TBL23 - Anal Canal Flashcards
What does rupture of the cloacal membrane enable? What is the epidermal lined anus derived from?
1) Rupture of the cloacal membrane enables ectoderm and endoderm to jointly line the anal canal
2) The anal fold forms the epidermal lined anus
What does the rectum join at the anorectal junction and what must it perforate to do so? What do the anal columns define and what are they formed by? What do the inferior ends of the anal columns mark?
1) The rectum perforates the levator ani to join the anal canal at the anorectal junction
2) The superior portion of the anal canal is defined by anal columns, formed by permanent longitudinal folds of the endoderm-derived mucosa
3) The inferior ends of the anal columns mark the location of the pectinate line
What is the hindgut-derived portion of the anal canal above the pectinate line supplied by? What is the ectoderm derived portion of the anal canal below the pectinate line supplied by?
1) The hindgut-derived portion of the anal canal above the pectinate line is supplied mainly by the superior rectal artery
2) The ectoderm derived portion of the anal canal below the pectinate line is supplied by the inferior rectal artery, a branch of internal pudendal artery
Where is the hindgut-derived portion of the anal canal in relation to the pelvic pain line? When this portion of the anal canal is distended, what do visceral sensory fibers refer? What does the inferior rectal nerve supply? What is it locally sensitive to?
1) The hindgut-derived portion of the anal canal is below the pelvic pain line
2) When distended, visceral sensory fibers from DRG at S2-S4 refer dull diffuse pain to the perineum and posterior thighs
3) The inferior rectal nerve (recall it is a branch of the pudendal nerve) supplies the anal canal below the pectinate line
4) Thus it is locally sensitive to pain, touch, and temperature
Above the pectinate line, where does venous blood from the anal canal enter? Below the pectinate line, where does venous blood drain into? Define lymph drainage from the anal canal above and below the pectinate line.
1) Above the pectinate line, venous blood from the anal canal enters the hepatic portal vein
2) Venous blood below the pectinate line drains into the IVC
3) Superior to the pectinate line, the lymphatic vessels drain deeply into the internal iliac lymph nodes, and through them into the common iliac and lumbar lymph nodes
4) Inferior to the pectinate line, the lymphatic vessels drain superficially into the superficial inguinal lymph nodes, as does most of the perineum
What creates internal and external hemorrhoids, respectively?
Distension of veins in the lamina propria above and below the pectinate line creates internal and external hemorrhoids, respectively
What are predisposing factors for hemorrhoids and why can external hemorrhoids be painful while internal hemorrhoids are painless?
1) Predisposing factors for hemorrhoids include pregnancy, chronic constipation and prolonged toilet sitting and straining,
and any disorder that impedes venous return, including increased intra-abdominal pressure
2) Regarding pain from and the treatment of hemorrhoids, it is important to note that the anal canal superior to the pectinate line is visceral; thus it is innervated by visceral afferent pain fibers, so that an incision or needle insertion in this region is painless. Internal hemorrhoids are not painful and can be treated without anesthesia. Inferior to the pectinate line, the anal canal is somatic, supplied by the inferior anal (rectal) nerves containing somatic sensory fibers. Therefore, it is sensitive to painful stimuli (e.g., to the prick of a hypodermic needle). External hemorrhoids can be painful but often resolve in a few days
What does the pectinate line mark the transition between in regards to epithelium? Above the pectinate line, what forms the involuntary internal anal sphincter?
1) The pectinate line marks the transition from endoderm derived simple columnar epithelium to ectoderm-derived nonkeratinized stratified squamous epithelium
2) Above the pectinate line, the inner circular smooth muscle layer of the muscularis externa thickens to form the involuntary internal anal sphincter
Below the pectinate line, what forms the external anal sphincter just superior to the anus?
Below the pectinate line, a circular layer of skeletal muscle forms the external anal sphincter just superior to the anus
What accompany branches of the superior rectal artery? What do these nerves maintain in the internal anal sphincter?
1) Lumbar splanchnic nerves accompany branches of the superior rectal artery
2) The nerves maintain tonic contraction of the internal anal sphincter
What happens when distention of the rectal ampulla activates the pelvic splanchnic nerves? What does fecal continence require and what nerves mediate this?
1) When distension of the rectal ampulla activates the pelvic splanchnic nerves, peristaltic contraction of the rectum and anal canal is reflexively evoked and tonus of the internal anal sphincter is reflexively inhibited
2) Fecal continence requires voluntary contraction of the external anal sphincter and puborectalis muscle, mediated by the inferior rectal nerve and spinal nerve S4, respectively
