Gross Anatomy ANS Pelvis Notes 2025 PDF

Gross Anatomy: ANS Pelvis Page 1 of 13 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately summarize the: 1. Autonomic Innervation of the Pelvic Viscera Summ...

Gross Anatomy: ANS Pelvis Page 1 of 13 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately summarize the: 1. Autonomic Innervation of the Pelvic Viscera Summarize the pelvic visceral efferent and afferent nerve fibers. Summarize the autonomic functional effects on pelvic viscera. Describe the gross anatomical organization of the pelvic visceral nerves. Explain the sympathetic innervation of the pelvic viscera. Explain the parasympathetic innervation of the pelvic viscera. Describe the afferents from the pelvic viscera. Describe the pelvic subsidiary plexuses. 2. Micturition reflex with application to clinical problems Describe the circuitry of the micturition reflex. Apply anatomical knowledge to discuss clinical problems affecting micturition. 3. Defecation reflex with application to clinical problems Describe the circuitry of the defecation reflex. Apply anatomical knowledge to discuss clinical problems affecting defecation. 4. Erection/Ejaculation with application to clinical problems Describe the circuitry of the erection/ejaculation reflex. Apply anatomical knowledge to discuss clinical problems affecting erection/ejaculation. Session Outline I. Innervation of the Pelvic Viscera A. Circuitry Overview B. Autonomic Effects C. Gross Anatomical Organization D. Sympathetic Innervation E. Parasympathetic Innervation F. Pelvic Subsidiary Plexuses G. Afferents II. Micturition A. Circuitry B. Clinical Problems III. Defecation A. Circuitry B. Clinical Problems IV. Erection/Ejaculation A. Circuitry B. Clinical Problems Gross Anatomy: ANS Pelvis Page 2 of 13 Dr. Paul Walker I. Innervation of the Pelvic Viscera A. Circuitry Overview Visceral efferent autonomic fibers innervate the pelvic viscera (rectum, bladder, and reproductive organs) to modulate reflexes involved in defecation, micturition, and reproduction. Visceral afferent fibers carry stretch and pain information to the spinal cord along the same nerve plexuses that convey the autonomic fibers. Knowledge of referred pain areas is useful to localize pathology of the pelvic viscera. PANS Preganglionic PANS fibers for pelvic organs originate from S2-S4 spinal levels and travel in Pelvic Splanchnic Nerves. SANS Preganglionic SANS fibers originate from T10-L2 spinal levels and travel in Thoracic, Lumbar, and Sacral Splanchnic Nerves. Fig 1 (Grays Anatomy for Students) Fig 2: Schematic diagram summarizing SANS vs. PANS innervation of the pelvic viscera including neurotransmitter receptors for pharmacological targeting. Gross Anatomy: ANS Pelvis Page 3 of 13 Dr. Paul Walker B. Autonomic Effects PANS Vasodilation Contract the bladder’s detrusor smooth muscle Stimulate engorgement of erectile tissues Increased colorectal peristalsis Relaxation of internal urethral & internal anal sphincters SANS Vasoconstriction (or maintain vasomotor tone) Contraction of internal urethral and internal anal sphincters. Smooth muscle contraction to move sperm along the male reproductive tract and stimulates secretions from seminal vesicles, prostate, and accessory glands. C. Gross Anatomical Organization Fig 3 shows the sacral plexus located on the posterior wall of the pelvic cavity with ventral rami emerging from anterior sacral foramina. The pudendal nerve is observed emerging from the ventral rami of S2-S4 in Fig 3. This nerve has important roles in bladder, rectal, and reproductive reflexes. The sympathetic trunk is also seen descending into the pelvic cavity in Fig 3. There are usually 4 sacral ganglia on each side that converge at the ganglion impar (covered by insertion of pubococcygeus muscle. Fig 3 (Gray’s Anatomy for Students) Gross Anatomy: ANS Pelvis Page 4 of 13 Dr. Paul Walker The addition of the hypogastric plexus to the drawing in Fig 4 increases the complexity of the nerves of the pelvic cavity. The superior hypogastric plexus is a highway of autonomic fibers that continue from the aortic plexus below the aortic bifurcation. Near the pelvic rim, the nerve fibers collect into right and left hypogastric nerves and descend into the pelvic cavity. Fibers spread apart in the pelvic cavity as the inferior hypogastric plexus (pelvic plexus). In addition to SANS fibers (thoracic, lumbar, and sacral splanchnics), pelvic splanchnic PANS fibers are also found in the inferior hypogastric plexus. Fig 4 (Gray’s Atlas of Anatomy) Gross Anatomy: ANS Pelvis Page 5 of 13 Dr. Paul Walker D. Sympathetic Innervation The below description refers to SANS innervation of the uterus, ovaries, vagina, external genitalia, and bladder of the female (left Fig 5) and the prostate, testes, external genitalia, and bladder of the male (right Fig 5). Preganglionic SANS axons from T10-L2 enter the sympathetic ganglion at their respective spinal levels. Fibers do not synapse in the sympathetic ganglia entered but instead travel through the ganglia to their postganglionic targets by several routes: Aortic Plexus Route: Preganglionic SANS fibers exit as thoracic and lumbar splanchnic nerves and descend in the aortic plexus of autonomic fibers toward their postsynaptic targets, which are small ganglia located in the inferior hypogastric plexus within the pelvis. To get there, they descend via the superior hypogastric plexus and use the left and right hypogastric nerves to reach the inferior hypogastric plexus. Sympathetic Trunk Route: Preganglionic SANS fibers DO NOT exit the sympathetic trunk, but instead descend in the sympathetic trunk inferiorly until they reach the sacral sympathetic ganglia of the pelvic cavity. This is shown in the right diagram of Fig 5 below. The nerves emerge from the sacral sympathetic ganglia as sacral splanchnic nerves and enter the inferior hypogastric plexus where they synapse in the same ganglia of the inferior hypogastric plexus as described above. Gonadal Route: Some preganglionic SANS fibers destined for the gonads exit as thoracic and lumbar splanchnic nerves and gather into the aortic plexus of autonomic fibers. Rather than descending into inferior hypogastric plexus, these fibers synapse in gonadal ganglia (spermatic and ovarian) located in the aortic plexus between SMA and IMA ganglia. Postganglionic fibers use the gonadal vessels to travel to the testes/ovaries. Fig 5 (Gray’s Atlas of Anatomy) Gross Anatomy: ANS Pelvis Page 6 of 13 Dr. Paul Walker Regardless of aortic plexus route or sympathetic trunk route, the sympathetic nerves gather in the inferior hypogastric plexus (with exception of gonadal fibers). The inferior hypogastric plexus is also called the pelvic plexus. The inferior hypogastric plexus contains pre- vs. post-ganglionic SANS nerves and preganglionic PANS fibers (discussed below). Why are the different routes important for clinicians to be aware of? Surgeons need to know where the autonomic pathways are located so they are not disrupted during abdominal or pelvic surgery. They carry fibers critical for bladder, rectal, and reproductive organ reflexes. Anesthesiologists need to know the various routes in which visceral pain information from pelvic organs is transported so appropriate anesthetic blocks can be performed. E. Parasympathetic Innervation Fig 6 (Gray’s Atlas of Anatomy) Preganglionic PANS axons originate from S2-S4 spinal levels and travel in the ventral rami of S2-S4 spinal nerves to enter the pelvic cavity. These axons come directly out of the S2-S4 ventral roots as pelvic splanchnic nerves. Pelvic splanchnic nerves join the other nerves of the inferior hypogastric plexus and travel to terminal PANS ganglia located in the pelvic viscera. How can you differentiate between pelvic splanchnics and sacral splanchnics in lab since both are located in the pelvic cavity? Pelvic splanchnics (PANS) emerge from S2- S4 ventral rami. Sacral splanchnics (SANS) emerge from sacral sympathetic ganglia. Gross Anatomy: ANS Pelvis Page 7 of 13 Dr. Paul Walker F. Pelvic Subsidiary Plexuses Fig 7 (Netter) PANS & SANS fibers in the inferior hypogastric (pelvic) plexus form sub-networks called pelvic subsidiary plexuses (Fig 8): 1. Rectal plexus 2. Vesical plexus 3. Uterovaginal plexus 4. Prostatic plexus Autonomic nerves from the uterovaginal plexus in females and the prostatic plexus in males continue into the perineum to innervate the erectile tissues. These are called cavernous nerves (as discussed in Dr. Goebel’s Perineum lecture). Visceral afferents are also located in the subsidiary plexuses. G. Afferents Fig 8 (Gray’s Atlas of Anatomy) Somatic afferents conveying touch and pain sensations from the external genitalia are transmitted to the S2-S4 spinal cord via the pudendal nerve. Visceral afferents conveying stretch and pain sensations from the pelvic organs are transmitted along the same plexuses used by the visceral efferent autonomic nerves. The ‘pelvic pain line’ refers to visceral pain relayed from pelvic structures that extend superiorly to contact the peritoneum vs subperitoneal pelvic cavity structures. Above the pelvic pain line, VA afferents travel via the hypogastric nerves and referred pain is felt in the abdominal region. Examples include superior bladder, uterus, ovaries. Below the pelvic pain line, VA afferents from these organs travel via the pelvic splanchnic nerves and referred pain is felt in the perineal region. Examples include inferior bladder, urethra, prostate, vagina. The location of the pelvic pain line in the colon is in the middle of the sigmoid colon. Gross Anatomy: ANS Pelvis Page 8 of 13 Dr. Paul Walker VIII. Micturition A. Circuitry of the Micturition Reflex Fig 9 1. Visceral afferent mechanoreceptors (stretch receptors) in the bladder wall are quiet during filling but become active as the walls are stretched beyond a certain threshold. 2. Two micturition centers of the CNS respond to VA signaling: A) S2-S4 Spinal Cord – center for reflexive coordination of PANS stimulation of bladder detrusor muscle coupled with relaxation of the external urethral sphincter. This reflex allows voiding regardless of circumstance. B) Brainstem (Pons) – center that receives VA signals relayed from the spinal cord regarding ‘urge’ to urinate. Connects to higher conscious centers for volitional decision making. When circumstances are appropriate to void, the brainstem micturition center sends the ‘go’ signal to the S2-S4 reflexive center. 3. There are 2 mechanisms that prevent voiding during bladder filling until the time is right: A) SANS (T10-L2) nerves maintain contraction of the internal urethral sphincter as well as inhibit detrusor muscles of the bladder wall. B) SE pudendal nerve (S2-S4) maintains contraction of the external urethral sphincter. 4. S2-S4 reflexive spinal cord center receives the ‘go’ signal from the brainstem. PANS signaling contracts the detrusor bladder muscles. The circuitry also inhibits SANS & SE effects on internal & external urethral sphincters such that both are relaxed to allow bladder emptying. Gross Anatomy: ANS Pelvis Page 9 of 13 Dr. Paul Walker B. Micturition- Clinical Problems Children master the volitional control of micturition between ages 2-5. Prior to this period, urination is entirely controlled by the spinal reflexive center. Enuresis is involuntary urination (e.g. noctural enuresis). Higher conscious centers are affected by emotions and because of their connections to the brainstem micturition center, may trigger urination in response to strong emotions (e.g. sudden intense fear). Paruresis (shy bladder) is the inability to void in certain settings (e.g. public restroom). Urinary tract infections (UTIs) irritate the VA fibers of the urethral and bladder wall and promote feelings of urgency to void even when urine volume is low. Urinary incontinence is the term for leaking urine. There are several types (Fig 10). Stress Incontinence is a condition in which individuals leak urine when sneezing, laughing, or other physical exertions. Urge Incontinence occurs in overactive bladder (OAB) in which there are excessive involuntary contractions of the detrusor muscles prior to filling. Overflow Incontinence occurs when the urethra is narrowed or blocked. Benign Prostatic Hyperplasia (BPH) is an enlarged prostate that narrows the urethra and promotes urinary retention and incomplete bladder emptying. Over time the bladder walls thicken, and the VA fibers become less efficient in relaying information to the spinal cord. Neurogenic Incontinence is due to a dysregulation of the neural circuitry. Spinal cord injury above the lumbosacral levels cuts off the descending brainstem pathways mediating voluntary control of micturition. Patients develop reflexive micturition but a problem of incoordination between detrusor and sphincter reflexes (detrusor- sphincter dysynergia) makes voiding inefficient and incomplete. Bladder dysfunction often accompanies dysautonomias that arise in various neurological diseases and manifest as urgency, incontinence, or retention. Fig 10 Gross Anatomy: ANS Pelvis Page 10 of 13 Dr. Paul Walker Below are examples of pharmacological targets for 2 common types of urinary incontinence: Overactive Bladder (OAB)- a type of urge incontinence Benign Prostatic Hypertrophy (BPH)- a type of overflow incontinence Do the agents developed for these targets fit your understanding of the micturition circuitry and autonomic innervation of the bladder? Fig 11 Vesicare is a muscarinic antagonist for OAB. It lessens overactive detrusor contractions by blocking PANS acetylcholine effects at the M3 receptor. Flomax is a alpha 1 adrenergic antagonist for BPH. It inhibits the constricting effects of SANS norepinephrine on the internal urethral sphincter. From Fowler et al. The Neural Control of Micturition, Nature Reviews Neuroscience, 19 (2008) 453-466. Modified Modified fromtoFowler include information et al. Theon pharmacological Neural Control of drug targeting. Nature Reviews Neuroscience, 19 Micturition, (2008) 453-466. Gross Anatomy: ANS Pelvis Page 11 of 13 Dr. Paul Walker III. Defecation A. Circuitry of the Defection Reflex Fig 12 1. Visceral afferent mechanoreceptors in rectum become active when a mass movement pushes feces into the rectum. A mass movement is a sustained high-intensity contraction of the circular muscles of the colon. 2. There are 2 mechanisms that prevent defecation until circumstances permit: Visceral efferent SANS fibers carried by hypogastric nerves maintain tonic activity of the internal anal sphincter. Somatic efferent fibers carried by the pudendal nerve maintain tonic stimulation of the external anal sphincter. In addition, tonic stimulation of the puborectalis muscle maintains the ‘sling-hold’ bend to the rectum. 3. If sufficient volume is present in the rectum, VA impulses trigger reflexive contractions of the colon via the PANS stimulation, which also regulates the relaxation of the internal anal sphincter. This promotes the ‘urge’ to defecate and reflexive contractions of the external anal sphincter prevent evacuation until circumstances permit. 4. If circumstances are not appropriate for defecation, continued contraction of the external anal sphincter causes ‘reverse peristalsis’ of the colon and pressure is relieved. The sense of urgency is diminished at that point until suitable conditions are found. 5. If circumstances are appropriate for defecation, S2-S4 spinal levels receive a ‘go’ signal from higher brain centers. Spinal interneurons inhibit pudendal nerve signals and the external anal sphincter relaxes. The puborectalis muscle also relaxes to promote a broader anorectal angle. Expulsion of feces is assisted by a Valsalva maneuver to increase intraabdominal pressure. Gross Anatomy: ANS Pelvis Page 12 of 13 Dr. Paul Walker B. Defectation- Clinical Problems Children master volitional control of defecation between ages 2-5. Some children develop a fear of defecation termed coprophobia, which usually occurs after a painful defecation. Similar to micturition, it is possible to lose control of defecation during periods of strong emotions (e.g. sudden intense fear). Fecal incontinence is the term for involuntary loss of rectal contents (feces, gas). A recent study reported that topical administration of oxymetazoline gel (alpha- adrenergic agonist) reduced fecal incontinence over placebo in a small study in spinal cord injured patients (Barak et al. Diseases of the Colon & Rectum 62: 2 (2019). The targeted mechanism is SANS mediated smooth muscle contraction of the internal anal sphincter. Constipation- inability to evacuate stools completely and spontaneously ≥3X per week. Normal Transit Constipation- feces are transported at normal rate through the colon and frequency of elimination is normal, but patients believe they are constipated because of difficulty in defecation or hard stools. Treatable with dietary fiber or osmotic laxative. Slow Transit Constipation- more common in young women with infrequent bowel movements (≤1X per week). May not be relieved with dietary fiber or osmotic laxative. Defecatory Disorders- usually due to impaired function (incoordination) of pelvic floor and/or anal sphincter musculature. Biofeedback training has been shown to be effective. Botox injections into the puborectalis muscle may work in patients who suffer spasms of pelvic floor musculature. Opioid Induced Constipation Fig 13 Disrupts peristalsis and causes sphincter spams, as well as fluid and electrolyte secretion. Opioid receptors are present in enteric neurons and alter Ach and NO/VIP transmission https://pubmed.ncbi.nlm.nih.gov/32547183/ Gross Anatomy: ANS Pelvis Page 13 of 13 Dr. Paul Walker IV. Erection/Ejaculation A. Circuitry Erection is PANS Mediated: center located at S2-S4 spinal levels. 1. Cutaneous SA and VA impulses travel in the pudendal and pelvic splanchnic nerves to S2-S4 spinal levels. Higher brain centers that process erotic signals also send projections to S2-S4 spinal cord. Fig 14 2. Preganglionic S2-S4 PANS neurons become activated and transmit a signal via pelvic splanchnic and cavernous nerves to the erectile tissues. PANS fibers stimulate the release of nitric oxide (NO) from the endothelium of helicine arteries, which promotes vasodilation and blood engorgement of the erectile tissues. Venous return is decreased due to distention of tunica albuginea against the circumferential veins (Fig 14). Ejaculation is SANS Mediated- center located T11-L2 spinal levels 1. Emission a) Contraction of the internal urethral sphincter to prevent retrograde semen into bladder. b) Movement of fluids from vas deferens, prostate gland, seminal vesicle. 2. Ejaculation a) Relaxation of the external urethral sphincter. b) Pulsatile contractions of prostatic smooth muscle, bulbospongiosus, ischiocavernosus, and transverse perineal muscles expel the semen. B. Clinical Problems Fig 15 Erectile Dysfunction- Viagra, Cialis, and Levitra are drugs (generic is Sildenafil) that prolong engorgement of erectile tissues via the NO/cGMP mechanism. Spinal cord injury above the lumbosacral region does not negate erectile tissue engorgement with cutaneous stimulation. The conus medullaris and peripheral nerves must be undamaged for preservation of reflex. Emotional state (fear, anxiety) has a large influence on the erection/ejaculation reflexes.