The sacral splanchnic nerves are part of the autonomic nervous system and consist mainly of sympathetic fibers. They arise from the sacral part of the sympathetic trunk and travel to the pelvic region. These nerves originate from the sacral sympathetic ganglia (S2 to S4 levels) and course toward the pelvic organs.
Located in the pelvis, the sacral splanchnic nerves connect to the inferior hypogastric plexus, where they contribute sympathetic innervation to the pelvic viscera, including the bladder, rectum, and reproductive organs. They run anteriorly, passing alongside the sacrum and coccyx, within the pelvic cavity.
Structure and Anatomy
Origin
The sacral splanchnic nerves originate from the sacral portion of the sympathetic trunk, specifically from the sacral sympathetic ganglia, located at the level of the second to fourth sacral vertebrae (S2 to S4). The sympathetic trunk runs vertically along the anterolateral aspect of the sacrum, and the sacral splanchnic nerves branch off from these sympathetic ganglia. These nerves mainly carry sympathetic postganglionic fibers, although some preganglionic fibers may be present as well.
Pathway
Once they branch off from the sacral sympathetic trunk, the sacral splanchnic nerves pass anteriorly toward the pelvic cavity. They travel through the anterior sacral foramina, which are openings in the sacrum that allow passage of nerves and blood vessels. As they course toward their target, they run alongside the pelvic organs and are closely related to the sacral bones.
The nerves move towards the midline of the pelvis and eventually converge with other autonomic fibers from the pelvic splanchnic nerves (which carry parasympathetic fibers) at the inferior hypogastric plexus (also known as the pelvic plexus). This plexus is located anterior to the sacrum and near the rectum, and it serves as a hub for autonomic innervation to the pelvic organs.
Connections with the Inferior Hypogastric Plexus
The sacral splanchnic nerves enter the inferior hypogastric plexus, a complex network of autonomic nerves that innervate the pelvic viscera. In the plexus, the sympathetic fibers from the sacral splanchnic nerves integrate with parasympathetic fibers from the pelvic splanchnic nerves (S2–S4). This convergence allows the sympathetic and parasympathetic systems to work together in regulating the functions of pelvic organs.
The inferior hypogastric plexus lies in close proximity to the rectum and the lateral walls of the pelvis. It also gives rise to subsidiary plexuses that innervate specific pelvic organs, such as the rectal plexus, vesical plexus (for the bladder), and prostatic plexus (in males).
Structure
The sacral splanchnic nerves are relatively small but complex in structure. They consist predominantly of postganglionic sympathetic fibers, which are axons that have already synapsed in the sacral sympathetic ganglia. These fibers travel to their target organs, where they regulate the tone of smooth muscles, blood vessels, and other autonomically controlled tissues.
The nerve fibers are bundled together and encased in connective tissue as they travel toward their destinations. The nerves are closely associated with other autonomic fibers in the pelvis, such as those of the pelvic splanchnic nerves (which carry parasympathetic innervation).
Relationship to Other Structures
The sacral splanchnic nerves are located in close proximity to several important anatomical structures in the pelvis:
- Sacral Sympathetic Trunk: The sacral splanchnic nerves arise directly from the sacral portion of the sympathetic trunk, which is a vertical chain of ganglia running along the sacrum.
- Sacrum and Coccyx: The nerves are positioned near the sacrum and coccyx, running anterior to these bones as they course through the pelvis.
- Pelvic Splanchnic Nerves: While the sacral splanchnic nerves carry sympathetic fibers, the pelvic splanchnic nerves carry parasympathetic fibers. Both sets of nerves converge at the inferior hypogastric plexus, allowing integration of autonomic control over the pelvic organs.
- Pelvic Organs: The sacral splanchnic nerves are closely associated with pelvic structures such as the rectum, bladder, and reproductive organs. They contribute to the innervation of these organs by regulating smooth muscle tone and blood vessel function.
Blood Supply
The sacral splanchnic nerves receive their blood supply from small branches of nearby arteries, such as the internal iliac artery and its branches. These arteries provide oxygen and nutrients to the nerve fibers and surrounding tissues, ensuring their proper function. Venous drainage occurs via corresponding veins, which eventually drain into the internal iliac vein.
Variability
Anatomical variations in the sacral splanchnic nerves can occur among individuals. The number of nerves branching from the sacral sympathetic trunk, their precise course, and their connections with other autonomic fibers may differ. However, the overall function and role of these nerves in the autonomic control of the pelvic organs remain consistent across individuals.
Function
The sacral splanchnic nerves are part of the autonomic nervous system, primarily involved in carrying sympathetic fibers to the pelvic region. These nerves play a crucial role in regulating the functions of smooth muscles, glands, and blood vessels within the pelvic organs. The functions of the sacral splanchnic nerves can be categorized into several key areas.
Sympathetic Innervation of Pelvic Organs
The sacral splanchnic nerves primarily provide sympathetic innervation to the pelvic organs, including the bladder, rectum, reproductive organs, and other structures in the lower pelvic region.
- Bladder Function: Sympathetic fibers from the sacral splanchnic nerves help regulate the urinary bladder by promoting relaxation of the detrusor muscle (the muscle in the bladder wall) and contraction of the internal urethral sphincter. This activity is crucial in preventing involuntary urination and maintaining continence, especially during the filling phase of the bladder. The sympathetic innervation helps keep the internal urethral sphincter closed, allowing the bladder to store urine.
- Rectal Function: In the rectum, sympathetic fibers inhibit peristalsis (the rhythmic contraction of the rectal smooth muscle) and promote contraction of the internal anal sphincter. This regulation helps maintain fecal continence by preventing the premature release of stool and slowing down the movement of content through the rectum. During times of stress or sympathetic activation, these fibers help prevent involuntary defecation.
- Reproductive System: The sacral splanchnic nerves also play a role in the reproductive organs. In males, sympathetic innervation is essential for ejaculation. The sympathetic fibers trigger the contraction of smooth muscle in the vas deferens, seminal vesicles, and prostate, allowing the release of semen during ejaculation. In females, the sympathetic system helps regulate smooth muscle tone in the reproductive organs, including the uterus and vagina, particularly in the context of blood vessel constriction and control over the uterine walls.
Vasomotor Regulation
The sacral splanchnic nerves play an important role in controlling the blood vessels within the pelvic organs. Sympathetic innervation causes vasoconstriction, which helps regulate blood flow to the pelvic tissues. This vasomotor control is essential for adjusting blood supply based on the needs of the organs and the overall state of the body.
- Blood Flow Control: Sympathetic activation results in the constriction of blood vessels, reducing blood flow to non-essential areas during periods of stress, such as the fight-or-flight response. This response diverts blood from pelvic organs like the rectum and bladder, and redirects it toward vital organs like the heart and muscles.
- Regulation of Blood Flow During Sexual Arousal: Sympathetic fibers from the sacral splanchnic nerves also help regulate blood flow to the reproductive organs. In males, while parasympathetic innervation controls the erection of the penis, the sympathetic system regulates the subsequent decrease in blood flow following arousal and ejaculation, helping to return the organ to its flaccid state. In females, sympathetic innervation influences vascular tone within the uterus and vagina, modulating blood flow during various stages of the reproductive cycle.
Modulation of Autonomic Reflexes
The sacral splanchnic nerves contribute to several autonomic reflexes that are essential for the regulation of pelvic organ function. These reflexes involve the coordinated action of both the sympathetic and parasympathetic nervous systems.
- Micturition Reflex (Urination): While the micturition reflex is primarily governed by parasympathetic innervation, the sacral splanchnic nerves play a regulatory role in bladder control. Sympathetic fibers suppress the parasympathetic stimulation of the detrusor muscle, thereby allowing the bladder to fill and maintain continence. During the storage phase of the bladder cycle, the sympathetic activity promoted by the sacral splanchnic nerves prevents the premature contraction of the bladder.
- Defecation Reflex: Similar to micturition, the defecation reflex is primarily controlled by the parasympathetic system. However, the sacral splanchnic nerves’ sympathetic fibers contribute by maintaining the tonic contraction of the internal anal sphincter, preventing involuntary defecation. This modulation is crucial for maintaining continence when defecation is not appropriate.
- Sexual Reflexes: The sacral splanchnic nerves are involved in the reflexes that control sexual function. Sympathetic innervation regulates smooth muscle contraction in the reproductive ducts, such as the vas deferens in males, facilitating ejaculation. This function is part of the ejaculatory reflex, in which sympathetic outflow causes the release of semen by contracting the smooth muscles of the reproductive ducts.
Stress and Fight-or-Flight Response
The sacral splanchnic nerves are integral to the body’s response to stress, often referred to as the fight-or-flight response. As part of the sympathetic nervous system, they prepare the pelvic organs for situations of heightened activity or danger by promoting specific physiological changes.
- Inhibition of Digestive and Urinary Functions: During stress, the sympathetic fibers of the sacral splanchnic nerves inhibit the activity of the digestive and urinary systems. This allows the body to divert energy and resources to more critical functions, such as increased heart rate and muscle activity, needed for immediate survival. In the pelvic region, this inhibition reduces rectal peristalsis and suppresses bladder contractions to prevent defecation or urination in times of emergency.
- Increased Blood Pressure and Vascular Resistance: Sympathetic activation through the sacral splanchnic nerves contributes to increasing systemic blood pressure by constricting blood vessels, especially those in the pelvic and abdominal regions. This redirection of blood flow supports the body’s need to focus resources on essential organs like the heart and muscles during stressful situations.
Integration with Other Autonomic Pathways
The sacral splanchnic nerves integrate sympathetic responses with parasympathetic inputs in the pelvic region, particularly through their connections to the inferior hypogastric plexus. This allows a fine-tuned regulation of pelvic organ function, where sympathetic and parasympathetic inputs either complement or oppose each other, depending on the situation.
Coordination with Pelvic Splanchnic Nerves: The sacral splanchnic nerves work in conjunction with the pelvic splanchnic nerves, which carry parasympathetic fibers from the sacral spinal cord segments (S2 to S4). While the sacral splanchnic nerves handle sympathetic outflow, the pelvic splanchnic nerves manage parasympathetic innervation to the same pelvic organs, such as the bladder, rectum, and reproductive organs. This coordination allows the autonomic nervous system to precisely regulate functions such as micturition, defecation, and sexual activity.
Clinical Significance
The sacral splanchnic nerves are crucial for regulating sympathetic functions in the pelvic region, and any disruption or injury to these nerves can lead to significant clinical issues. Dysfunction of the sacral splanchnic nerves may result in disturbances in bladder control, leading to conditions like urinary retention or incontinence, as they help regulate the detrusor muscle and internal urethral sphincter. Similarly, they play a role in controlling defecation, and damage to these nerves can cause fecal incontinence or constipation.
These nerves are also important in the regulation of sexual function. Disruption can lead to sexual dysfunction, including ejaculatory dysfunction in males. Furthermore, the sacral splanchnic nerves contribute to the transmission of pelvic pain, so nerve blocks targeting them can be employed in the treatment of chronic pelvic pain, including pain due to conditions like endometriosis or pelvic inflammatory disease.
