Rectum and Anal Canal Anatomy PDF

Rectum and anal canal Prof R Ndou Anatomy Department Lecture Layout Revision of the anatomical features of the colon The anatomy of the rectum Position, structure, peritoneal coverings, lymphatic drainage and neurovascular supply The anatomy of the anal canal it...

Rectum and anal canal Prof R Ndou Anatomy Department Lecture Layout Revision of the anatomical features of the colon The anatomy of the rectum Position, structure, peritoneal coverings, lymphatic drainage and neurovascular supply The anatomy of the anal canal its position, structure, relations and neurovascular supply. Colon Atlas of Human Anatomy. 4th ed. Netter F.  Rectum The rectum begins at the level of the S3 (as a continuation of the sigmoid colon). It is macroscopically distinct from the colon, with an absence of taenia coli, haustra, and omental appendices. Two major flexures: Sacral flexure –follows the curve of the sacrum and coccyx. Anorectal flexure – anteroposterior curve with convexity anteriorly. Rectum There are additionally three lateral flexures superior, middle and inferior, which are formed by transverse folds of the internal rectum wall. The final segment of the rectum, the ampulla, relaxes to accumulate and temporarily store faeces until defecation occurs. It is continuous with the anal canal; which passes through the pelvic floor to end as the anus. Netter’s, Atlas of Human Anatomy, Section 5, 355-429 Copyright © 2014 by Saunders, an imprint of Elsevier Inc.  Peritoneal Coverings In the superior third of the rectum, the anterior surface and lateral sides are covered by peritoneum. The middle third only has an anterior peritoneal covering, and the lower 1/3 has no peritoneum associated with it. In males, the reflection of peritoneum from the rectum to the posterior bladder wall forms the rectovesical pouch. In females, the peritoneum reflects to the posterior vagina and cervix, forming the rectouterine pouch (pouch of Douglas). Anatomical Relations The rectum is located within the pelvic cavity, and is the most posterior of the pelvic viscera. Its anatomical relations are different in men and women: Anterior Posterior Male Female Rectovesical Rectouterine Sacrum and coccyx pouch pouch Sigmoid colon Sigmoid colon Piriformis Ileum Ileum Coccygeus Bladder Vagina Levator ani Prostate Cervix Sacral plexus Seminal vesicles  Blood Supply Superior rectal artery the continuation of the inferior mesenteric artery, supplies the proximal part of rectum. Middle rectal artery branch of the internal iliac artery supplies the middle part of rectum Inferior rectal arteries arising from the internal pudendal arteries, supply the anorectal junction and the anal canal. Netter’s, Atlas of Human Anatomy, Section 5, 355-429 Copyright © 2014 by Saunders, an imprint of Elsevier Inc. Venous drainage The rectal venous plexus surrounds the distal rectum and anal canal consists of an internal rectal plexus deep to the epithelium of the anal canal and an external rectal plexus external to the muscular coats of the wall of the anal canal. The superior rectal vein drains into the portal system, and the middle and inferior veins drain into the systemic system; this is an important area of portacaval anastomosis Netter’s, Atlas of Human Anatomy, Section 5, 355-429 Copyright © 2014 by Saunders, an imprint of Elsevier Inc.  Lymphatic Drainage Lymphatic drainage of the rectum is via the pararectal lymph nodes, which drain into the inferior mesenteric nodes. Additionally, the lymph from the lower aspect of the rectum drains directly into the internal iliac lymph nodes. Innervation The rectum receives sensory and autonomic innervation. Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses. Visceral afferent (sensory) fibres follow the parasympathetic supply. Grant’s Atlas of anatomy 13th ed, Copyright © 2013, Lippincott Williams & Wilkins. Pg 218 Innervation The rectum receives sensory and autonomic innervation. Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses. Visceral afferent (sensory) fibres follow the parasympathetic supply. Grant’s Atlas of anatomy 13th ed, Copyright © 2013, Lippincott Williams & Wilkins. Pg 218 The Anal Canal Anal canal The anal canal is the final segment of the gastrointestinal tract. It has an important role in defecation and maintaining faecal continence. It has internal and external sphincters that provide subconscious and conscious control over the process of defecation. It prevents defecation until a socially appropriate time Defecation Mechanism Colonic mass movements and peristalsis move intestinal contents distally into the rectum. Rectal distention stimulates stretch receptors, with the signals spreading to the descending colon, sigmoid, and rectum via the myenteric plexus. The process initiates the defecation reflex and forces feces toward the anus. Inhibitory myenteric plexus signals relax the internal anal sphincter as the peristaltic waves approach the anus. Defecation occurs when the external anal sphincter is voluntarily relaxed. The myenteric defecation reflex is weak on its own. However, parasympathetic impulses bolster the myenteric signals. Craniosacral involvement starts with rectal wall distention sending afferent signals via the pelvic nerve to the defecation center in the spinal cord. The spinal defecation center sends back motor impulses to the descending colon, sigmoid, and rectum via pelvic nerve efferents. Parasympathetic signals cause strong sigmoid and rectal contractions and internal anal sphincter relaxation. Anatomical Position The anal canal is located within the anal triangle of the perineum between the right and left ischioanal fossae. It is the final segment of the gastrointestinal tract, around 4cm in length. The canal begins as a continuation of the rectum and passes inferoposteriorly to terminate at the anus.  Anatomical Structure Anal Sphincters The internal and external anal sphincters maintain of fecal continence Internal anal sphincter – surrounds the upper 2/3 of the anal canal. It is formed from a thickening of the involuntary circular smooth muscle in the bowel wall. External anal sphincter – voluntary muscle that surrounds the lower 2/3 of the anal canal (and so overlaps with the internal sphincter). It blends superiorly with the puborectalis muscle of the pelvic floor. At the junction of the rectum and the anal canal, there is a muscular ring – known as the anorectal ring. It is formed by the fusion of the internal anal sphincter, external anal sphincter and puborectalis muscle. Netter, Frank H., MD, Netter Atlas of Human Anatomy: Classic Regional Anatomy Approach, 6, 393- 468.e12Anorectal Musculature; Copyright © 2023 Copyright © 2022 by Elsevier Inc. Anatomical Relations Anteriorly Posteriorly Laterally Male Female Anococcygeal Ischioanal ligament fossae Coccyx and sacrum Perineal body Perineal body Urogenital diaphragm Urogenital diaphragm Urethra Vagina Bulb of the penis  Histology The superior aspect of the anal canal has the same epithelial lining as the rectum (columnar epithelium). However, in the anal canal, the mucosa is organized into longitudinal folds, known as anal columns These are joined at their inferior ends by anal valves. anal sinuses – these secrete mucus. The anal valves collectively form an irregular circle – known as the pectinate line (or dentate line). This line divides the anal canal into upper and lower parts, which differ in both structure and neurovascular supply. This is a result of their different embryological origins: Above the pectinate line – derived from the embryonic hindgut endoderm. Below the pectinate line – derived from the ectoderm of the proctodeum (ectodermal). Inferior to the pectinate line, the anal canal is lined by non- keratinised stratified squamous epithelium. transitions at the level of the intersphincteric groove to true skin (keratinised stratified squamous).  Histology of the Anorectal Junction The pectinate line marks the boundary between rectum and anal canal. In this area, simple columnar epithelium changes abruptly to an intervening transitional zone of stratified columnar epithelium, becomes nonkeratinized stratified squamous. Submucosa below the pectinate line has a Lower Digestive System plexus of inferior hemorrhoidal veins. The Ovalle, William K., PhD, Netter's Essential Histology, 13, 307-332 inner circular smooth muscle layer in the muscularis externa is thickened—the. involuntary internal sphincter. At the lower end of the anal canal, the epithelium Beyond this sphincter, skeletal muscle fibers of becomes keratinized and is continuous externally the levator ani form the voluntary external with epidermis of skin. sphincter. Large apocrine sweat glands, the circumanal glands, are in the dermis. Neurovascular Supply and Lymphatics Modality Above Pectinate line Below Pectinate line As discussed above, the Arterial Supply Superior rectal artery (branch of inferior mesenteric artery) Inferior rectal artery (branch of the internal pudendal artery) pectinate line divides the Anastomosing branches from the Anastomosing branches from the middle rectal artery. middle rectal artery. anal canal into two parts Venous Drainage Superior rectal vein, which empties into Inferior rectal vein, which empties – which have a different the inferior mesenteric vein (portal venous system). into the internal pudendal vein (systemic venous arterial supply, venous system). drainage, innervation and Nerve Supply Visceral innervation via the inferior hypogastric plexus. Somatic innervation via the inferior rectal nerves (branches of the lymphatic drainage. pudendal nerve) Sensitive to stretch. Sensitive to pain, temperature, touch and pressure. Lymphatics Internal iliac lymph nodes Superficial inguinal lymph nodes Test your knowledge !! What is the vertebral level at the start of the rectum? Describe the peritoneal covering of the rectum in each one third section. What are the differences in the anterior relations of the rectum in the male and female? Explain the systemic and portal venous drainage of the rectum Describe the lymphatic drainage of the rectum Describe the autonomic innervation of the rectum What forms the anorectal ring? Describe the anal columns, pectinate line and embryological derivatives of the parts above and below the pectinate line Describe the histology of the Anorectal Junction