20210827_Cooke_Pelvis_Perineum_notes.pdf

Pelvis and Perineum Lecture Notes Siobhán B. Cooke, Ph.D. Study Objectives This lecture serves as an introduction to the pelvis and perineum. Orientation, osteology, major muscles, fascial layers, pelvic circulation, and...

Pelvis and Perineum Lecture Notes Siobhán B. Cooke, Ph.D. Study Objectives This lecture serves as an introduction to the pelvis and perineum. Orientation, osteology, major muscles, fascial layers, pelvic circulation, and certain nerves are introduced here. You should know the skeletal components of the bony pelvis, the pelvic musculature, the male and female pelvic viscera, the course and major branches of the iliac vessels and pudendal nerve, the structure and function of the pelvic diaphragm, and the structures in the urogenital region. Terms Osteology levator ani rectouterine pouch innominate pubococcygeus pudendal canal ilium puborectalis ischium iliococcygeus Organs and Associated pubis internal and external anal Structures sacrum sphincters uterus coccyx bulbospongiosus broad ligament anterior superior iliac ischiocavernosus ovary spine suspensory ligament of ischial spine Nerves and Vessels ovary (IP ligament) ischial tuberosity pudendal nerve and branches transverse (cardinal) ligament acetabulum sacral plexus uterosacral ligament pubic symphysis lumbar splanchnic nerves pubocervical ligament pubic tubercle sacral splanchnic nerves vagina greater sciatic pelvic splanchnic nerves labia majora notch/foramen hypogastric plexus labia minora lesser sciatic external iliac artery/vein clitoris notch/foramen internal iliac artery/vein ductus deferens obturator foramen internal pudendal seminal vesicles sacral promontory artery/vein ejaculatory duct pelvic inlet/outlet testicular/ovarian prostate true/false pelvis artery/vein corpora cavernosa pampiniform plexus corpus spongiosum Ligaments scrotum sacrospinous ligament Spaces and Regions epididymis sacrotuberous ligament perineum urinary bladder sacroiliac ligament deep and superficial perineal ureter pouch urethra Muscles anal triangle perineal membrane piriformis muscle urogenital triangle perineal body obturator internus muscle rectovesical pouch pelvic diaphragm vesicouterine pouch Pelvis and Perineum Lecture Notes 2 LECTURE PART 1 I. Pelvic wall 1. Bony pelvis (slides 2-7) a. The innominate has three centers of ossification the ilium, ischium and pubis, which fuse over the course of development. i. Ilium is the superior portion of the innominate and articulates with the sacrum at the sacroiliac joint. ii. Ischium is the posterior-inferior portion of the innominate and includes a large ischial tuberosity posteriorly and includes the ischial spine, an important landmark and point of attachment for ligaments and muscles. iii. Pubis is the anterior-inferior portion of the innominate and includes the pubic tubercle, which is site of attachment of inguinal ligament. b. Sacrum is form by five fused vertebrae and forms much of the posterior-superior wall of the bony pelvis. It articulates with the ilium at the sacroiliac joint and with the last of the lumbar vertebrae at the lumbosacral joints. Its anterior-most point forms the sacral promontory. It Includes four pairs of sacral foramina which serve as exit points for nerves leaving the spinal cord in the sacral region. c. Coccyx is formed by four rudimentary vertebrae three of which are fused. d. The pelvic inlet (or superior aperture) is the opening bounded anteriorly by the superior margin of the pubic symphysis and posteriorly by the sacral promontory. The pelvic outlet (or inferior aperture) is the opening bounded anteriorly by the inferior margin of the pubic symphysis and posteriorly by the tip of the coccyx. 2. Ligaments (slides 8-11) a. Sacrospinous ligament runs between sacrum and the spinous process of the ischium and divides the sciatic notch into greater and lesser sciatic foramina. b. Sacrotuberous ligament runs between the sacrum and the ischial tuberosity and closes off the posterior-inferior margin of the lesser sciatic foramen. c. Sacroiliac ligaments run between sacrum and ilium on anterior and posterior sides of both bones help restrict movement of the sacrum when standing. 3. Joints (slides 12-15) a. Pubic symphysis is a relatively immobile joint held in place by the superior and inferior pubic ligament. The joint is encapsulated by interwoven layers of collagen fibers. b. The lumbosacral and sacroiliac joints are relatively stable as body weight is transmitted to the lower limbs through the pelvis. Clinical Note: Many of the ligaments of the pelvis loosen significantly under the influence of hormones released during childbirth allowing expansion of the pelvic outlet. Pelvis and Perineum Lecture Notes 3 Figure 1. Major anatomical features of the bony pelvis. 4. Muscles of the pelvic wall (slide 16) a. There are two main muscles in the pelvic wall. i. The piriformis originates on the posterior wall before leaving the pelvis posteriorly via the greater sciatic foramen en route to the femur. In the gluteal region, this muscle will serve as an important anatomical landmark. ii. The obturator internus covers the inside of the obturator foramen at its origin. It then exits the pelvis posteriorly via the lesser sciatic foramen en route to the femur. 5. Pelvic diaphragm (slide 17-19) a. The pelvic diaphragm forms cone-like partition at the bottom of the pelvis and is made up of the levator ani and coccygeus muscles. The levator ani is paired muscle which is usually treated as a unit, but is made up of three parts: the puborectalis, pubococcygeus, and iliococcygeus. The pelvic diaphragm helps to support viscera, regulate abdominal pressure, and maintain closure of the rectum and vagina. Males have two openings through the diaphragm: one for the anus and one for the urethra. Females have three including an opening for the vagina. Pelvis and Perineum Lecture Notes 4 Figure 2. The pelvic diaphragm. Pelvis and Perineum Lecture Notes 5 LECTURE PART 2 I. Orientation to the pelvic cavity and urinary and gastrointestinal systems 1. Peritoneal reflections (slides 2-4) a. As the peritoneum passes over the pelvic viscera, it dips between structures forming peritoneal pouches: i. rectouterine pouch and vesicouterine pouch in females ii. rectovesical pouch in males. b. The peritoneum forms folds around structures. These folds are often referred to as “ligaments” such as the broad ligament of the uterus, which serve supportive functions. II. Viscera within the Pelvis (slides 5-12) 1. The Rectum is the continuation of the sigmoid colon. Note the vascular supply comes from three different sources: the inferior mesenteric, internal iliac, and pudendal arteries. 2. The urinary system within the pelvis includes the ureters, muscular tubes passing from kidneys to posterior surface of the urinary bladder. The urinary bladder sits immediately posterior to the pubic bones and peritoneum covers its superior surface. The ureters enter the bladder on its posterior surface. Urine exits the bladder via the urethra. a. In females, the urethra immediately proceeds to the perineal region. b. In males, the urethra first traverses the prostate before reaching the perineal region. c. The urethra takes the name of the structure it traverses. In males, it is referred to as the prostatic urethra, membranous urethra, and spongy urethra d. In females, it goes directly from membranous urethra to its external opening. Pelvis and Perineum Lecture Notes 6 LECTURE PART 3 I. Typical female pelvic organs and supporting structures (slides 2-15) 1. Uterus (slides 4-8) a. A muscular organ in females responsible for the reception, retention, and nutrition of fertilized eggs. It is divided into body and cervix with the latter joining with the vagina. The uterus is supported by the pelvic diaphragm and the broad ligament of the uterus. b. The typical position is anteflexed and anteverted, but there is variation (slide 7). 2. Ovaries (slides 9-10) a. Glands responsible for the production of ova and female sex hormones. They are are located along the lateral wall of the pelvic cavity and sit immediately adjacent to the distal ends of the uterine tubes. b. They are supported by two ligaments visible as peritoneal folds – the suspensory ligament of the ovary, which contains the ovarian vessels and is in the lateral-most portion of the broad ligament of the uterus. 3. Vagina (slide 11) a. A musculomembranous tube running from the cervix of the uterus to the vestibule (the space between the labia minora). It normally collapses in on itself. except at superior end where uterine cervix holds it open. 4. Supporting structures (slides 12-15) a. Broad ligament of the uterus i. Covers and supports the uterus and surround structures ii. Has three parts a. Mesometrium – covers uterus b. Mesovarium – covers ovary c. Mesosalpinx – covers uterine tube iii. Cervical ligaments a. The uterus and cervix are supported by the pubocervical, uterosacral, and transverse cervical ligaments. Clinical note: During pregnancy, the uterus becomes greatly enlarged. By the 9th month of pregnancy, the tip of the uterus frequently reaches the xiphoid process at the end of the sternum. Pelvis and Perineum Lecture Notes 7 II. Typical female perineum (slides 16-24) 1. Perineal boundaries and pouches (slides 17-19) a. The perineum is a diamond-shaped region inferior to the pelvic diaphragm containing the external genitalia. The area anterior to the ischial tuberosities is the urogenital triangle, and the area posterior to the ischial tuberosities is the anal triangle. b. Perineal membrane is a thick, fibrous perineal membrane is attached laterally to the pubic arch. The membrane’s posterior border is mostly free but is anchored in the midline to the perineal body. It serves as an important boundary for the deep and superficial perineal pouches. i. The deep perineal pouch (or deep perineal space) is superior to perineal membrane. It contains: the external sphincter urethrae muscle, deep transverse perineal muscles, bulbourethral glands (males only), urethra, and vagina (female only). ii. The superficial perineal pouch contains the urethra, superficial transverse perineal muscles, erectile tissues and surrounding muscles, pudendal nerve and vasculature, the vagina, and vestibular glands (females only). 2. Urogenital Triangle (slide 20) a. The contents of the urogenital triangle include the external genitalia, and the superficial and deep perineal pouches. The external genitalia are the most superficial structures in UG triangle. 3. Female external genitalia (slides 20-23) a. The labia majora and labia minora are superficial folds of skin protecting urethral and vaginal openings. b. The clitoris is an erectile organ located where the labia minora meet anteriorly. i. It is composed of two erectile bodies, the corpora cavernosa. Each corpus is covered by ischiocavernosus muscle. c. The bulb of the vestibule splits to sit on either side of the vagina and covered by the bulbospongiosus muscle. d. Vestibular glands secrete mucus to moisten and lubricate labia and vagina. e. The external genitalia are supplied by branches of the internal and external pudendal arteries (external pudendal branches are branches of the femoral artery). Venous return mirrors arterial supply. f. The external genitalia is innervated by pudendal nerve, various superficial branches from the sacral plexus, plus autonomics. Pelvis and Perineum Lecture Notes 8 LECTURE PART 4 I. Viscera within the male pelvis (slides 2-6) 1. Ductus deferens or vas deferens (slides 3-4) a. thick, muscular tubes that carry sperm from the testes to the prostatic urethra. b. They run about 45 cm from the tail of the epididymis in the scrotum, through the inguinal canal (as part of the spermatic cord), down the lateral wall of the pelvis, to join the duct from the seminal vesicle to form the ejaculatory duct on the posterior surface of the prostate. 2. Seminal vesicles (slides 4-5) a. coiled, tubular organs which sit immediately posterior to the prostate. b. Join with the terminal end of the ductus deferens to form the ejaculatory duct, which immediately enters the prostate. Seminal vesicles do not store sperm but produce a thick alkaline fluid that mixes with other components of the seminal fluid and helps to nourish the sperm. 3. Prostate (slide 5) a. a glandular structure in males that lies immediately inferior the urinary bladder. The ejaculatory ducts enter the prostate’s posterior surface. The prostate contributes additional alkaline fluid to the seminal fluid that helps neutralize acidity in the vagina. Figure 5. The male pelvic viscera. Pelvis and Perineum Lecture Notes 9 II. Typical male perineum (slides 7-17) 1. Penis (slides 11-13) a. outlet for both urine and semen. b. It is composed of three cylindrical bodies of cavernous erectile tissue: i. corpus spongiosum a. a single erectile body containing the spongy urethra ii. corpora cavernosa a. two erectile bodies fused in the median plane. c. The erectile bodies are covered by muscles that take their names from the erectile bodies: i. bulbospongiosus ii. ischiocavernosus. iii. The blood supply to the penis primarily the internal pudendal artery from the anterior trunk of the internal iliac artery. It is drained primarily by the deep dorsal vein which, unlike the artery, does not follow the pudendal vessels, but instead passes under the pubic symphysis to drain into the anterior branch of the internal iliac vein. iv. It is innervated by pudendal nerve and autonomics. 2. Scrotum (slides 14-17) a. a cutaneous fibromuscular sac for the testes and associated structures. i. Testes a. produce sperm and hormones. Before birth, they descend retroperitoneally from posterior abdominal wall into scrotum ii. Epididymis a. a convoluted duct that receives sperm from the testis and transmits them to the ductus deferens. iii. Due to descent of testes, arterial supply is split between the testicular artery (testes and epididymis) and external and internal pudendal arteries (scrotum) The external pudendal artery is a branch of the femoral artery. Venous return mirrors arterial supply. iv. The testes and epididymis are innervated by autonomic branches from the vagus nerve and the lower thoracic region. The scrotum itself and the penis are innervated by pudendal nerves and superficial branches from the sacral plexus. III. Anal Triangle (slides 18-20) 1. The contents of the anal triangle include: a. The ischiorectal (or ischioanal) fossa – a wedge-shaped space between skin and pelvic diaphragm skirted by the pudendal nerves and vessels. The fossa itself is filled mainly with fat. b. The anal canal – the portion of the rectum inferior to the pelvic diaphragm including the external anal sphincter muscle (voluntary) and the internal anal sphincter (involuntary). Pelvis and Perineum Lecture Notes 10 LECTURE PART 5 I. Vasculature (slides 3-16) 1. Pelvic cavity – arteries (slides 3-5; 7-8; 12; 14-16) a. The descending aorta splits into right and left common iliac arteries in the lowermost lumbar region. This then splits into the external and internal iliac arteries. b. External iliac artery passes along the pelvic brim. It gives off the inferior epigastric artery before passing under the inguinal ligament to become the femoral artery. c. The Internal iliac artery supplies most of pelvic viscera, pelvic wall, perineum, and buttocks. Branching patterns are variable. It splits into the anterior and posterior trunks. i. Anterior trunk a. supplies most structures of interest in the pelvis; branches of interest include the uterine and vaginal arteries, the obturator artery (passes through obturator foramen en route to medial thigh), middle rectal artery (supplies the middle portion of the rectum), the internal pudendal artery which reaches the must perineum, via the greater sciatic foramen, then immediately passes through the lesser sciatic foramen to pass beneath the pelvic diaphragm, travels along lateral wall of ischiorectal fossa in the pudendal canal, a fascial sling provided by the obturator internus muscle. ii. Posterior trunk iii. Supplies posterior wall of pelvic cavity and part of the gluteal region. d. Superior rectal artery (a branch of the inferior mesenteric artery) supplies the superior portion of the rectum. e. The ovarian and testicular arteries both originate from the aorta in the abdomen. The ovarian artery passes under the broad ligament of the uterus to reach the ovary. The testicular artery skirts pelvic brim to enter inguinal canal en route to the scrotum as part of the spermatic cord. 2. Pelvic cavity – veins (slides 6, 10-11; 13, 14-16) a. The veins of the pelvic cavity mirror the arteries. b. Most of these are part of the inferior vena cava system. The exception is the rectum which drains into both the caval system and the portal system i. Superior rectal vein drains into the portal system ii. Middle rectal vein drains into the caval system iii. Inferior rectal vein drains into the caval system Clinical Note: This dual drainage makes the rectal veins a prime spot for evidence of portal hypertension via hemorrhoids. c. As with the arterial supply, the remainder of the venous return from the pelvis involves external and internal iliac veins joining to form the common iliac veins which then join to form the inferior vena cava. i. The external iliac vein is a continuation of the femoral vein from the lower limb, skirts the pelvis and doesn’t drain pelvic viscera. Pelvis and Perineum Lecture Notes 11 ii. The internal iliac vein is formed by union of tributaries corresponding to the branches of the internal iliac artery, including: the uterine and vaginal veins, the obturator vein, the internal pudendal vein. iii. The ovarian and testicular veins (or gonadal veins) are the same as arteries and travel much further superiorly than iliac veins. 3. Vasculature of the perineum (slides 9-11) a. Internal pudental artery largely supplies the perineum i. The clitoris is supplied bu the dorsal clitoral artery and drained by the deep dorsal clitoral vein. ii. The penis is supplied by the dorsal penile artery and drained by the deep dorsal penile vein. iii. Branches of the internal pudendal artery drain the labia and scrotum. iv. The testicular veins from the pampiniform plexus which allows for countercurrent heat exchange between the veins and testicular artery. II. Nerves of the pelvic cavity (slides 17-28) 1. Most of the nerves of the pelvic cavity come from the lumbar and sacral regions of the spinal cord, although certain structures (e.g., the ovaries) receive some innervation from lowermost thoracic nerves and even the vagus nerve due to their fetal origin in the lower thoracic region. a. Somatic innervation (slides 19-23) i. The sacral plexus is composed of nerves from spinal segments L4-S4. It is mainly a plexus of somatic nerves, but some sympathetic nerves are carried in its branches. ii. The pudendal nerve is responsible for much of the innervation of the external genitalia (e.g., dorsal nerve of the clitoris, dorsal nerve of the penis). It takes a circuitous route to the perineum, which you will trace in lab. b. Autonomic nerves in the pelvis (slides 24-28) i. Located deep in the pelvis on the anterior surface of the sacrum medial to the sacral foramina. ii. Sympathetic nerves a. The presynaptic fibers originate from the lowermost portion of the sympathetic trunk via the lumbar splanchnic nerves and sacral splanchnic nerves. Some fibers join nerves of the sacral plexus to be distributed to blood vessels and smooth muscle in lower limb. Most of fibers join hypogastric plexuses to supply the pelvic viscera. iii. Parasympathetic nerves a. The parasympathetic* nerves enter the pelvis as pelvic splanchnic nerves which are derived from sacral levels of spinal cord (S2-4). Most join hypogastric plexuses to supply pelvic viscera, however, some also travel up to provide parasympathetic innervations to the hindgut. Pelvis and Perineum Lecture Notes 12 Figure 6. Nerves of the pelvis. *Nota bene: There is now some debate in the literature about pelvic splanchnic nerves. A paper was published two years ago (Espinosa-Medina et al., 2016) calling for pelvic splanchnic nerves to be reclassified as sympathetic on the basis that the neurons share a cellular phenotype and transcriptional similarities with the other autonomic outflow from the spinal cord, the thoracolumbar sympathetic neurons. These authors argue that pelvic splanchnic nerves should be reclassified as sympathetic despite having 1) no connection to the sympathetic chain, 2) opposing physiological function to sympathetics with respect to pelvic viscera, 3) similar neurotransmitters to parasympathetics from the head. Responses to this claim were subsequently published (Neuhuber et al., 2017; Janig et al., 2017), asserting that the transcriptional similarities indicate “spinal” autonomics, rather than “sympathetic” autonomics, and highlighting the functional and anatomical differences between sacral and thoracolumbar autonomic outflow. Currently, there is no definitive answer, but if you read the Science paper, you should also read the responses, and be prepared for further discussion and debate in the literature. For now, this course will follow the text book and consider the pelvic splanchnic nerves to be parasympathetic. Espinosa-Medina I, Saha O, Boismoreau F, Chettouh Z, Rossi F, Richardson WD, Brunet J-F. 2016. The sacral autonomic outflow is sympathetic. Science 354:893-897. Neuhuber W, McLachlan E, Janig W. 2017. The sacral autonomic outflow is spinal, not “sympathetic”. Anatomical Record 300:1369- 1370. Janig W, Keast JR, McLachlan EM, Neuhuber WL, Southard-Smith M. 2017. Renaming all spinal autonomic outflows as sympathetic is a mistake. Autonomic Neuroscience 206:60-62.