The hypogastric nerves are a fundamental component of the autonomic nervous system, the body’s involuntary control system, specifically operating within the pelvic region. They function primarily as a crucial pathway, transferring signals between the upper abdominal nervous centers and the complex network of nerves that directly innervate the pelvic organs. While often referred to in the singular, the system involves a pair of nerves—the right and the left—that act as conduits for both sympathetic and parasympathetic fibers. This dual-fiber structure allows them to manage the involuntary functions of the bladder, reproductive organs, and the lower digestive tract.
Where the Hypogastric Nerves Are Located
The hypogastric nerves originate high in the lower abdomen from a broad, unpaired network called the superior hypogastric plexus. This plexus is situated centrally, lying on the vertebral bodies anterior to where the abdominal aorta divides into the iliac arteries. The superior plexus then splits into the distinct right and left hypogastric nerves, which begin their descent into the pelvis.
These nerves course inferiorly, traveling along the front surface of the sacrum. They are located deep within the retroperitoneal space, positioned medially to the internal iliac blood vessels. As they travel downward, the hypogastric nerves typically appear as a delicate meshwork rather than thick, single cords.
The hypogastric nerves connect the superior hypogastric plexus with the inferior hypogastric plexus, also known as the pelvic plexus. Sympathetic nerve fibers, which originate mainly from the T12 to L3 spinal cord segments, travel down the hypogastric nerves to reach the pelvic organs. Conversely, some parasympathetic fibers ascend from the pelvic splanchnic nerves (S2-S4) to meet the hypogastric nerves, contributing to the complexity of the inferior plexus.
Control Over Pelvic Organ Functions
The hypogastric nerves exert control by delivering sympathetic signals to the pelvic organs, balancing storage and relaxation with the parasympathetic system’s signals for emptying and contraction. This involuntary balance is essential for maintaining normal bodily function. The sympathetic fibers carried by these nerves often cause smooth muscle relaxation in organ walls and contraction in sphincter muscles.
In the urinary system, the sympathetic input from the hypogastric nerves promotes the storage phase of the micturition cycle. These signals cause the detrusor muscle in the bladder wall to relax, allowing it to fill with urine without increasing pressure. Simultaneously, the sympathetic fibers cause the internal urethral sphincter to contract, securing the flow of urine and maintaining continence.
The reproductive system relies on this sympathetic pathway for aspects of sexual function, particularly in males where the nerves innervate the prostate and seminal vesicles. The hypogastric nerves are responsible for semen emission, which is the movement of sperm and glandular fluid into the posterior urethra before expulsion. In the female pelvis, these nerves provide innervation to the uterus and vagina, where they help modulate uterine tone.
Regarding the gastrointestinal tract, the hypogastric nerves control the function of the distal colon and rectum. Sympathetic signals induce smooth muscle relaxation in the walls of the rectum, which aids in fecal storage. They also contribute to the contraction of the internal anal sphincter, necessary for maintaining fecal continence. The balance of sympathetic and parasympathetic signaling, integrated through the inferior hypogastric plexus, coordinates the defecation reflex.
Clinical Relevance in Surgery and Disease
Due to their location deep within the pelvis, the hypogastric nerves and their associated plexuses are vulnerable to injury during major surgical procedures. Surgical interventions such as radical prostatectomy, radical hysterectomy, and extensive rectal resections all carry a risk of damaging these nerves. Surgeons often utilize nerve-sparing techniques, guided by anatomical landmarks, to preserve the integrity of the hypogastric nerves.
Damage to these nerves can result in significant postoperative dysfunction because the involuntary control over the pelvic organs is compromised. A common consequence is neurogenic bladder, which manifests as an inability to properly store or empty the bladder, leading to urinary retention or incontinence. Similarly, patients may experience sexual dysfunction, such as ejaculatory issues in males, and difficulties with bowel control.
The hypogastric nerve pathways are also implicated in chronic pain syndromes affecting the pelvic area. The superior hypogastric plexus, from which the nerves arise, carries visceral sensory fibers that transmit pain signals from the pelvic organs back to the central nervous system. For patients suffering from pelvic pain, a procedure known as a superior hypogastric plexus block may be performed to interrupt these pain signals for therapeutic relief.