Myenteric Plexus

The myenteric plexus, also known as the Auerbach plexus, is one of the principal neural networks of the enteric nervous system (ENS). It consists of interconnected enteric ganglia and nerve fibers distributed throughout much of the gastrointestinal tract.

The plexus is located primarily between the inner circular and outer longitudinal layers of the muscularis externa. This strategic position places its neurons in close relationship with the smooth muscle responsible for gastrointestinal movement.

The myenteric plexus is particularly important in the regulation of gastrointestinal motility. It participates in coordinating smooth muscle contraction and relaxation, peristaltic activity, intestinal propulsion, muscle tone, and the behavior of gastrointestinal sphincters.

Enteric Nervous System

The enteric nervous system is an extensive network of neurons and supporting cells contained within the wall of the gastrointestinal tract. It can coordinate many gastrointestinal functions locally while also communicating with the central nervous system through sympathetic and parasympathetic pathways.

Two major enteric plexuses are classically recognized:

• Myenteric plexus, located primarily between the circular and longitudinal muscle layers
• Submucosal plexus, located principally within the submucosa

Location of the Myenteric Plexus

The myenteric plexus lies within the muscular wall of the gastrointestinal tract, typically between the circular and longitudinal layers of the muscularis externa.

Its location allows enteric neurons to communicate extensively with gastrointestinal smooth muscle and coordinate motor activity over segments of the digestive tract.

Layers of the Gastrointestinal Wall

Layer	Relationship to Myenteric Plexus
Mucosa	Located internal to the plexus
Submucosa	Contains the major submucosal neural plexus
Inner circular muscle	Myenteric plexus lies immediately external to this layer
Myenteric plexus	Located between the principal muscular layers
Outer longitudinal muscle	Lies external to the myenteric plexus
Serosa or adventitia	Forms the outer covering depending on the gastrointestinal region

Distribution

The myenteric plexus extends along much of the gastrointestinal tract. Its organization and density vary among different regions according to local anatomy and functional requirements.

Rather than forming a single continuous nerve, it forms an interconnected network of ganglia, interganglionic nerve bundles, and smaller neural pathways distributed through the muscular wall.

Myenteric Ganglia

Myenteric ganglia are collections of enteric neuronal cell bodies located within the plexus. Individual ganglia are connected to neighboring ganglia by bundles of nerve fibers, creating an extensive neural network.

The ganglia contain several functional classes of neurons involved in sensory processing, integration, and motor control.

Interganglionic Connections

Nerve fiber bundles connect myenteric ganglia along the length and circumference of the gastrointestinal tract. These connections allow neural signals to travel between different regions of the bowel wall.

This interconnected architecture is essential for coordinating patterns of smooth muscle activity across distances greater than a single ganglion.

Cellular Components

The myenteric plexus contains neurons together with specialized supporting glial cells. The neuronal population is functionally diverse rather than consisting of a single type of autonomic motor neuron.

Major functional neuronal categories include:

• Intrinsic primary afferent neurons
• Interneurons
• Excitatory motor neurons
• Inhibitory motor neurons

Enteric Neurons

Enteric neurons receive information about conditions within the gastrointestinal tract, communicate with other enteric neurons, and regulate smooth muscle and other target cells.

Complex local circuits allow the enteric nervous system to coordinate gastrointestinal activity without requiring every response to be initiated directly by the brain or spinal cord.

Enteric Glial Cells

Enteric glial cells are non-neuronal cells associated with enteric neurons and nerve fibers. They provide structural and metabolic support and participate in maintaining the local neural environment.

Enteric glia are distributed throughout the enteric plexuses and are important components of enteric ganglia.

Intrinsic Sensory Neurons

Intrinsic sensory neurons detect mechanical and chemical conditions within the gastrointestinal tract. Distension of the bowel wall and changes associated with luminal contents can activate enteric sensory pathways.

This sensory information can initiate local reflex circuits controlling gastrointestinal motor activity.

Interneurons

Enteric interneurons connect different populations of neurons within the enteric nervous system. Their processes can extend orally or anally along the gastrointestinal tract.

Ascending and descending neural pathways are particularly important in coordinating polarized motor responses during propulsion of intestinal contents.

Motor Neurons

Myenteric motor neurons innervate the smooth muscle layers of the gastrointestinal tract. They can produce either excitatory or inhibitory effects depending on their neurotransmitters and target cells.

The balance between excitatory and inhibitory motor pathways allows precise regulation of contraction and relaxation.

Excitatory Motor Neurons

Excitatory enteric motor neurons promote contraction of gastrointestinal smooth muscle. Acetylcholine is an important excitatory neurotransmitter within enteric motor pathways.

Other signaling molecules can participate depending on the gastrointestinal region and neural circuit.

Inhibitory Motor Neurons

Inhibitory enteric motor neurons promote relaxation of gastrointestinal smooth muscle. Important inhibitory signaling molecules include nitric oxide and vasoactive intestinal peptide.

Inhibitory pathways are essential for coordinated propulsion because smooth muscle ahead of advancing intestinal contents must relax appropriately.

Neurotransmitters

The enteric nervous system uses a large variety of neurotransmitters and neuromodulators.

Signaling Molecule	General Enteric Role
Acetylcholine	Important excitatory transmitter affecting gastrointestinal smooth muscle
Nitric oxide	Important inhibitory mediator producing smooth muscle relaxation
Vasoactive intestinal peptide	Participates in inhibitory and regulatory pathways
Serotonin	Participates in gastrointestinal sensory and reflex signaling
Substance P	Participates in excitatory enteric pathways

Primary Function

The myenteric plexus is primarily associated with the motor control of the gastrointestinal tract. It coordinates the activity of smooth muscle rather than simply producing generalized contraction.

Its functions include regulation of:

• Peristalsis
• Propulsive movement
• Segmental contractions
• Muscle tone
• Strength and frequency of contractions
• Coordination between circular and longitudinal muscle
• Sphincter relaxation and contraction

Control of Gastrointestinal Motility

Gastrointestinal motility requires precisely timed contraction and relaxation of smooth muscle. The myenteric plexus integrates local sensory information and generates coordinated motor outputs to the muscularis externa.

Because the plexus extends longitudinally through the gastrointestinal wall, activity can be coordinated across adjacent segments rather than being limited to isolated muscle contractions.

Peristalsis

Peristalsis is a coordinated pattern of contraction and relaxation that propels luminal contents through the gastrointestinal tract.

Distension caused by a bolus or intestinal contents can activate local enteric circuits. These circuits coordinate contraction on the oral side and relaxation on the anal side, promoting movement of the contents in the appropriate direction.

Ascending Excitatory Pathway

During a typical peristaltic response, enteric neural circuits activate excitatory motor pathways on the oral side of the stimulated region.

Contraction of circular smooth muscle behind the luminal contents helps generate propulsive force.

Descending Inhibitory Pathway

At the same time, descending inhibitory pathways promote relaxation of smooth muscle on the anal side of the stimulated region.

This relaxation reduces resistance ahead of the advancing contents and allows coordinated propulsion.

Peristaltic Reflex

Region	Neural Response	Mechanical Effect
Oral side	Predominantly excitatory motor activity	Circular muscle contraction
Stimulated segment	Local sensory and integrative activity	Coordinates response to distension
Anal side	Predominantly inhibitory motor activity	Smooth muscle relaxation

Segmentation

Not all intestinal contractions propel contents over long distances. Segmentation contractions mix luminal contents and increase their contact with the mucosal surface.

Enteric neural circuits interact with intrinsic smooth muscle electrical activity and other regulatory mechanisms to influence these patterns of intestinal movement.

Gastrointestinal Muscle Tone

The myenteric plexus contributes to regulation of the baseline contractile state of gastrointestinal smooth muscle. Changes in excitatory and inhibitory neural activity can increase or decrease muscle tone.

This function is particularly important in regions where sustained muscular activity must be coordinated with periods of relaxation.

Sphincter Control

Enteric neural pathways contribute to control of several gastrointestinal sphincters. Appropriate relaxation of sphincteric smooth muscle is necessary for movement of luminal contents between adjacent regions.

Inhibitory enteric neurons are particularly important in producing relaxation when passage through a sphincter is required.

Relationship to Interstitial Cells of Cajal

Interstitial cells of Cajal are specialized cells within the gastrointestinal muscular wall that participate in generation and propagation of electrical slow waves and in communication between enteric neurons and smooth muscle.

They are closely associated anatomically and functionally with components of the enteric nervous system but are distinct from enteric neurons.

Slow Waves

Gastrointestinal smooth muscle exhibits rhythmic electrical activity known as slow waves. Interstitial cells of Cajal play an important role as electrical pacemaker cells in many gastrointestinal regions.

The myenteric nervous system modifies the resulting motor activity through excitatory and inhibitory neural signals rather than serving as the sole generator of gastrointestinal rhythmicity.

Myenteric and Submucosal Plexuses

The myenteric and submucosal plexuses are interconnected but have different predominant anatomical relationships and functions.

Feature	Myenteric Plexus	Submucosal Plexus
Alternative name	Auerbach plexus	Meissner plexus
Primary location	Between circular and longitudinal muscle layers	Within the submucosa
Major association	Gastrointestinal motility	Mucosal secretion, absorption and local blood flow
Relationship to smooth muscle	Closely associated with muscularis externa	More closely associated with mucosal and submucosal functions

Intrinsic Control

A major characteristic of the enteric nervous system is its capacity to organize local reflexes independently of direct central nervous system commands.

Sensory neurons, interneurons, and motor neurons can form complete reflex circuits entirely within the gastrointestinal wall.

Extrinsic Autonomic Innervation

Although enteric circuits can operate locally, gastrointestinal function is modified extensively by parasympathetic and sympathetic innervation.

Extrinsic autonomic pathways influence enteric neurons, smooth muscle, glands, blood vessels, and other gastrointestinal structures.

Parasympathetic Influence

Parasympathetic pathways generally support gastrointestinal activity and communicate extensively with enteric neural circuits.

Preganglionic parasympathetic fibers reach the gastrointestinal tract through the vagus nerve and pelvic splanchnic nerves, depending on the region, and synapse with neurons within enteric ganglia.

Vagus Nerve

The vagus nerve provides parasympathetic innervation to a large portion of the gastrointestinal tract. Its preganglionic fibers communicate with enteric neurons rather than simply innervating gastrointestinal smooth muscle directly.

This organization allows central autonomic signals to modify complex local enteric circuits.

Pelvic Splanchnic Nerves

Pelvic splanchnic nerves provide parasympathetic innervation to the distal portion of the gastrointestinal tract. Their preganglionic fibers arise from sacral spinal cord levels and communicate with enteric neurons.

Sympathetic Influence

Sympathetic pathways generally reduce gastrointestinal motility and secretion while influencing vascular tone and sphincteric activity. Many sympathetic effects occur through modulation of enteric neural circuits.

Postganglionic sympathetic fibers reaching the gastrointestinal tract originate principally from prevertebral sympathetic ganglia.

Enteric Reflexes

Enteric reflexes can be entirely contained within the gut wall or can involve extrinsic autonomic pathways and the central nervous system.

Short reflexes contained within the enteric nervous system are particularly important for rapid local responses to changes in luminal contents and distension.

Local Reflex Arc

A simplified local enteric reflex can include:

1. Detection of mechanical or chemical changes within the gastrointestinal tract.
2. Activation of intrinsic sensory neurons.
3. Transmission through enteric interneurons.
4. Activation or inhibition of enteric motor neurons.
5. Contraction or relaxation of gastrointestinal smooth muscle.

Histological Appearance

In histological sections, myenteric ganglia can be identified between the circular and longitudinal layers of smooth muscle. Ganglia contain neuronal cell bodies surrounded by smaller supporting cells and nerve fibers.

The exact appearance varies with section orientation, gastrointestinal region, staining technique, and the portion of the neural network included in the section.

Identification on Histology

Important features used to recognize the myenteric plexus include:

• Location between layers of the muscularis externa
• Clusters of relatively large neuronal cell bodies
• Smaller enteric glial cells
• Associated bundles of nerve fibers
• Close relationship with circular and longitudinal smooth muscle

Development

Most enteric neurons and glial cells develop from neural crest-derived precursor cells that migrate into and along the developing gastrointestinal tract.

These cells proliferate, differentiate, and organize into the interconnected ganglia and nerve networks of the mature enteric nervous system.

Neural Crest Migration

Proper migration and colonization of the developing bowel by enteric neural crest-derived cells are essential for formation of functional enteric plexuses.

Failure of enteric ganglion cells to populate a segment of bowel can produce severe abnormalities of gastrointestinal motility.

Hirschsprung Disease

Hirschsprung disease is characterized by congenital absence of enteric ganglion cells from a variable length of distal bowel. Both myenteric and submucosal ganglia are absent from the affected segment.

The aganglionic bowel cannot coordinate normal relaxation and propulsion, producing functional obstruction and dilation of the normally innervated bowel proximal to the affected segment.

Aganglionosis

The term aganglionosis describes absence of ganglion cells. In Hirschsprung disease, the aganglionic segment begins distally and extends proximally for a variable distance.

The anatomical length of the affected bowel influences the clinical pattern and surgical management.

Achalasia

Achalasia is an esophageal motility disorder associated with impaired relaxation of the lower esophageal sphincter and abnormal esophageal peristalsis. Loss or dysfunction of inhibitory neurons within the myenteric plexus is an important component of its pathophysiology.

Failure of appropriate inhibitory signaling interferes with coordinated relaxation during swallowing.

Enteric Neuropathies

Disorders affecting enteric neurons can alter gastrointestinal motility. Depending on the affected neuronal populations and gastrointestinal region, abnormalities may involve propulsion, sphincter function, transit, or coordinated muscular activity.

Enteric neuropathies can be congenital or acquired and may occur independently or as part of broader neurological or systemic disorders.

Intestinal Pseudo-Obstruction

Intestinal pseudo-obstruction describes a syndrome in which intestinal contents fail to progress normally despite the absence of a mechanical blockage. Abnormalities of enteric nerves, smooth muscle, or related regulatory systems can contribute to this condition.

Neuropathic forms can involve dysfunction or degeneration of enteric neural circuits.

Myenteric Plexus and Motility Disorders

Condition	Relationship to Enteric Neural Function
Hirschsprung disease	Congenital absence of enteric ganglion cells in affected bowel
Achalasia	Loss or dysfunction of inhibitory myenteric neurons contributes to impaired sphincter relaxation
Neuropathic intestinal pseudo-obstruction	Enteric neural dysfunction can impair coordinated propulsion

Relationship to Smooth Muscle

The anatomical position of the myenteric plexus between the major layers of the muscularis externa reflects its close functional relationship with gastrointestinal smooth muscle.

Enteric motor neurons influence circular and longitudinal muscle activity directly and through interactions with other cellular components of the gastrointestinal neuromuscular apparatus.

Circular Muscle

The circular muscle layer narrows the intestinal lumen when it contracts. Coordinated circular muscle contraction is particularly important in segmentation and propulsive activity.

The myenteric plexus lies immediately external to this layer in the classic organization of the gastrointestinal wall.

Longitudinal Muscle

The longitudinal muscle layer lies external to the myenteric plexus. Contraction shortens segments of the gastrointestinal tract and participates in coordinated motility.

Integration of circular and longitudinal muscle activity allows complex changes in bowel shape and luminal dimensions during movement of gastrointestinal contents.

Functional Organization

Component	Principal Role
Intrinsic sensory neurons	Detect local mechanical and chemical conditions
Ascending interneurons	Transmit information toward oral segments
Descending interneurons	Transmit information toward anal segments
Excitatory motor neurons	Promote smooth muscle contraction
Inhibitory motor neurons	Promote smooth muscle relaxation
Enteric glia	Support enteric neurons and local neural environment

Myenteric Plexus and the Autonomic Nervous System

The myenteric plexus illustrates the distinctive organization of the enteric division of the autonomic nervous system. Unlike a simple peripheral motor pathway, it contains complete local circuits capable of sensing conditions, processing information, and producing coordinated responses.

Sympathetic and parasympathetic pathways regulate these circuits but do not replace the intrinsic neural organization of the bowel wall.

Clinical Importance

The myenteric plexus is clinically important because normal gastrointestinal propulsion depends on coordinated enteric neural activity. Structural absence, degeneration, or dysfunction of myenteric neurons can produce substantial disturbances of motility.

Its location is also important in histopathology. Examination of enteric ganglion cells can be necessary in the evaluation of congenital and acquired disorders affecting gastrointestinal innervation.

Functional Importance

The myenteric plexus is the principal enteric neural network associated with the motor activity of the gastrointestinal tract. Its position between the circular and longitudinal muscle layers allows it to regulate the muscular apparatus responsible for propulsion and mixing.

Through networks of sensory neurons, interneurons, excitatory motor neurons, and inhibitory motor neurons, the plexus can organize local reflexes and coordinate contraction with relaxation across adjacent intestinal segments. These mechanisms are fundamental to peristalsis and normal gastrointestinal transit.

The plexus also demonstrates the relative autonomy of the enteric nervous system. Local neural circuits can coordinate complex gastrointestinal responses while remaining subject to modulation by sympathetic and parasympathetic pathways.

Understanding the myenteric plexus is fundamental to the study of enteric nervous system anatomy, autonomic innervation, gastrointestinal motility, peristalsis, smooth muscle control, enteric reflexes, Hirschsprung disease, achalasia, and gastrointestinal histology.