Abdominal Midgut And Hindgut

Question 1

Describe the jejunum

Answer 1

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

Question 2

Describe the ileum

Answer 2

Intraperitoneal

Supplied by compound arterial arcades with short vasa recta (straight arteries) branches of SMA

Question 3

What are some external features of the colon?

Answer 3

Outer 3 bands of longitudinal smooth muscle (tenia coli) causing outpocketings (haustra) with attached outer fatty appendages (appendices epiploicae)

Question 4

Describe the cecum

Answer 4

Blind pouch that contains ileal papilla (cone like projection of the ileum)
Ileocecal valve regulates passage of ileal contents into cecum

Question 5

Describe the appendix

Answer 5

Attached to posterior medial part of cecum and supported by mesoappendix
Position is variable
Contains several lymphoid nodules (part of immune system)

Question 6

Describe the transverse colon

Answer 6

Approximately 10-14” in length

Intraperitoneal and suspended to posterior wall by transverse mesocolon mesentery

Question 7

Describe the sigmoid colon

Answer 7

Approximately 12” in length

Joins rectum at level of S3 and suspended by sigmoid mesocolon

Question 8

Describe the rectum

Answer 8

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

Question 9

What is the internal anal sphincter comprised of?

Answer 9

Smooth muscle (controlled by ANS)

Question 10

What is the external anal sphincter comprised of?

Answer 10

Skeletal muscle (controlled by somatic efferents)

Question 11

Describe the anal canal

Answer 11

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

Question 12

What is the primary arterial supply to the midgut and hindgut?

Answer 12

SMA and IMA

Question 13

What does the superior mesenteric artery supply?

Answer 13

Pancreas, duodenum, SI, cecum, ascending and transverse colon

Question 14

What are some SMA relationships?

Answer 14

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

Question 15

What are the proximal branches of the SMA?

Answer 15

Anterior/posterior inferior pancreaticoduodenal arteries anastomose with branches of gastroduodenal (superior pancreaticoduodenal arteries) from the celiac trunk
20% gives rise to right hepatic A

Question 16

What are the intestinal branches of the SMA?

Answer 16

Jejunal and ileal (intraperitoneal in the mesentery), ileocolic, right colic, middle colic (intraperitoneal in transverse mesocolon)

Question 17

The IMA comes off the aorta at the level of what?

Answer 17

L3

Question 18

What does the IMA supply?

Answer 18

Descending and sigmoid colon

Superior rectum

Question 19

What are the intestinal branches of the IMA?

Answer 19

Left colic, sigmoidal A and superior rectal A

Question 20

What is the marginal artery of Drummond?

Answer 20

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

Question 21

Which arteries provide blood supply to the rectum?

Answer 21

Superior rectal A (off IMA)
Middle rectal A (off internal iliac in pelvic cavity)
Inferior rectal A (off internal pudendal A in perineum)

Question 22

Which two veins form the hepatic portal vein?

Answer 22

Superior mesenteric and splenic vein

Receives blood from most of the GI system and brings it to liver

Question 23

Describe the portal systemic anastomoses

Answer 23

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)

Question 24

What provides sympathetic innervation to the GI tract?

Answer 24

Thoracic, lumbar and sacral splanchnics

Question 25

What are thoracic splanchnics?

Answer 25

Carry presynaptic axons from the SC to postsynaptic neurons located in aortic collateral ganglia
Greater, lesser, and least

Question 26

What are the greater thoracic splanchnic fibers?

Answer 26

T5-9
Go to celiac ganglia where they synapse with postsynaptic neurons that will innervate the stomach, pancreas, liver and gallbladder

Question 27

What are the lesser thoracic splanchnic fibers?

Answer 27

T10-11

Go to SMA ganglia where they synapse with postsynaptic neurons and follow the SMA to innervate LI and SI

Question 28

What are the least thoracic splanchnic fibers?

Answer 28

T12

To aorticorenal ganglia where they synapse with postsynaptic fibers that innervate the kidney and suprarenal glands

Question 29

Describe lumbar splanchnic sympathetic fibers

Answer 29

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

Question 30

Describe sacral splanchnic sympathetic fibers

Answer 30

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

Question 31

What provides parasympathetic innervation (postsynaptic neurons in walls of organs) to the GI tract?

Answer 31

Vagus nerve (presynaptic) and pelvic splanchnic fibers (S2-4)

Question 32

Describe the parasympathetic innervation from the vagus nerve

Answer 32

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

Question 33

Describe pelvic splanchnic parasympathetic fibers

Answer 33

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

Question 34

Where do pelvic splanchnic neurons synapse?

Answer 34

In the descending colon, sigmoid colon, rectum, anal canal, urogenital organs and organs in the perineum

Question 35

What is the somatic afferent pathway?

Answer 35

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)

Question 36

What is the visceral afferent pathway?

Answer 36

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