The pelvic splanchnic nerves, also known as nervi erigentes, are part of the parasympathetic nervous system. These nerves arise from the S2, S3, and S4 spinal nerve roots in the sacral region and are responsible for providing parasympathetic innervation to the pelvic organs. Unlike the thoracic and lumbar splanchnic nerves, which are sympathetic, the pelvic splanchnic nerves carry parasympathetic fibers.
Location
The pelvic splanchnic nerves are located in the pelvic region, emerging from the ventral rami of the sacral spinal nerves S2 to S4. They travel through the pelvic plexus (inferior hypogastric plexus) and extend to innervate structures in the pelvis, including the bladder, rectum, and reproductive organs. These nerves are distinct from the sympathetic splanchnic nerves and are key components of the parasympathetic outflow in the pelvis.
Structure and Anatomy
The pelvic splanchnic nerves are a critical component of the parasympathetic nervous system, emerging from the sacral spinal nerves to provide innervation to various pelvic organs. Below is a detailed description of the anatomy of these nerves.
Origin
The pelvic splanchnic nerves arise from the ventral rami of the sacral spinal nerves, specifically from S2, S3, and S4. These nerves are unique in that they carry parasympathetic fibers, unlike the thoracic and lumbar splanchnic nerves, which are sympathetic. The cell bodies of the preganglionic parasympathetic neurons are located in the intermediolateral cell column of the sacral spinal cord (S2-S4).
Course
After originating from the ventral rami of S2-S4, the pelvic splanchnic nerves exit the spinal canal through the anterior sacral foramina. They then travel in the pelvic cavity and converge to form part of the inferior hypogastric plexus (also known as the pelvic plexus). This plexus lies on the lateral walls of the pelvis, adjacent to the rectum in males and the vagina in females.
Relation to Other Nerves
The pelvic splanchnic nerves form an integral part of the inferior hypogastric plexus, where they mix with sympathetic fibers from the sacral splanchnic nerves (which arise from the sympathetic chain). While the pelvic splanchnic nerves are parasympathetic, they interact with the sympathetic nerves in the plexus to coordinate autonomic functions in the pelvic region. The close proximity of these parasympathetic and sympathetic fibers allows for integrated autonomic control of the pelvic organs.
Pelvic Plexus
The pelvic splanchnic nerves contribute significantly to the inferior hypogastric plexus. This plexus is positioned on the lateral sidewalls of the pelvis and extends its fibers to various pelvic organs. The parasympathetic fibers from the pelvic splanchnic nerves synapse with postganglionic neurons in ganglia located within or near the target organs, such as the bladder, rectum, and genitalia.
Branches and Distribution
The pelvic splanchnic nerves distribute parasympathetic fibers to several key structures within the pelvic cavity:
- Bladder: The nerves innervate the detrusor muscle of the bladder, playing a role in its contraction.
- Rectum: Parasympathetic fibers extend to the smooth muscle of the rectum to regulate motility.
- Reproductive Organs: The pelvic splanchnic nerves also provide innervation to the male and female reproductive organs. In males, they extend to the erectile tissue of the penis, while in females, they innervate the clitoris and other structures involved in sexual function.
- Uterus: In females, the nerves extend to the uterus, providing autonomic control.
- Prostate Gland: In males, the nerves provide autonomic innervation to the prostate.
Relations to the Hypogastric Plexus
The pelvic splanchnic nerves join the hypogastric nerves, which arise from the superior hypogastric plexus. Together, these nerves form the inferior hypogastric plexus, which provides autonomic innervation to the pelvic viscera. The hypogastric nerves are primarily sympathetic, while the pelvic splanchnic nerves are parasympathetic, allowing for the coordinated function of the sympathetic and parasympathetic nervous systems in the pelvis.
Ganglia
The postganglionic neurons of the parasympathetic nervous system, supplied by the pelvic splanchnic nerves, are located in ganglia that are either embedded in the walls of the target organs (such as the bladder or rectum) or positioned near them. These ganglia serve as relay stations for the preganglionic parasympathetic fibers to synapse with the postganglionic fibers, which then innervate the smooth muscle and glands of the pelvic organs.
Histological Structure
Microscopically, the pelvic splanchnic nerves consist of myelinated preganglionic fibers originating from the spinal cord. These fibers travel through the ventral rami of S2-S4 and into the inferior hypogastric plexus. The postganglionic fibers that arise after synapsing in ganglia are mostly unmyelinated and innervate the smooth muscle and glands of the target organs.
Connection with Other Autonomic Plexuses
The pelvic splanchnic nerves are connected to several other autonomic plexuses, including:
- Superior Hypogastric Plexus: Located higher up in the pelvic region, this plexus contains sympathetic fibers that join with the parasympathetic fibers from the pelvic splanchnic nerves in the inferior hypogastric plexus.
- Rectal Plexus: The parasympathetic fibers from the pelvic splanchnic nerves contribute to the rectal plexus, which innervates the smooth muscle of the rectum.
- Vesical Plexus: This plexus, associated with the bladder, receives parasympathetic input from the pelvic splanchnic nerves to control bladder contraction and urinary function.
- Prostatic Plexus: In males, parasympathetic fibers from the pelvic splanchnic nerves innervate the prostate and surrounding structures through the prostatic plexus.
Function
The pelvic splanchnic nerves play a crucial role in the parasympathetic regulation of the pelvic organs. They are responsible for a variety of involuntary functions related to urinary, gastrointestinal, and reproductive systems. Below is a detailed explanation of the functions of the pelvic splanchnic nerves.
Bladder Control (Micturition)
One of the primary functions of the pelvic splanchnic nerves is in the regulation of the bladder during urination.
Bladder Contraction
The pelvic splanchnic nerves provide parasympathetic innervation to the detrusor muscle of the bladder. Stimulation of these nerves causes the detrusor muscle to contract, facilitating the emptying of the bladder. When the bladder fills, stretch receptors in the bladder wall send signals to the spinal cord, which activates the parasympathetic pathways of the pelvic splanchnic nerves. This results in the detrusor muscle contracting, pushing urine out through the urethra.
Relaxation of the Internal Urethral Sphincter
Simultaneous with the contraction of the detrusor muscle, the pelvic splanchnic nerves cause the internal urethral sphincter to relax. This relaxation allows urine to pass through the urethra, completing the process of urination. This coordinated action ensures proper voiding of urine from the bladder.
Rectal Control (Defecation)
The pelvic splanchnic nerves are also essential for the regulation of bowel movements, providing parasympathetic innervation to the rectum and anal canal.
Stimulation of Peristalsis in the Rectum
Parasympathetic stimulation via the pelvic splanchnic nerves increases peristaltic activity in the rectum. This wave-like contraction of smooth muscle helps move fecal material from the sigmoid colon into the rectum. The presence of feces in the rectum triggers the defecation reflex, in which the pelvic splanchnic nerves stimulate the contraction of the rectal muscles, pushing the stool toward the anus.
Relaxation of the Internal Anal Sphincter
The pelvic splanchnic nerves also facilitate the relaxation of the internal anal sphincter during defecation. Once the rectum is distended, signals are sent through the pelvic splanchnic nerves to relax the smooth muscle of the internal anal sphincter, allowing the passage of feces. Voluntary control of the external anal sphincter by somatic nerves completes the process.
Sexual Function and Reproduction
The pelvic splanchnic nerves are involved in sexual arousal and function, providing parasympathetic innervation to the reproductive organs in both males and females.
Erection (Male Sexual Function)
In males, the pelvic splanchnic nerves play a key role in erection. Parasympathetic stimulation causes the relaxation of the smooth muscle in the walls of the penile arteries, leading to increased blood flow into the corpora cavernosa. This results in the swelling and hardening of the penis, facilitating erection. The parasympathetic action is essential for the vasodilation required for erection.
Vaginal Lubrication and Clitoral Erection (Female Sexual Function)
In females, the pelvic splanchnic nerves contribute to clitoral erection and vaginal lubrication during sexual arousal. Parasympathetic stimulation increases blood flow to the clitoral tissues, causing clitoral engorgement. Additionally, parasympathetic innervation increases glandular secretions in the vagina, providing lubrication during sexual activity.
Stimulation of the Uterus
The pelvic splanchnic nerves also innervate the uterus, particularly influencing uterine contractions during sexual activity. This may contribute to reproductive processes, such as sperm transport through the female reproductive tract.
Gastrointestinal Regulation
In addition to their role in defecation, the pelvic splanchnic nerves play a role in regulating certain functions of the lower gastrointestinal tract.
Colon Motility
The pelvic splanchnic nerves stimulate smooth muscle contraction in the distal colon and rectum, promoting the forward movement of fecal matter. This regulation of peristalsis helps maintain efficient digestive transit and ensures that waste is properly moved through the colon toward the rectum.
Secretion of Digestive Fluids
The parasympathetic fibers of the pelvic splanchnic nerves also stimulate the secretion of mucus and other digestive fluids in the distal colon, helping to lubricate the passage of stool and protect the lining of the intestine.
Reproductive Gland Function
The pelvic splanchnic nerves also influence the function of reproductive glands.
Prostate Gland
In males, the pelvic splanchnic nerves innervate the prostate gland. Parasympathetic stimulation promotes the secretion of prostatic fluid, which is a key component of semen. This function is important during ejaculation, ensuring the fluid is mixed with sperm to form semen.
Vaginal and Cervical Secretions
In females, parasympathetic stimulation through the pelvic splanchnic nerves enhances cervical and vaginal secretions, which play a role in reproductive health by helping to transport sperm through the female reproductive tract and maintain a healthy vaginal environment.
Coordination with Sympathetic Nervous System
The pelvic splanchnic nerves often work in coordination with the sympathetic nervous system to balance the autonomic regulation of pelvic organs.
Bladder and Rectal Sphincter Control
While the pelvic splanchnic nerves control the relaxation of the internal sphincters during urination and defecation, the sympathetic nervous system contributes to contraction of these sphincters during non-voiding phases. This coordination ensures that the internal sphincters remain contracted to retain urine and feces, and only relax when elimination is necessary.
Reproductive Function
The parasympathetic and sympathetic systems work together to regulate sexual function. For example, while the pelvic splanchnic nerves (parasympathetic) mediate erection and lubrication, the sympathetic nervous system is involved in the ejaculatory reflex in males, ensuring that sexual function is appropriately balanced.
Sensory Input and Reflexes
The pelvic splanchnic nerves also carry sensory fibers that contribute to several reflexes in the pelvic region.
Micturition Reflex
Sensory fibers from the pelvic splanchnic nerves relay information about bladder distension to the spinal cord. This input is critical for initiating the micturition reflex, which coordinates the contraction of the bladder and the relaxation of the internal urethral sphincter, allowing urination to occur.
Defecation Reflex
Similarly, sensory fibers detect rectal distension and send signals to the spinal cord, triggering the defecation reflex. This reflex ensures that the rectal muscles contract and the internal anal sphincter relaxes when defecation is necessary.
Clinical Significance
The pelvic splanchnic nerves are vital for parasympathetic control of the pelvic organs, and their dysfunction or damage can lead to significant clinical issues.
Bladder Dysfunction
Damage to the pelvic splanchnic nerves can result in neurogenic bladder, leading to problems such as urinary retention or incontinence due to impaired control of bladder contraction and internal sphincter relaxation.
Bowel Dysfunction
Injury to these nerves can cause fecal incontinence or constipation. The inability to effectively control the internal anal sphincter and stimulate peristalsis in the rectum can lead to difficulty in defecation.
Sexual Dysfunction
Damage to the pelvic splanchnic nerves can lead to erectile dysfunction in males and reduced vaginal lubrication and clitoral engorgement in females, affecting sexual health and function.
Surgical Considerations
Pelvic surgeries, such as prostatectomies or colorectal surgeries, may put the pelvic splanchnic nerves at risk, potentially leading to postoperative complications such as bladder and sexual dysfunction. Surgeons need to carefully preserve these nerves during procedures to maintain normal autonomic functions.
