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1. Diagram the boundaries and features of the perineum, and understand the location and function of the perineal body.

The perineum is the region of the pelvis superficial to the pelvic floor, and may be best conceptualized as a diamond-shaped structure consisting of two non-coplanar triangles: the urogenital triangle and the anal triangle. The inter-ischial line (the plane between ischial tuberosities) forms the boundary between the urogenital and anal triangles.

The urogenital triangle contains structures relating to the distal urinary tract and external features of the reproductive system (e.g. root of penis & scrotum, or labia majora, labia minora, clitoris, and vestibule of vagina).

The anal triangle contains the anal canal and anal sphincters, and the ischio-anal (ischiorectal) fossae with pudendal canals.

The perineal body, a fibromuscular mass, sits on the midline between the urogenital and anal triangles, at the level of the muscles within the superficial perineal space and external anal sphincter. It may be palpated through the skin, but it may only be seen upon dissection into the superficial space of the urogenital triangle and anal triangle.

2. Review the structures of the urogenital triangle with an emphasis on structures of the superficial perineal space/pouch.

The superficial perineal space is deep to the superficial fascia (subcutaneous), and superficial to the perineal membrane. Contained within the superficial perineal space are the erectile bodies and associated musculature. 

The urogenital triangle of the perineum consist of the following layers (superficial-to-deep):

• Skin

• Subcutaneous

  • Superficial fatty (Camper’s) fascia

  • Deep membranous (Colles’) fascia

• Superficial perineal space (pouch)

  • Ischiocavernosus mm. & Crura of clitoris/penis

  • Bulbospongiosus mm. & Vestibular bulbs/bulb of the penis

  • Perineal body (anterior portion)

  • Greater vestibular (Bartholin’s) glands

• Perineal membrane

• Deep perineal space (pouch)

  • Bulbo-urethral (Cowper’s) glands

• Pelvic floor (Levator ani m.)

There are two sets of erectile bodies: corpus spongiosum and corpus cavernosum. Within close association with the portions of the erectile tissues within the superficial perineal space are muscles: bulbospongiosus & ischiocavernosus mm. These muscles apply pressure to erectile tissue. Bulbospongiosis mm. associate with corpus spongiosum derivatives, whereas ischiocavernosus mm. associate with corpus cavernosum derivatives

AFAB erectile bodies and associated muscles:

The corpus spongiosum manifests as the bulbs of the vestibule (vestibular bulbs), masses of erectile tissue flanking the vestibule of the vagina and attached to the perineal membrane. The vestibular bulbs are united anteriorly by a slight commissure, which is associated with the body of the clitoris. The vestibular bulbs are served by the arteries of the bulb (of vestibule). The vestibular bulbs are covered by the bulbospongiosus mm.

The clitoris is a mass of erectile tissues important for sexual response. Most of the clitoris consists of paired elements of erectile tissue called corpora cavernosa (sing. = corpus cavernosum). The corpora cavernosa of clitoris consist of the crura of clitoris (which run adjacent to the inferior margins of the ischiopubic rami) and the body of clitoris (which consists of the paired, yet separated termini of the corpora cavernosa). The body of clitoris (often referred to as the shaft) may be palpated through the skin, and the glans (of) clitoris is composed of corpus spongiosum, and sits atop the inferior portion of the body of clitoris. The glans clitoris is densely covered in sensory receptors and free nerve endings. The erectile tissues of the clitoris and the superficial tissues of the glans clitoris are served by the branches of the internal pudendal a. The crura of clitoris are covered by the ischiocavernosus mm.

AMAB erectile bodies and associated muscles:

The corpus spongiosum manifests as a midline mass proximally (within the deep perineal space) as the bulb of the penis, continuing distally (surrounding the urethra in the body of the penis) as the corpus spongiosum penis, and terminating as the glans penis. The bulb of the penis is covered by the bulbospongiosus mm.

The corpora cavernosa manifest as the crura of the penis proximally, along the ischiopubic rami (within the deep perineal space), continuing into the body of the penis as dorsally paired masses of erectile tissue. The crura of the penis are covered by the ischiocavernosus mm.

The superficial and deep spaces of the urogenital triangle are variously traversed by branches of the pudendal n. and internal pudendal a. (the dominant neurovascular supply to the perineum). Pudendal is derived from the Latin pudere, ‘to be ashamed.’ [As anatomists, we contend that no structure is inherently shameful, so chin up!]

3. Describe the location of the ischio-anal fossae, and diagram the contents located within.

The ischio-anal fossae occupy the majority of the superficial anal triangle, and laterally flank the anal canal and sphincters. The main content of these fossae is adipose tissue, in addition to pudendal neurovasculature.

Boundaries of the ischio-anal fossa:

• Anterior: inter-ischial line

• Posterior: gluteus maximus m. and sacrotuberous ligament

• Medial: external anal sphincter and portions of levator ani m.

• Lateral: lateral pelvic wall, obturator fascia, and obturator internus m. This includes the pudendal (Alcock’s) canal.

Pudendal (Alcock’s) canal is located within the internal obturator fascia of the lateral wall of the ischio-anal fossa. The pudendal canal transmits neurovasculature that serves the perineum, specifically,

• Pudendal n. & brs.

• Internal pudendal a. & brs., and

• Internal pudendal v. & tributaries.

4. Diagram and describe the locations, muscular layers, and structures of the rectum and anal canal.

Rectum:

The sigmoid colon transitions into the rectum at the rectosigmoid junction at  approximately S3, and the more proximal portions of the rectum are similar in diameter to the sigmoid colon. The distal portion of the rectum is dilated and referred to as the rectal ampulla. The distal rectum transitions into the anal canal at the recto-anal junction, adjacent to the puborectalis part of levator ani m.

The outer longitudinal muscle layer of the rectum is continuous, which differs from the distinct, separated taeniae coli of the large intestine.

The internal rectum typically has three transverse (rectal) folds. These folds are permanent and are most distinguishable in rectal distension.

The rectum is supplied by the superior rectal a. (from the inferior mesenteric a.) and the middle rectal aa. (from the internal iliac aa.). The rectum, as part of the hindgut, is innervated by the inferior mesenteric plexus. Parasympathetic fibers to the rectum are sourced from pelvic splanchnic nn., branches of the ventral primary rami of S2-4. 

Anal canal:

The anal canal begins at the anorectal junction and ends at the anus. The puborectalis m. (part of levator ani m.) forms a sling around the anorectal junction that imparts a flexure between the anus and rectum. As a result, the anal canal is oriented posteriorly.

The mucosal lining of the proximal two-thirds of the anal canal contains numerous infoldings called anal columns. The anal columns contain terminal brs. of the superior rectal a. The spaces between the anal columns are the anal sinuses. The anal columns & sinuses terminate at the pectinate line.

The proximal two-thirds of the anal canal is surrounded by three layers of muscle: the internal anal sphincter (nearest to the alimentary canal), the longitudinal musculature, and the external anal sphincter (surrounding the other two layers).

• The internal anal sphincter is a specialization of the circular layer of the muscularis of the colon, and is, therefore, composed of smooth muscle innervated by fibers from the inferior hypogastric plexus. At ‘rest,’ the internal anal sphincter is constricted.

  • Sympathetics: maintain tone/constriction; decreases peristalsis

  • Parasympathetics: relaxes smooth muscle; increases peristalsis

• The longitudinal musculature is a continuation of the longitudinal smooth musculature of the rectum. The longitudinal musculature atrophies with age, and is replaced by connective tissues.

• The external anal sphincter is composed of skeletal muscle tissue innervated by the inferior rectal nn. (from pudendal nn.). The external anal sphincter is served by the inferior rectal aa.

Both the internal and external anal sphincters (in addition to the puborectalis m.) must relax to allow defecation.

The pectinate (dentate) line demarcates the proximal two-thirds from the distal one-third of the anal canal. The pectinate line is useful to distinguish regions of the anal canal that vary with respect to: embryological origin, neurovascular service, lymphatic drainage, and classification of hemorrhoid.

5. Diagram and describe the origins of the internal iliac a. and its branches associated with the pelvis. Diagram and describe the branches of the internal pudendal a. and its targets in the perineum.

The common iliac a. bifurcates into its terminal branches, external & internal iliac aa., at approximately the level of the pelvic brim or the L5/S1 intervertebral disc.

The external iliac aa. are larger than the internal iliac aa., and supply the lower limb. The internal iliac aa. supply structures associated with pelvic and gluteal regions.

The internal iliac a. divides into anterior and posterior divisions at approximately the level of greater sciatic notch.

The posterior division of internal iliac a. primarily serves musculature of the back and hip. The anterior division of internal iliac a. supplies pelvic viscera & musculature.

Posterior division of internal iliac a. - branches

• Iliolumbar a.

• Lateral sacral aa.

• Superior gluteal a.

  • Continuation of posterior division

  • Largest branch of internal iliac a.

  • Exits greater sciatic foramen superior to piriformis m.

  • Supplies muscles associated with the hip joint: gluteus maximus, medius, and minimus, piriformis, and obturator internus mm.

Anterior division of internal iliac a. branches

• Umbilical a.

  • Typically 1st branch of anterior division

  • Gives off:

    • Superior vesical aa.: supplies superior bladder, distal ends of ureters, and proximal ends of ductus deferens and seminal vesicles

  • Continues as:

    • Medial umbilical ligament: obliterated umbilical a.

      • Associated with medial umbilical fold

• Obturator a.

  • Runs medially on the obturator fascia to exit the pelvis via the obturator canal

  • Supplies the medial compartment of thigh (adductors of hip), some pelvic muscles, skin

• Inferior vesical a.

  • Supplies inferior portion of the bladder

  • Variable in origin. Possible branching points include:

    • Directly off anterior division, umbilical a., vaginal a., uterine a., & middle rectal a.

  • Prostatic brs. supplying the prostate, seminal vesicles, and ductus deferens

• Vaginal a.

  • Variable in origin

  • Will often have branches that supply the inferior bladder (inferior vesical brs.)

• Uterine a.

  • At the internal os of cervix, the ureter crosses inferior (‘under’) the uterine a.

    • This relationship is particularly important in uterine surgeries when the uterine a. is ligated.

      • ‘Water under the bridge’

    • Anastomose with ovarian a. and vaginal a. branches

• Internal pudendal a.

  • Exits through the greater sciatic foramen, and then enters the ischio-anal fossa via the lesser sciatic foramen and pudendal canal

  • Primary source of blood to the perineum

  • Branches include:

    • Inferior rectal (anal) aa.

      • Supplies: external anal sphincter

    • Perineal aa.

      • Supplies: ischiocavernosus mm., bulbospongiosus mm., skin and subcutaneous layer of the perineum (and labia majora in AFAB/ scrotum in AMAB)

    • Arteries of the bulb (vestibule/penis)

      • Supplies: Corpus spongiosum

    • Deep aa. of clitoris/penis:

      • Supplies: corpora cavernosa through helicine and straight artery branches

    • Dorsal aa. of clitoris/penis:

      • Supplies: superficial tissues of the clitoris/penis; deep perineal pouch

• Inferior gluteal a.

  • Supplies portions of the pelvic diaphragm, muscles of the hip (gluteus maximus m., piriformis m., quadratus femoris m.) and some portion of the hamstring mm.

    • Provides collateral circulation to the thigh

  • Exits through the greater sciatic foramen inferior to the piriformis m.

  • May be a terminal branch of posterior division

6. Describe the location and source of the sacral plexus

The sacral plexus is a somatic nervous plexus (somatic afferent & efferent fibers). This plexus is composed of the lumbosacral trunk, ventral primary rami (VPR) of S1-S3, and a portion of the ventral primary ramus of S4. The ventral primary rami of the sacral spinal nerves enter the pelvic cavity through the anterior sacral foramina, and lie anterior to the piriformis m.

The lumbosacral trunk is formed by a portion of the VPR of L4, L5, & S1. It is visible medial to the psoas major m., and the lumbar portion joins S1 anterior to the sacro-iliac (SI) joint.

The sacral plexus has numerous branches, including the superior & inferior gluteal nn., sciatic nn., and pudendal nn. Most branches will be discussed in more detail in the musculoskeletal sequence. The pudendal nn. and its branches will be discussed in another learning objective.

The pudendal n. arises from VPR of S2-4 and follows the internal pudendal a. out the greater sciatic foramen, back through the lesser sciatic foramen (where there is a close relationship with the ischial spine), and through the pudendal (Alcock’s) canal to serve as the primary nerve of the perineum. The ischial spine is a palpable landmark for a pudendal block because of its close relationship with the pudendal nerve.

The pudendal n. has several major brs., including the:

• Inferior rectal (anal) nn.

  • Efferent to external anal sphincter

  • Afferent from peri-anal skin, and potentially inferior vagina

• Perineal nn. 

  • Deep perineal nn. - efferent to mm. of perineum (ischiocavernosus, bulbospongiosus, etc.)

  • Posterior labial/scrotal nn. - afferent from skin of posterior labia majora/ scrotum & inferior vagina

• Dorsal nn. of the clitoris/penis - afferent from the skin of the shaft of clitoris / penis; sympathetic motor to skin of shaft of clitoris/penis

7. Understand basics of the autonomic supply of the pelvis.

There are three autonomic plexuses/nerves associated with the pelvis: superior hypogastric plexus, hypogastric nn., and inferior hypogastric plexuses. It is important to note that these are continuous with one another and with abdominal autonomic plexuses. The inferior hypogastric plexuses are considered the main sources of autonomics to the pelvis.

The superior hypogastric plexus lies within extraperitoneal connective tissue in the midline, anterior to the abdominal aortic bifurcation. It is derived from three sources: lumbar splanchnic nn. (sympathetic), hypogastric nn. (sympathetic and parasympathetic), and the aortic plexus (sympathetic and parasympathetic). The organization of this plexus is widely variable, with possible presentations ranging from a web-like appearance to distinct nerve trunks.

The typically paired hypogastric nn. are located between the superior & inferior hypogastric plexuses, medial to internal iliac vasculature and lateral to the anterior sacral foramina. While these are referred to as nerves, they typically present as condensations of nerve fibers. These nerves consist of both sympathetic and parasympathetic fibers, entering and leaving the pelvis.

The inferior hypogastric plexuses are located on either side of the rectum. The main sources of autonomics for this plexus are derived from the pelvic splanchnic nn. (parasympathetic) and sacral splanchnic nn. (sympathetic). Sacral splanchnic nn. are typically derived from the 1st and 2nd sympathetic sacral ganglia. Pelvic splanchnic nn. consist of parasympathetic fibers derived from the ventral primary rami of S2-S4. The main destination of pelvic splanchnic nn. is the inferior hypogastric plexus, but some fibers travel to the hypogastric nn. or directly to the descending and sigmoid colon.

It is important to note that the inferior hypogastric plexus sends sub-plexuses to most internal pelvic viscera, these include:

• Vesical plexus - urinary bladder & seminal vesicles

• Prostatic plexus - prostate gland

• Uterovaginal plexus - uterus & vagina

• Rectal plexus - rectum & anal canal

Parasympathetic fibers’ main roles:

• Contraction of smooth muscle of bladder and rectum/anal canal

• Relaxation of smooth muscle that allows for increased blood flow into erectile tissues, leading to tumescence & erection

• Initiates glandular secretions

Sympathetic fibers’ main roles:

• Vasoconstriction

• Inhibition of rectal/anal smooth muscle contraction

• Initiating emission (seminal fluid into urethra)

• Contraction of smooth muscle of genital organs leading to ejaculation (in conjunction with dominant somatic innervation - pudendal n.)

General Visceral Afferent fibers

• Reflexive sensations all travel with pelvic splanchnic nn. (parasympathetic)

• Pain sensation is dependent on relationship with ‘pelvic pain line’

  • In the pelvis, the pelvic pain line is synonymous with the inferior-most extent of peritoneum

  • Superior to pelvic pain line: travel with sympathetic fibers

  • Inferior to pelvic pain line: travel with parasympathetic fibers

Visceral pain from pelvic organs refers to the suprapubic region.