Pelvic floor
Pelvic walls and floor: levator ani, perineal body, pelvic fascia and their nerve supply
The pelvic floor is a muscular hammock, chiefly the levator ani, pierced by a hiatus for the urethra, vagina and rectum, and slung from fascial lines on the side walls. Childbirth stretches it, tears it from the pubis and stretches its nerves, which is the main reason women develop prolapse and incontinence. CPSP asks its parts, nerve supply, the fascial layers and how each prolapse arises.
Pelvic walls
| Wall | Muscle | Key point |
|---|---|---|
| Lateral | Obturator internus | Covers the obturator membrane; tendon leaves by the lesser sciatic foramen; its fascia gives origin to levator ani |
| Posterior | Piriformis | From the front of S2–S4; leaves by the greater sciatic foramen; the sacral plexus lies on it |
| Floor | Levator ani and coccygeus | Pelvic diaphragm |
- Obturator externus is a thigh muscle, not a wall muscle.
Pelvic diaphragm
- Formed by levator ani plus coccygeus.
| Part of levator ani | Origin | Action and importance |
|---|---|---|
| Pubococcygeus (pubovisceral) | Back of pubis | Slings urethra, vagina and rectum; fibres to vagina and perineal body; avulsed by forceps or long second stage |
| Puborectalis | Pubis | U-shaped sling behind the anorectal junction, creating the anorectal angle (faecal continence) |
| Iliococcygeus | Tendinous arch on obturator fascia and ischial spine | Thin posterior sheet to anococcygeal body and coccyx |
- Coccygeus runs from the ischial spine to sacrum and coccyx on the sacrospinous ligament; it is in the pelvic diaphragm but not part of levator ani.
- Levator hiatus: the anterior gap between the two pubococcygeus edges, transmitting urethra and vagina (and rectum). It is wider in women, and prolapse passes through it.
- Levator plate: fused fibres between anorectal junction and coccyx, near horizontal; the upper vagina and rectum rest on it. It sags in prolapse.
- Anococcygeal body: midline band behind the anus; the perineal body lies in front.
Nerve supply and function
- Nerve to levator ani (S3–S4) on its pelvic surface, plus perineal and inferior rectal branches of the pudendal nerve (S2–S4) from below. Coccygeus: S4–S5.
- Levator ani is striated and mostly type I slow-twitch fibres: constant tone, even in sleep, with fast fibres for cough. Because it is voluntary skeletal muscle, pelvic floor training improves stress incontinence.
- It contracts reflexly just before abdominal pressure rises on coughing, closing the urethra; at voiding, the floor relaxes first, then the detrusor contracts.
- Its forward-sloping gutter turns the leading part of the head anteriorly; under a dense epidural, OP positions persist more often.
- The pudendal nerve hooks round the ischial spine, where it is tethered and stretched in a long second stage, giving denervation and a descending perineum.
- Vaginismus: involuntary spasm of pubococcygeus with normal anatomy.
Perineal body
- A fibromuscular node between the vagina and anal canal, derived from the urorectal septum. Attached: bulbospongiosus, superficial and deep transverse perinei, external anal sphincter, levator ani fibres and the perineal membrane.
- It is the level III support of the lower vagina; a median episiotomy cuts into it, dividing bulbospongiosus and the transverse perineal muscles.
Pelvic fascia
- Parietal fascia lines the walls and is continuous above the brim with the transversalis, iliac and psoas fascia.
- Visceral (endopelvic) fascia wraps the organs and condenses into ligaments.
- Arcus tendineus fasciae pelvis (white line): pubis to ischial spine; the pubocervical fascia under the bladder attaches to it. Lateral detachment gives a paravaginal cystocele (lost lateral sulci, rugae kept). The tendinous arch of levator ani lies just above it.
- Rectovaginal fascia (septum): between vagina and rectum; a defect gives a rectocele.
- Pubourethral ligaments anchor the mid-urethra to the pubis; mid-urethral slings replace them.
- Spaces: supralevator (pelvirectal) space above the levator, below the peritoneum (pelvic sepsis); ischioanal fossa below it (anal sepsis).
Prolapse
| Type | Organ | Defect |
|---|---|---|
| Cystocele | Bladder | Pubocervical fascia, central or paravaginal |
| Urethrocele | Urethra | Lower anterior wall, pubourethral support |
| Rectocele | Rectum | Rectovaginal fascia; low rectocele: perineal body |
| Enterocele | Small bowel in pouch of Douglas | Upper posterior wall; common after hysterectomy |
| Uterine or vault | Uterus or vault | Cardinal–uterosacral complex (level I) |
- On rectovaginal examination the finger enters a rectocele but not an enterocele.
- A large cystocele leaves residual urine, causing infection, and can kink the ureters.
- Causes: childbirth (levator avulsion, pudendal stretch), menopause (collagen loss), raised abdominal pressure. In nulliparous women think of spina bifida (denervated floor) or collagen disease. In pregnancy, progesterone and relaxin loosen pelvic tissue and cause early stress leakage.
- On dynamic MRI, descent is measured below the pubococcygeal line (lower symphysis to last coccygeal joint).
Remember The levator ani is the active floor; ligaments only take the load after the muscle fails, which is why prolapse appears years after the birth.
Exam trap Coccygeus is in the pelvic diaphragm but not part of levator ani; obturator internus is a wall muscle.
Exam trap The perineal body lies in front of the anus and the anococcygeal body behind it.
Test yourself on this topic
5 of the 31 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Electromyography shows that the levator ani is electrically active all day, even during sleep, keeping the hiatus closed and the pelvic organs supported. Which feature of its muscle fibres allows this constant activity without fatigue?
Answer: Mostly type I slow-twitch fibres
The levator ani is skeletal muscle with a high proportion of type I (slow-twitch, oxidative) fibres, rich in mitochondria and myoglobin and resistant to fatigue. This suits its constant postural role of supporting the pelvic organs. A smaller fast-twitch population produces strong, quick contractions when intra-abdominal pressure rises suddenly, such as on coughing.
Levator ani fibres
- Skeletal striated muscle
- Mostly type I slow-twitch
- Constant tonic support
- Fast fibres for coughing
Remember Postural muscles are slow-twitch.
Exam trap Constant tone does not mean smooth muscle; the levator is striated but slow-twitch.
Why the other options are wrong
- Mostly type IIb fast-twitch fibres Fatigue quickly; only a minority for sudden contractions.
- Mostly smooth muscle cells Levator ani is striated muscle.
- Mostly cardiac-like branched fibres Not a feature of skeletal muscle.
- Mostly denervated fibres Healthy muscle is innervated; denervation weakens it.
During vaginal vault repair, a surgeon places sutures into the thin posterior part of the levator ani that arises from the tendinous arch over the obturator internus fascia and from the ischial spine and inserts into the anococcygeal body and coccyx. Which part of the levator ani is this?
Answer: Iliococcygeus
The levator ani has three parts. Puborectalis and pubococcygeus arise from the back of the pubic body; puborectalis slings around the anorectal junction, and pubococcygeus (with its pubovaginalis fibres) runs back to the anococcygeal body and coccyx. Iliococcygeus arises from the tendinous arch of the levator ani, a thickening of obturator internus fascia, and the ischial spine, and is thin and posterior. Coccygeus is a separate muscle.
Levator ani parts
- Puborectalis: pubis, slings rectum
- Pubococcygeus: pubis to coccyx
- Iliococcygeus: tendinous arch and spine
- Tendinous arch on obturator fascia
Remember Ilio-coccygeus starts from the ilium side wall arch.
Exam trap Coccygeus is part of the pelvic diaphragm but not part of levator ani.
Why the other options are wrong
- Puborectalis Arises from the pubis and loops round the rectum.
- Pubococcygeus Arises from the pubic body, not the tendinous arch.
- Coccygeus A separate muscle on the sacrospinous ligament.
- Pubovaginalis Medial fibres of pubococcygeus around the vagina.
In a continent woman, pelvic floor EMG shows a burst of activity in the levator ani and urethral sphincter that begins just before intra-abdominal pressure rises when she coughs. What is the role of this reflex contraction?
Answer: Closing the urethra before pressure rises
A sudden cough or sneeze raises abdominal pressure, which pushes on the bladder. In continent women the pelvic floor and striated urethral sphincter contract reflexly, slightly before the pressure peak, lifting and compressing the urethra so urethral pressure stays above bladder pressure. Weak or delayed contraction after childbirth leads to stress incontinence, and pelvic floor training teaches women to pre-contract deliberately before coughing.
Guarding reflex
- Cough raises bladder pressure
- Pelvic floor contracts just before
- Keeps urethral pressure higher
- Weak reflex = stress incontinence
Remember Squeeze before you sneeze.
Exam trap Stress incontinence is a failure of urethral closure, not of the detrusor.
Why the other options are wrong
- Starting the micturition reflex Micturition needs relaxation, not contraction, of the floor.
- Relaxing the detrusor to store urine Detrusor relaxation is sympathetic and not this reflex.
- Increasing renal urine output No effect on urine production.
- Opening the bladder neck for voiding The opposite of what this contraction does.
A para 5 woman has an anterior vaginal wall prolapse in which the lateral vaginal sulci are lost, but the rugae over the bulge are preserved. At surgery, the pubocervical fascia has torn away from a white fascial line on the side wall running from the pubis to the ischial spine. Which structure is this line?
Answer: Arcus tendineus fasciae pelvis
The arcus tendineus fasciae pelvis (the white line) is a thickening of pelvic fascia stretching from the back of the pubis to the ischial spine. The anterior vaginal wall and pubocervical fascia attach laterally to it, forming a hammock under the bladder. Detachment from it gives a paravaginal (lateral) defect cystocele, with loss of the lateral sulci but preserved rugae. Central defects, by contrast, show smooth, rugae-less bulges.
Paravaginal defect
- White line = arcus tendineus fasciae pelvis
- Pubis to ischial spine
- Lateral detachment = paravaginal cystocele
- Lost lateral sulci, rugae kept
Remember Lateral sulci gone: the vagina has pulled off the white line.
Exam trap The arcus tendineus fasciae pelvis is different from the tendinous arch of levator ani, which lies just above it.
Why the other options are wrong
- Tendinous arch of levator ani Lies just above; gives origin to iliococcygeus, not the vaginal attachment.
- Pectineal (Cooper) ligament Lies along the pecten pubis; used in Burch-type repairs.
- Sacrospinous ligament Runs from the ischial spine to the sacrum, posteriorly.
- Perineal membrane edge Lies below the pelvic floor in the urogenital triangle.
In a long second stage, the descending fetal head pushes the pelvic floor downwards, while the pudendal nerve is held at a fixed bony point as it leaves and re-enters the pelvis. Where is the nerve tethered and most stretched?
Answer: Around the ischial spine
The pudendal nerve (S2–S4) leaves the pelvis through the greater sciatic foramen below piriformis, hooks around the ischial spine and sacrospinous ligament, and re-enters through the lesser sciatic foramen into the pudendal canal. It is relatively fixed at the spine, so when the pelvic floor descends during a prolonged second stage the nerve is stretched there. Repeated stretch neuropathy contributes to faecal and urinary incontinence and prolapse.
Pudendal nerve course
- From S2–S4
- Out via greater sciatic foramen
- Hooks round the ischial spine
- In via lesser foramen to Alcock canal
Remember The spine is where the pudendal nerve is pinned and stretched.
Exam trap The same spine is the landmark for a pudendal block, so the nerve is close to it.
Why the other options are wrong
- Within the obturator canal Transmits the obturator nerve, not the pudendal.
- At the deep inguinal ring The inguinal canal carries the round ligament and ilioinguinal nerve.
- Over the sacral promontory The pudendal nerve does not cross the promontory.
- Under the arcuate pubic ligament The dorsal nerve of the clitoris passes here distally, but the main tether is the spine.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Pelvis, pelvic floor and perineum, Pelvic walls and floor: levator ani, perineal body, pelvic fascia and their nerve supply