Abdominal Midgut & Hindgut (Brauer) Flashcards
What structures/organs comprise the midgut and hindgut?
- small intestines: jejunum, ileum
- large intestines: cecum, ascending colon, transverse colon, descending colon, sigmoid colon
- rectum and anus
- comprises the proximal 2/5’s of the SI
- intraperitoneal (supported by mesentery)
- contains mucosal folds (plicae circulares)
- mesenteric fat increases from proximal to distal
- supp by arterial arcades w/ long vasa recta (straight arteries) off branches of the SMA
jejunum
- comprises distal 3/5’s of SI
- intraperitoneal (supported by mesentery)
- supp by compound arterial arcades w/ short vasa recta (straight arteries) branches of SMA
ileum
- approx 1.5 m long and begins at ileocecal junction
- external features: outer 3 bands of longitudinal smooth muscle (tenia coli) causing outpocketings (haustra) w/ attached outer fatty appendages (appendices epiploicae)
colon
What are the different parts of the colon?
- cecum: blind pouch that contains ileal papilla (cone-like projection of ileum); ileocecal valve regulates passage of ileal contents into cecum
- appendix
- ascending/descending colon: both secondarily retroperitoneal
- transverse colon: approx 10-14” in length; intraperitoneal and suspended to posterior wall by transverse mesocolon mesentery
- sigmoid colon: approx 12” in length; joins rectum at level of approx S3; suspended by sigmoid mesocolon
- organ that is attached to posterior-medial part of cecum and supported by mesoappendix
- position is variable
- contains several lymphoid nodules (part of the immune system)
appendix
- both retroperitoneal and sub-peritoneal
- located in pelvic cavity
- approx 5” in length
- contains 3 transverse folds (superior, middle, inferior)
- c/w anal canal
rectum
- approx 1.5” in length
- surrounded by 2 sphincters: internal (comprised of smooth muscle) and external (comprised of skeletal muscle)
- contains columns, valves, and sinuses
- pectinate line: located at inferior ends of anal valves
- external opening is the anus
anal canal
What is the main arterial supply to the midgut and hindgut?
- SMA: pancreas and duodenum, SI, cecum, ascending and transverse colon
- IMA: descending and sigmoid colon, superior rectum
What is the general course of the SMA?
- SMA arises from aorta at L1 level posterior to neck of pancreas and then passes inferiorly and anterior to horizonal portion of duodenum
- proximal braches: inferior pancreaticoduodenal arteries anastomose w/ branches of gastroduodenal artery from celiac trunk; 20% gives rise to right hepatic artery
What are branches off the SMA and IMA?
SMA
- jejunal and ileal (intraperitoneal)
- ileocolic
- right colic
- middle colic (intraperitoneal in transverse mesocolon)
IMA
- left colic
- sigmoidal artery
- superior rectal artery
multiple anastomoses w/ each other
What is the marginal artery of drummond and what is its clinical significance?
- it is an arterial anastamosis between SMA and IMA, forming one prominent artery
- depending on health of anastamosis and speed of vessel obstruction it helps prevent intestinal ischemia by providing alternate route (dot represents site of an occlusion)
What is the blood supply to the rectum?
- superior rectal A.: branch of inferior mesenteric A.
- middle rectal A.: branch of internal iliac A. (in pelvic cavity)
- inferior rectal A.: branch of internal pudendal A. (in perineum)
Diagram the hepatic portal system:
What are the systemic anastamoses of the hepatic portal system?
- portal venous system has anastamoses w/ systemic system at: gastroesophageal plexus, umbilicus, and rectal venous plexus
- varices can occur at these sites due to decrease blood flow through liver (e.g. hepatic hypertension)
Describe the sympathetic nerve system organization within the midgut and hindgut:
sympathetics:
thoracic portion of SC houses presynaptic nerves for sympathetics (fibers come from T5-L2) w/ the ganglion being in the sympathetic trunk; the main difference is most of the neurons DO NOT synapse w/ post-ganglionic cell bodies in the trunk, rather their fibers pass through the trunk and synapse on ganglion along abdominal aorta or in the pelvis
thoracic splanchnics: carry presynapetic axons from SC to postsyaptic neurons located in aortic collateral ganglia
- greater (level of T5-9): #4 in figure, to celiac ganglion and SMG
- lesser (level of T10-11): #5 in figure, to SMG
- least (level of T12): #6 in figure, to aorticorenal ganglia (not shown)
lumbar splanchnics (level of L1-5): #7,8 in figure, carry presynpatic axons from SC to SMG, IMG, and hypogastric ganglia
sacral splanchnics (level of S1-5): #9 in figure, carry presynaptic axons from sacral sympathetic chain to hypogastric ganglia
What is the sympathetic innervation of the midgut and hindgut?
thoracic splanchnics:
- greater (level of T5-9): #4 in figure, to celiac ganglion and SMG > foregut (liver, gallbladder, pancreas, spleen, adrenal medulla)
- lesser (level of T10-11): #5 in figure, to SMG > SI, LI, ascending and transverse colon
- least (level of T12): #6 in figure, to aorticorenal ganglia (not shown) > kidneys and adrenal glands
lumbar splanchnics (level of L1-5): #7,8 in figure, to SC to SMG, IMG, and hypogastric ganglia >
level of L1-2: IMG > descending/sigmoid colon and rectum
level of L3-5: hypogastric plexus > rectum and anus
sacral splanchnics (level of S1-5): #9 in figure, from sacral sympathetic chain to hypogastric ganglia > seminal vesicles, prostate, uterus, vagina
Describe the parasympathetic organization and innervation of the midgut:
parasympathetics: postsynaptic neurons within walls of organs
- vagus nerve: presynaptic fibers travel through SMG and IMG > SI, cecum, appendix, ascending/transverse colon
- pelvic splanchnics (level of S2-4): #10 in figure, carry presynaptic axons from sacral spinal nerves to join hypogastric plexus > travel to organs > synapse w/ post-synaptic parasympathetic neurons within organs (rest of colon, rectum, anus)
What are the major plexuses and nerves within the autonomic nervous system of the GI and where are they located?
- preaortic plexus: celiac ganglia, SMA ganglia, aorticorenal ganglia, IMA ganglia
- superior hypogastric plexus
- inferior hypogastric plexus (in pelvis, not in figure)
- lumbar and sacral splanchnics
- pelvic splanchnics
Describe the organization of sacral splanchnic nerves:
- originate from the spinal cord
- a/w spinal nerves S1-5, however fibers arise from sympathetic trunk
- synapse in inferior hypogastric plexus ganglia
- neurons distributed w/ branches of internal iliac artery to the pelvic viscera and perineum
Describe the organization of pelvic splanchnic nerves:
- parasympathetic preganglionic neurons originate in spinal cord (S2-4)
- neurons synapse in the: descending/sigmoid colon, rectum, anal canal, urogenital organs, organs in perineum
- presynaptic fibers S2-4 join hypogastric plexus and ascend to supply rectum, sigmoid/descending colon
How is pain referred via the somatic afferent pathway?
- pain is caused by irritation of parietal peritoneum
- brain interprets pain as though it is occuring in the skin of the region supplied by the same sensory ganglia and SC segments as the region of the affected peritoneum
- acute pain, often well localized
- sensitive to stretching of parietal peritoneum (“rebound tenderness”)
- example: inflamed liver irritating diaphragmatic peritoneum that has afferents carried via the phrenic nerve (C3-5)
