The inferior mesenteric plexus is a network of autonomic nerves that is part of the sympathetic nervous system. It is associated with the inferior mesenteric artery and supplies nerve fibers to the structures of the hindgut.
Location
The inferior mesenteric plexus is located around the origin of the inferior mesenteric artery, which branches off the abdominal aorta at the level of the L3 vertebra. The plexus extends towards the left side of the abdomen, following the course of the inferior mesenteric artery. It provides innervation to the descending colon, sigmoid colon, and rectum.
Structure and Anatomy
The inferior mesenteric plexus is an important component of the autonomic nervous system, particularly the sympathetic branch, and is intricately connected with surrounding structures.
Formation and Composition
The inferior mesenteric plexus is formed by contributions from the sympathetic chain and the aortic plexus. It is composed of autonomic nerve fibers—primarily sympathetic fibers—along with a small number of parasympathetic fibers. The nerve fibers within the plexus are derived from the lumbar splanchnic nerves, specifically from the second and third lumbar ganglia (L2 and L3) of the sympathetic trunk.
Nerve Fibers
- Sympathetic Fibers: The majority of the fibers within the inferior mesenteric plexus are sympathetic. These fibers originate from the lumbar splanchnic nerves and are responsible for regulating the autonomic functions of the hindgut organs. After synapsing in the inferior mesenteric ganglia, postganglionic sympathetic fibers travel along the branches of the inferior mesenteric artery.
- Parasympathetic Fibers: A few parasympathetic fibers are present, primarily from the pelvic splanchnic nerves (S2-S4), although the bulk of parasympathetic innervation to the organs supplied by this plexus comes directly from the pelvic plexus.
Ganglia
The inferior mesenteric plexus contains small ganglia embedded within the nerve network. These ganglia serve as relay points where preganglionic sympathetic fibers synapse with postganglionic neurons that subsequently extend their fibers to the target organs.
Branches and Distribution
The inferior mesenteric plexus distributes nerve fibers along the branches of the inferior mesenteric artery. These branches include:
- Left Colic Plexus: Supplies the descending colon.
- Sigmoid Plexus: Extends fibers to the sigmoid colon.
- Superior Rectal Plexus: Supplies the upper part of the rectum.
Each of these smaller plexuses follows the corresponding arteries as they course toward the target organs. The nerve fibers travel alongside the blood vessels, forming intricate networks of innervation along the vascular structures.
Relations
The inferior mesenteric plexus is closely related to the following anatomical structures:
- Abdominal Aorta: The plexus wraps around the origin of the inferior mesenteric artery, which arises from the abdominal aorta.
- Inferior Mesenteric Artery: The artery serves as a pathway for the nerves of the plexus, as they follow its branches to the descending colon, sigmoid colon, and rectum.
- Adjacent Plexuses: It is continuous superiorly with the intermesenteric plexus and inferiorly with the superior hypogastric plexus, allowing communication between different parts of the autonomic nervous system within the abdomen.
Connections
- Lumbar Splanchnic Nerves: The main source of sympathetic fibers, these nerves emerge from the sympathetic trunk and enter the plexus.
- Aortic Plexus: The inferior mesenteric plexus receives contributions from the aortic plexus, a large network of autonomic fibers that surrounds the abdominal aorta.
- Hypogastric Plexus: Inferiorly, the plexus is continuous with the superior hypogastric plexus, allowing for an extended distribution of autonomic fibers to the pelvic organs.
This highly organized network of nerves forms an essential link between the central nervous system and the organs of the lower abdomen.
Function
The inferior mesenteric plexus plays a crucial role in regulating the autonomic control of the abdominal organs, particularly the hindgut. It mediates involuntary functions related to digestion and other processes in the large intestine. Its actions are primarily mediated through the sympathetic and, to a lesser extent, parasympathetic fibers. Below is a detailed explanation of its functional roles.
Sympathetic Control
The sympathetic fibers within the inferior mesenteric plexus are the dominant functional component. They originate from the lumbar splanchnic nerves (L2 and L3) and are responsible for various involuntary actions that generally inhibit or reduce digestive activities.
Vasoconstriction
The sympathetic fibers play a significant role in controlling blood flow to the large intestine. By causing vasoconstriction of the arteries (including the inferior mesenteric artery and its branches), the plexus reduces blood flow to the descending colon, sigmoid colon, and rectum during times of increased sympathetic activity, such as stress or physical exertion. This redirection of blood helps prioritize vital organs like the brain and muscles during a “fight or flight” response.
Inhibition of Peristalsis
The sympathetic fibers of the inferior mesenteric plexus inhibit peristalsis, the wave-like contractions that move food through the digestive tract. This inhibition slows the transit of fecal material through the descending and sigmoid colon. During stressful situations, this suppression of gut motility prevents unnecessary digestion, conserving energy for more critical functions.
Reduction of Glandular Secretion
The sympathetic input from the inferior mesenteric plexus reduces the secretion of digestive fluids and mucus in the large intestine. This reduction in glandular secretion minimizes the digestive activity in the lower gut, further supporting the overall inhibitory function of the sympathetic nervous system in stressful conditions.
Sphincter Contraction
Sympathetic fibers contribute to contraction of the internal anal sphincter, a smooth muscle located in the rectum. This action aids in maintaining fecal continence by keeping the sphincter contracted until it is appropriate to relax it during defecation.
Parasympathetic Control
Although the inferior mesenteric plexus is primarily sympathetic, it also contains a smaller number of parasympathetic fibers, which originate from the pelvic splanchnic nerves (S2-S4). These fibers provide a complementary role in promoting digestive activity, especially in the distal parts of the large intestine.
Stimulation of Peristalsis
The parasympathetic fibers stimulate peristalsis, enhancing the rhythmic contractions of the large intestine. This function is crucial in promoting the movement of fecal matter through the descending colon, sigmoid colon, and rectum, facilitating the process of defecation.
Increased Glandular Secretion
Parasympathetic activation stimulates the secretion of mucus and digestive enzymes in the large intestine, helping to lubricate and soften fecal material. This secretion is essential for proper digestion and the formation of stool, making the process of elimination more efficient.
Relaxation of the Internal Anal Sphincter
The parasympathetic fibers contribute to the relaxation of the internal anal sphincter, a necessary step in the defecation process. When the body is ready to defecate, the parasympathetic input relaxes this smooth muscle, allowing for the passage of stool into the rectum and subsequently out of the body.
Coordination with Other Plexuses
The inferior mesenteric plexus functions in coordination with other autonomic plexuses within the abdomen to ensure smooth regulation of digestive processes. These include:
- Superior Mesenteric Plexus: Involved in the innervation of the midgut, it works with the inferior mesenteric plexus to provide a seamless transition in autonomic control between different segments of the intestines.
- Hypogastric Plexus: The inferior mesenteric plexus is continuous with the superior hypogastric plexus, ensuring coordinated autonomic control between the lower abdomen and pelvic organs.
The synchronized activity between these plexuses enables the autonomic nervous system to modulate digestive processes efficiently across the entire gastrointestinal tract.
Response to Physiological States
The inferior mesenteric plexus adapts its function according to different physiological states:
Fight or Flight Response
During stress, the sympathetic output from the plexus is heightened, reducing blood flow, peristalsis, and secretion in the large intestine to conserve energy for more critical bodily functions.
Rest and Digest Response
In contrast, during periods of relaxation, parasympathetic activity increases, promoting blood flow, stimulating digestive secretions, and facilitating the movement of fecal material through the large intestine.
This dynamic modulation allows the body to shift between conserving energy during stress and optimizing digestion during rest.
Clinical Significance
The inferior mesenteric plexus plays a critical role in regulating the autonomic functions of the large intestine, and its dysfunction or involvement in certain medical conditions can have significant clinical implications.
Chronic Abdominal Pain
Disorders affecting the inferior mesenteric plexus, such as irritation or compression of its nerve fibers, can result in chronic abdominal pain, particularly in the lower abdomen. Conditions like irritable bowel syndrome (IBS) and chronic inflammatory bowel diseases (e.g., Crohn’s disease, ulcerative colitis) may involve abnormal autonomic regulation mediated through this plexus.
Surgical Considerations
During abdominal surgeries, particularly those involving the large intestine (e.g., colorectal surgeries, resection for cancer, or diverticulitis), the inferior mesenteric plexus may be at risk of injury. Damage to the plexus can lead to postoperative complications such as impaired bowel motility, chronic pain, or altered defecation patterns. Surgeons must take care to preserve the plexus and its nerve fibers to prevent such complications.
Autonomic Neuropathy
In conditions like diabetes mellitus, where autonomic neuropathy can occur, the inferior mesenteric plexus may be affected. This can lead to gastrointestinal dysfunction, including slowed transit of fecal material, constipation, and alterations in bowel habits due to impaired nerve signaling in the large intestine.
Pelvic Cancer Involvement
Cancers of the colon, rectum, or other pelvic organs can invade or compress the inferior mesenteric plexus, leading to pain and dysfunction of the lower gastrointestinal tract. In advanced cases, nerve-block procedures targeting the plexus may be used to manage severe pain in patients with metastatic cancer.
