The enteric nervous system (ENS) controls the digestive system, connecting through the central nervous system (CNS) and sympathetic nervous system.
In particular, the enteric nervous system determines the movements of the gastrointestinal tract, regulates gastric acid secretion, changes in local blood flow and the gut hormones release, and interacts with the immune system in the gut. It forms during the last few months of gestation (in humans) and continues to develop after birth.
Similar to the other peripheral nervous systems, the enteric nervous system connections between the gut and the brain contain both afferent (sensory) nerves and efferent (motor) nerves.
The enteric nervous system contains between 200 and 600 million neurons and 20 different types of neurons (and possibly nundreds or thousands more in the brain) have been identified based different criteria such as the morphology, chemical properties and physiology and functional roles. With such complexity, the enteric nervous system is sometimes referred as the "second brain" or "the brain in the gut".
Its neurons are grouped in thousands of ganglia (group of neurons) that are divided into myenteric and submucosal.
The myenteric ganglia are organised in a network around the gut, which spans the length between the upper oesophagus and the internal anal sphincter and contains mostly motor neurons.
The submucosal ganglia are localised in the small and large intestine with most of the primary afferent sensory neurons.
The enteric nervous system neurons can be classified in three groups: intrinsic primary afferent neurons, interneurons and motor neurons.
The intrinsic primary afferent neurons detect chemical and mechanical stimuli from ingestion, usually food and drink.
The motor neurons in the enteric nervous system are responsible for coordinating peristalsis, rhythmic muscle contractions that move material along the digestive tract.
Enteric neurons can regulate the vomiting reflex, which is particularly critical in those cases in which vomiting arises as a side-effect of medical treatments, including chemotherapies.
Interestingly, recent studies show that the connection between the gut, the enteric nervous system and the central nervous system is stronger than previously thought. Indeed, an altered gut microbiota, also known as intestinal flora, has been linked to mental illness, such as depression, and some neurodegenerative diseases, including Alzheimer’s and Parkinson’s diseases (Kim et al., 2018)
Author: Alessandra Donato from the Hilliard Lab
