  2. The gastrocolic reflex
  3. The gastroileal reflex

Long and Short Reflexes

Long reflexes to the digestive system involve a sensory neuron that sends information to the brain. This sensory information can come from within the digestive system, or from outside the body in the form of emotional response, danger, or a reaction to food.

These alternative sensory responses from outside the digestive system are also known as feedforward reflexes. Emotional responses can also trigger GI responses, such as the butterflies in the stomach feeling when nervous.

Control of the digestive system is also maintained by enteric nervous system (ENS), which can be thought of as a digestive brain that helps to regulate motility, secretion, and growth. The enteric nervous system can act as a fast, internal response to digestive stimuli. When this occurs, it is called a short reflex.

Three Main Types of Gastrointestinal Reflex

The Enterogastric Reflex

The enterogastric reflex is stimulated by the presence of acid levels in the duodenum at a pH of 3–4 or in the stomach at a pH of 1.5. When this reflex is stimulated, the release of gastrin from G- cells in the antrum of the stomach is shut off. In turn, this inhibits gastric motility and the secretion of gastric acid (HCl). Enterogastric reflex activation causes decreased motility.

The Gastrocolic Reflex

This is an animated diagram of the gastrocolic reflex, one of a number of physiological reflexes that control the motility, or peristalsis, of the gastrointestinal tract. The bolus is seen descending the tube-like esophagus, as circular muscle contraction and relaxation move it down.

Peristalis: The gastrocolic reflex is one of a number of physiological reflexes that control the motility, or peristalsis, of the gastrointestinal tract.

The gastrocolic reflex is the physiological reflex that controls the motility, or peristalsis, of the gastrointestinal tract. It involves an increase in motility of the colon in response to stretch in the stomach and the byproducts of digestion in the small intestine. Thus, this reflex is responsible for the urge to defecate following a meal. The small intestine also shows a similar motility response. The gastrocolic reflex also helps make room for food in the stomach.

The Gastroileal Reflex

The gastroileal reflex is a third type of gastrointestinal reflex. It works with the gastrocolic reflex to stimulate the urge to defecate. This urge is stimulated by the opening of the ileocecal valve and the movement of the digested contents from the ileum of the small intestine into the colon for compaction.

GI Peptides that Contribute to Gastrointestinal Signals

GI peptides are signal molecules that are released into the blood by the GI cells themselves. They act on a variety of tissues that include the brain, the digestive accessory organs, and the GI tract.

The effects range from excitatory or inhibitory effects on motility and secretion, to feelings of satiety or hunger when acting on the brain. These hormones fall into three major categories:

  1. The gastrin family
  2. The secretin family
  3. A third family that is composed of the hormones that do not fit into either of these two families