The superior mesenteric plexus is a complex network of autonomic nerve fibers, primarily involved in supplying the midgut region of the gastrointestinal tract. It is a continuation of the celiac plexus and is associated with the superior mesenteric artery. The plexus primarily consists of sympathetic fibers from the thoracic splanchnic nerves, along with some parasympathetic fibers from the vagus nerve. These nerve fibers are responsible for regulating the autonomic functions of the small intestine, part of the large intestine, and associated abdominal structures.
Location
The superior mesenteric plexus is located surrounding the origin of the superior mesenteric artery, which arises from the abdominal aorta just below the celiac artery, at the level of the L1 vertebra. It extends downward along the course of the superior mesenteric artery and distributes fibers to the jejunum, ileum, ascending colon, and transverse colon.
Structure and Anatomy
The superior mesenteric plexus is a vital structure in the autonomic nervous system, responsible for innervating parts of the midgut. Its anatomy is complex and involves numerous components that allow it to relay autonomic signals to the abdominal organs.
Formation and Composition
The superior mesenteric plexus is formed by contributions from the celiac plexus and aortic plexus, with additional fibers from the greater and lesser splanchnic nerves (thoracic sympathetic chain) and vagal parasympathetic fibers. This mixture of sympathetic and parasympathetic fibers allows the plexus to regulate involuntary processes in the midgut.
Nerve Fibers
- Sympathetic Fibers: These fibers arise from the thoracic splanchnic nerves (primarily the greater and lesser splanchnic nerves) and enter the superior mesenteric plexus after synapsing in the celiac ganglia. The postganglionic sympathetic fibers from the celiac ganglia form a significant part of the superior mesenteric plexus.
- Parasympathetic Fibers: Parasympathetic fibers are derived from the vagus nerve. These preganglionic fibers travel through the plexus without synapsing, as they synapse directly in the walls of the organs they innervate, mainly in the small intestine and proximal large intestine.
Ganglia
The superior mesenteric plexus contains small ganglia, primarily sympathetic in nature, where some preganglionic fibers synapse with postganglionic neurons. These ganglia are embedded within the plexus and distributed around the branches of the superior mesenteric artery. They serve as relay stations for the transmission of sympathetic signals to the organs.
Branches and Distribution
The nerve fibers of the superior mesenteric plexus follow the branches of the superior mesenteric artery, forming smaller plexuses around these vessels. These include:
- Jejunal and Ileal Plexuses: Innervate the jejunum and ileum.
- Ileocolic Plexus: Supplies the terminal ileum, cecum, and appendix.
- Right Colic Plexus: Innervates the ascending colon.
- Middle Colic Plexus: Extends fibers to the transverse colon.
Each branch of the superior mesenteric artery is accompanied by a corresponding nerve plexus that provides autonomic control to the respective segments of the gastrointestinal tract.
Relations
The superior mesenteric plexus is closely related to several key anatomical structures:
- Celiac Plexus: Located superiorly, the superior mesenteric plexus is a direct continuation of the celiac plexus, ensuring a smooth transition of autonomic nerve supply between the upper and mid-abdominal organs.
- Aortic Plexus: Located anterior to the abdominal aorta, the superior mesenteric plexus is part of the broader aortic plexus, which supplies autonomic innervation to the abdominal organs.
- Inferior Mesenteric Plexus: Located inferiorly, the superior mesenteric plexus connects with the inferior mesenteric plexus, facilitating coordinated autonomic control between the midgut and hindgut regions.
Connections
- Celiac Plexus: The superior mesenteric plexus receives its primary sympathetic input from the celiac plexus, which serves as the upper abdominal autonomic hub.
- Aortic Plexus: The superior mesenteric plexus is connected to the broader aortic plexus, a large network of autonomic fibers surrounding the abdominal aorta.
- Vagal Nerve: Parasympathetic fibers from the vagus nerve pass through the superior mesenteric plexus, allowing for the regulation of digestive processes in the midgut.
Continuity with Other Plexuses
The superior mesenteric plexus is continuous with adjacent autonomic plexuses:
- Superiorly with the celiac plexus.
- Inferiorly with the inferior mesenteric plexus.
- Laterally it communicates with the renal plexuses, providing a pathway for autonomic signals between different abdominal organs.
Function
The superior mesenteric plexus is essential in regulating the autonomic functions of the midgut, which includes most of the small intestine, as well as parts of the large intestine. It controls various involuntary processes related to digestion and blood flow through both sympathetic and parasympathetic pathways. Below is a detailed description of its key functions.
Sympathetic Control
The majority of the fibers in the superior mesenteric plexus are sympathetic in nature, derived from the thoracic splanchnic nerves. These fibers play a vital role in the “fight or flight” response by inhibiting digestion during times of stress or danger.
Vasoconstriction
One of the primary sympathetic functions of the superior mesenteric plexus is to regulate blood flow to the midgut organs. The sympathetic fibers cause vasoconstriction of the superior mesenteric artery and its branches, which reduces blood supply to the jejunum, ileum, and ascending colon. This response is crucial during periods of increased sympathetic activity, such as stress or physical exertion, when blood flow is diverted to essential organs like the heart and muscles.
Inhibition of Peristalsis
The sympathetic fibers from the superior mesenteric plexus inhibit peristalsis, the wave-like contractions of the intestinal muscles that move food through the digestive tract. This reduction in intestinal motility slows the passage of food and reduces digestive activity, conserving energy for more immediate needs during times of stress.
Reduced Glandular Secretions
Sympathetic fibers also suppress the secretion of digestive enzymes and mucus in the intestines. By reducing glandular secretions, the superior mesenteric plexus decreases the digestive process, again prioritizing energy conservation for other physiological demands.
Contraction of Sphincters
Sympathetic stimulation leads to the contraction of intestinal sphincters, particularly the ileocecal valve, which controls the flow of material from the small intestine to the large intestine. This contraction prevents the movement of intestinal contents during stressful situations when digestion is not a priority.
Parasympathetic Control
While the superior mesenteric plexus is predominantly sympathetic, it also contains parasympathetic fibers from the vagus nerve. These fibers promote digestion and are active during the “rest and digest” state.
Stimulation of Peristalsis
Parasympathetic fibers in the superior mesenteric plexus stimulate peristalsis, enhancing the rhythmic contractions that propel food and waste through the intestines. This promotes efficient digestion and absorption of nutrients in the jejunum and ileum, as well as the movement of waste toward the large intestine.
Increased Glandular Secretions
Parasympathetic activity increases the secretion of digestive enzymes and fluids, which are necessary for the breakdown of food. This increase in glandular secretions helps the intestines absorb nutrients and properly process food for digestion.
Relaxation of Sphincters
Parasympathetic fibers induce the relaxation of the ileocecal valve and other sphincters in the gastrointestinal tract, allowing for the smooth passage of digested material from the small intestine to the large intestine. This relaxation is critical during normal digestion to ensure efficient movement through the digestive tract.
Coordination with Other Plexuses
The superior mesenteric plexus functions in coordination with adjacent autonomic plexuses to ensure seamless control over digestion and other autonomic functions throughout the abdomen.
Celiac Plexus Coordination
The superior mesenteric plexus is closely linked with the celiac plexus, which supplies the foregut (including the stomach, liver, and pancreas). Together, these plexuses regulate the transition of food from the stomach to the small intestine and ensure coordinated digestive activity across the foregut and midgut regions.
Inferior Mesenteric Plexus Coordination
The superior mesenteric plexus connects with the inferior mesenteric plexus, which innervates the hindgut (descending colon, sigmoid colon, and rectum). This coordination allows for a smooth transition of autonomic control as material moves from the midgut (supplied by the superior mesenteric plexus) to the hindgut (supplied by the inferior mesenteric plexus).
Regulation of Blood Flow
The superior mesenteric plexus plays a significant role in regulating the blood supply to the midgut organs through both vasoconstriction and vasodilation mechanisms.
Blood Flow During Rest
During periods of rest or after eating, the parasympathetic fibers of the plexus ensure that blood flow to the small intestine and ascending colon is maximized. This increase in blood flow is crucial for delivering oxygen and nutrients to support digestion and absorption processes.
Blood Flow During Stress
In contrast, during periods of stress or exercise, sympathetic fibers trigger vasoconstriction to reduce blood flow to the midgut, ensuring that blood is diverted to more critical organs like the heart and brain. This process ensures that the body can prioritize functions needed for survival, while temporarily reducing digestive activity.
Response to Physiological States
The superior mesenteric plexus adjusts its function based on the body’s physiological state:
Fight or Flight Response
During stress, the sympathetic fibers of the superior mesenteric plexus take over, inhibiting digestive activity, reducing blood flow to the intestines, and decreasing peristalsis. These actions help the body focus on immediate survival needs, conserving energy and resources for critical functions like muscle contraction and cardiovascular activity.
Rest and Digest Response
In a relaxed state, the parasympathetic fibers of the plexus dominate, promoting digestion by increasing blood flow to the intestines, enhancing peristalsis, and stimulating digestive secretions. This allows for the efficient processing of food and nutrient absorption when the body is at rest and does not need to respond to external stressors.
Clinical Significance
The superior mesenteric plexus plays a crucial role in the autonomic regulation of the midgut, and its dysfunction can lead to various clinical conditions.
Chronic Abdominal Pain
Disorders affecting the superior mesenteric plexus, such as irritation or compression of its nerve fibers, can lead to chronic abdominal pain or visceral discomfort, particularly in conditions like mesenteric ischemia, where blood flow to the intestines is compromised.
Ischemic Bowel Disease
Due to its role in regulating blood flow, the superior mesenteric plexus is involved in conditions like acute mesenteric ischemia, where reduced blood supply to the intestines can lead to severe pain and necrosis of intestinal tissue. Dysfunction or injury to this plexus can exacerbate the ischemic process.
Post-Surgical Complications
In surgeries involving the superior mesenteric artery, such as procedures for abdominal aneurysms or resections for tumors, inadvertent injury to the superior mesenteric plexus can result in complications like impaired gut motility, chronic pain, or autonomic dysregulation in the affected bowel segments.
Autonomic Neuropathy
Conditions like diabetic autonomic neuropathy can affect the superior mesenteric plexus, leading to issues like altered bowel motility, delayed gastric emptying, and other digestive dysfunctions, making it a key player in gastrointestinal manifestations of systemic diseases.
