Enteric Nervous System

The enteric nervous system is the large network of neurons and glia embedded in the wall of the gastrointestinal tract that controls digestion. Containing hundreds of millions of neurons arranged in interconnected plexuses, it can generate and coordinate reflexes such as peristalsis and local secretion on its own, without input from the brain or spinal cord, which has earned it the nickname the second brain.

Definition

The enteric nervous system is the intrinsic, semi-autonomous division of the autonomic nervous system, consisting of neurons and glial cells distributed through the gut wall in the myenteric and submucosal plexuses, which controls gastrointestinal motility, secretion, and blood flow through local reflex circuits.

Scope

The topic covers the organisation of the enteric nervous system into the myenteric and submucosal plexuses, the main classes of enteric neurons and their roles, the local reflex circuits that drive motility and secretion, and the system's relationship to extrinsic autonomic nerves. It is a physiology reference entry and does not give clinical management advice.

Core questions

• How is the enteric nervous system organised within the gut wall?
• Which neuron types sense the lumen, relay information, and drive muscle and glands?
• How does the system produce coordinated reflexes such as peristalsis on its own?
• How do brain and spinal connections modulate intrinsic enteric activity?

Key concepts

• Myenteric (Auerbach's) plexus
• Submucosal (Meissner's) plexus
• Intrinsic primary afferent neurons
• Interneurons and motor neurons
• Peristaltic reflex
• Enteric glia
• Semi-autonomous (intrinsic) control

Mechanisms

The enteric nervous system is arranged in two principal ganglionated plexuses: the myenteric plexus, between the longitudinal and circular muscle layers, which chiefly governs motility, and the submucosal plexus, which chiefly governs secretion and local blood flow. Sensory neurons, often called intrinsic primary afferent neurons, detect stretch and the chemistry of the luminal contents; they connect through interneurons to excitatory and inhibitory motor neurons that act on smooth muscle, secretory cells, and blood vessels. These circuits generate the peristaltic reflex, in which contraction above and relaxation below a bolus propel contents along the gut. Enteric glia support and modulate this signalling. The system operates largely independently but is tuned by extrinsic sympathetic and parasympathetic nerves, allowing the central nervous system to adjust digestion to the needs of the whole body.

Clinical relevance

The enteric nervous system is central to understanding gut motility and its disorders, and its degeneration or developmental absence is associated with motility problems. This entry describes its normal physiology as reference knowledge and is not a basis for diagnosing or treating individual patients.

History

The ganglionated plexuses of the gut were described anatomically in the nineteenth century by Auerbach and Meissner, whose names remain attached to the myenteric and submucosal plexuses. Early twentieth-century physiologists demonstrated the law of the intestine, the local reflex underlying peristalsis. Through the later twentieth century the system's complexity and partial independence from the central nervous system were established, and the popular term second brain, associated with Gershon, reflects this recognition.

Key figures

• Michael Gershon
• John Furness
• Leopold Auerbach
• Georg Meissner

Seminal works

• furness-2012
• rao-gershon-2016

Frequently asked questions

Why is the enteric nervous system called the second brain?

Because it contains a very large number of neurons and can generate and coordinate digestive reflexes on its own, without instructions from the brain or spinal cord, although the brain can still modulate it.

What are the two main plexuses of the enteric nervous system?

The myenteric (Auerbach's) plexus, located between the muscle layers and mainly controlling motility, and the submucosal (Meissner's) plexus, mainly controlling secretion and local blood flow.