Abdominal Midgut And Hindgut Flashcards
Describe the jejunum
Intraperitoneal (supported by mesentery)
Contains mucosal folds (pilcae circulares)
Mesenteric fat increases from proximal to distal
Supplied by arterial arcades with long vasa recta (straight arteries) off branches of the SMA
Describe the ileum
Intraperitoneal
Supplied by compound arterial arcades with short vasa recta (straight arteries) branches of SMA
What are some external features of the colon?
Outer 3 bands of longitudinal smooth muscle (tenia coli) causing outpocketings (haustra) with attached outer fatty appendages (appendices epiploicae)
Describe the cecum
Blind pouch that contains ileal papilla (cone like projection of the ileum)
Ileocecal valve regulates passage of ileal contents into cecum
Describe the appendix
Attached to posterior medial part of cecum and supported by mesoappendix
Position is variable
Contains several lymphoid nodules (part of immune system)
Describe the transverse colon
Approximately 10-14” in length
Intraperitoneal and suspended to posterior wall by transverse mesocolon mesentery
Describe the sigmoid colon
Approximately 12” in length
Joins rectum at level of S3 and suspended by sigmoid mesocolon
Describe the rectum
Both retroperitoneal and sub-peritoneal (portion of rectum is below the peritoneum)
Located in pelvic cavity
Contains 3 transverse rectal folds (superior, middle, and inferior)
Continuous with anal canal
What is the internal anal sphincter comprised of?
Smooth muscle (controlled by ANS)
What is the external anal sphincter comprised of?
Skeletal muscle (controlled by somatic efferents)
Describe the anal canal
Surrounded by 2 anal sphincters (internal and external)
Contains anal columns, anal valves and anal sinuses
Pectinate line located at inferior ends of anal valve
External opening is known as the anus
What is the primary arterial supply to the midgut and hindgut?
SMA and IMA
What does the superior mesenteric artery supply?
Pancreas, duodenum, SI, cecum, ascending and transverse colon
What are some SMA relationships?
Arises from aorta at level of L1 posterior to the neck of the pancreas and then passes inferiorly and anterior to horizontal portion of the duodenum
What are the proximal branches of the SMA?
Anterior/posterior inferior pancreaticoduodenal arteries anastomose with branches of gastroduodenal (superior pancreaticoduodenal arteries) from the celiac trunk
20% gives rise to right hepatic A
What are the intestinal branches of the SMA?
Jejunal and ileal (intraperitoneal in the mesentery), ileocolic, right colic, middle colic (intraperitoneal in transverse mesocolon)
The IMA comes off the aorta at the level of what?
L3
What does the IMA supply?
Descending and sigmoid colon
Superior rectum
What are the intestinal branches of the IMA?
Left colic, sigmoidal A and superior rectal A
What is the marginal artery of Drummond?
Arterial anastomosis exists between SMA and IMA forming a prominent artery called the marginal artery
Depending on health of anastomosis and speed of vessel obstruction it helps prevent intestinal ischemia by providing an alternate route to blood flow
Which arteries provide blood supply to the rectum?
Superior rectal A (off IMA)
Middle rectal A (off internal iliac in pelvic cavity)
Inferior rectal A (off internal pudendal A in perineum)
Which two veins form the hepatic portal vein?
Superior mesenteric and splenic vein
Receives blood from most of the GI system and brings it to liver
Describe the portal systemic anastomoses
Portal venous system has anastomoses with systemic system at gastroesophageal plexus, umbilicus and rectal venous plexus
Varices can occur at these sites due to decreased blood flow through the liver (e.g. due to hepatic HTN)
What provides sympathetic innervation to the GI tract?
Thoracic, lumbar and sacral splanchnics
What are thoracic splanchnics?
Carry presynaptic axons from the SC to postsynaptic neurons located in aortic collateral ganglia
Greater, lesser, and least
What are the greater thoracic splanchnic fibers?
T5-9
Go to celiac ganglia where they synapse with postsynaptic neurons that will innervate the stomach, pancreas, liver and gallbladder
What are the lesser thoracic splanchnic fibers?
T10-11
Go to SMA ganglia where they synapse with postsynaptic neurons and follow the SMA to innervate LI and SI
What are the least thoracic splanchnic fibers?
T12
To aorticorenal ganglia where they synapse with postsynaptic fibers that innervate the kidney and suprarenal glands
Describe lumbar splanchnic sympathetic fibers
T1-5
Carry presynaptic axons from the SC to SMA, IMA and hypogastric ganglia
L1-2 postsynaptic fibers follow IMA to innervate descending and sigmoid colon + rectum
L3-5 postsynaptic fibers head to hypogastric plexus to synapse with fibers that innervate the rectum and anus
Describe sacral splanchnic sympathetic fibers
S1-5
Carry presynaptic axons from sacral sympathetic chain and synapse in the inferior hypogastric plexus ganglia
Neurons distributed with branches of the internal iliac A to the pelvic organs and perineum
Non-splanchnic postganglionic to spinal nerves
What provides parasympathetic innervation (postsynaptic neurons in walls of organs) to the GI tract?
Vagus nerve (presynaptic) and pelvic splanchnic fibers (S2-4)
Describe the parasympathetic innervation from the vagus nerve
Pass through celiac and superior mesenteric ganglion then follows the SMA to the SI cecum, appendix, ascending and transverse colon to synapse with postganglionic neurons within the organs
Also follows branches fo celiac trunk/A to supply organs of the foregut
Describe pelvic splanchnic parasympathetic fibers
S2-4
Carry presynaptic axons from sacral spinal nerves to join hypogastric plexus
Ascend to travel into organs and synapse with postsynaptic parasympathetic neurons within anus, rectum, sigmoid and descending colon
Where do pelvic splanchnic neurons synapse?
In the descending colon, sigmoid colon, rectum, anal canal, urogenital organs and organs in the perineum
What is the somatic afferent pathway?
Pain from irritation of parietal peritoneum
Brain interprets pain as if the irritation occurred in the skin of the region supplied by the same sensory ganglia and SC segments as that region of the affected peritoneum
Acute pain often well localized
Sensitive to stretching of parietal peritoneum (rebound tenderness)
Ex: inflamed liver irritating diaphragmatic peritoneum that has afferents carried via the phrenic nerve (C3-5)
What is the visceral afferent pathway?
Pain from organ itself
Pain originating within organ is transmitted via splanchnic nerves to SC
Poorly localized but radiates to same dermatome level receiving visceral afferents from the organ
Ex: stomach organ pain carried via greater splanchnic nerve perceived in T5-9 dermatome regions
