S1: general overview of GI tract function & basic anatomy

Question 1

What is the purpose of the gut?

Answer 1

Ingest, digest, absorb what’s needed & egest what we don’t

Question 2

Outline the broad functions of the various regions of the GI tract

Answer 2

Mouth – physical breakdown of food, initial digestive enzymes released & infection control
Oesophagus – rapid transport of bolus to stomach through thorax
Stomach – chyme production, secrete intrinsic factor
Duodenum – neutralisation/osmotic stabilisation of chyme
Jejunum/ileum – final digestion, nutrient & water absorption
Large bowel – final water absorption
Rectum/anus – defaecation

Question 3

Describe the anatomy and function of the peritoneal cavity

Answer 3

Continuous membrane which lines the abdominal cavity and covers the abdominal organs
Two layers
-parietal peritoneum: lines the internal surface of the abdominopelvic wall
-visceral peritoneum: invaginates to cover most of the abdominal viscera
Peritoneal cavity – potential space between the two layers
-normally contains a small amount of lubricating fluid

Question 4

What is a mesentery?

Answer 4

Double layer of visceral peritoneum
Connects an intraperitoneal organ to (usually) the posterior abdominal wall
Provides a pathway for nerves, blood vessels & lymphatics

Question 5

Describe the basic structure of the autonomic nervous system in relation to its influence on the gastrointestinal tract

Answer 5

Parasympathetic – vagus nerve & pelvis splanchnic nerves
-preganglionic fibres synapse in walls of the viscera
-innervate smooth muscle/endocrine & secretory
-stimulate GI function
Sympathetic – T5-L2
-form abdominopelvic presynaptic splanchnic nerves
->synapse with prevertebral ganglia
-mainly innervate blood vessels
-generally inhibits GI function

Question 6

Describe the enteric nervous system

Answer 6

Can operate completely independently but has extensive connection with the ANS
From oesophagus to anus
Two main plexuses
-submucosal (Meissner’s): secretions & blood flow (in submucosa)
-myenteric (Auerbach’s): gut motility (between circular and longitudinal muscle)

Question 7

Describe the function of gastrin

Answer 7

G cells in antrum of stomach

Increases gastric acid secretion

Question 8

Describe the function of cholecystokinin

Answer 8

I cells in duodenum & jejunum
Increases pancreatic/gallbladder secretions 
-stimulated by fat & protein
-gall bladder contracts
-pancreas stimulated

Question 9

Describe the function of secretin

Answer 9

S cells in the duodenum
Stimulated by H+ & fatty acids
Increases HCO3 from pancreas/gallbladder
Decreases gastric acid secretion

Question 10

Describe the function of gastric inhibitory polypeptide

Answer 10

Released by cells in the duodenum and jejunum
Stimulated by sugars, amino acids & fatty acids
Increases insulin & decreases gastric acid secretion

Question 11

Describe the function of motilin

Answer 11

Cyclically released during the fasted state & released by Mo cells in the upper small intestine
Stimulates gastric & small intestine motility

Question 12

Describe the function of somatostatin

Answer 12

Produces predominantly neuroendocrine inhibitory effects

Inhibit GI secretions

Question 13

Describe the epithelial cell types of the major divisions of the alimentary tract

Answer 13

Stratified squamous in oesophagus & distal anus

Everything in between is simple columnar

Question 14

Describe the microstructure of the stomach

Answer 14

Surface mucous cells
-line gastric mucosa/gastric pits
->secrete mucus/HCO3 that forms barrier to stomach acid 
Neck mucous cells 
-smaller & irregular shapes

Question 15

Describe the microstructure of the small intestine

Answer 15

Enterocyte (one cell thick)
-need to transport nutrients through apical membrane & basolateral membrane (blood vessels/lymphatics lie immediately below the enterocyte)
Microvilli
-large surface area
Goblet cells
-produce mucus which protects epithelia from friction, chemical damage, bacterial inflammation

Question 16

Describe the microstructure of the intestinal crypts

Answer 16

Enteroendocrine cells 
-secrete hormones that control the function of the gut 
->gastrin, cholecystokinin & secretin 
Stem cells 
-constantly divide to replace epithelia 
Paneth cells
-secrete antibacterial proteins
->protects the stem cell

Question 17

List the four layers of the gut

Answer 17

Mucosa
Submucosa
External muscle layers (outer and inner)
Serosa

Question 18

List the three layers of the mucosa and describe the main features of each

Answer 18

Epithelial layer
-selectively permeable barrier
-facilitate transport and digestion of food
-promote absorption, production of hormones & mucus
Lamina propria
-lymphoid nodules & macrophages
-produce antibodies (mainly IgA – protect against bacterial/viral invasion)
Muscularis mucosae
-layers of smooth muscle orientated in different directions
-keeps epithelium in contact with gut contents

Question 19

What are the main features of the submucosa?

Answer 19

Contains dense connective tissue, blood vessels, glands, lymphoid tissue
Contains the submucosal plexus (Meissner’s)

Question 20

What are the main features of the serosa?

Answer 20

Contains blood & lymph vessels + adipose tissue

Continuous with mesenteries

Question 21

What are the main features of the external muscle layers?

Answer 21

Inner circular muscle
-myenteric (Auerbach’s) plexus
Outer longitudinal muscle

Question 22

List the four anterolateral abdominal wall muscles

Answer 22

External oblique
Internal oblique
Transversus abdominis
Rectus abdominis

Question 23

Describe the origin, insertion, action and nerve innervation of external oblique

Answer 23

Origin: ribs 5-12
Insertion: iliac crest and pubic tubercle
Action: contralateral rotation of the torso
Nerve innervation: thoracoabdominal nerves & subcoastal nerve

Question 24

Describe the origin, insertion, action and nerve innervation of internal oblique

Answer 24

Origin: inguinal ligament, iliac crest & lumbodorsal fascia
Insertion: ribs 10-12
Action: bilateral contraction compresses the abdomen, while unilateral contraction ipsilaterally rotates the torso
Nerve innervation: thoracoabdominal nerves, subcoastal nerve & branches of the lumbar plexus

Question 25

Describe the origin, insertion, action and nerve innervation of transversus abdominis

Answer 25

Origin: inguinal ligament, costal cartilages 7-12, iliac crest & thoracolumbar fascia
Insertion: conjoint tendon, xiphoid process, linea alba & pubic crest
Action: compression of abdominal contents
Nerve innervation: thoracoabdominal nerves, subcoastal nerve & branches of lumbar plexus

Question 26

Describe the origin, insertion, action and nerve innervation of rectus abdominis

Answer 26

Origin: crest of the pubis
Insertion: xiphoid process of the sternum & costal cartilages of ribs 5-7
Action: assists in compression of the abdominal viscera, stabilises the pelvis during walking & depresses the ribs
Nerve innervation: thoracoabdominal nerves

Question 27

Describe the rectus sheath

Answer 27

Formed by the aponeurosis of the three flat muscles
Anterior wall – formed by the aponeuroses of the external oblique & half of the internal oblique
Posterior wall – formed by the aponeuroses of half the internal oblique & of the transversus abdominis
Midway between the umbilicus & pubic symphysis – all the aponeuroses move to the anterior wall of the rectus sheath
-no posterior wall to the sheath at this point & rectus abdominis is in direct contact with the transversalis fascia
Arcuate line: posterior layer of the rectus sheath ends

Question 28

Describe the basic organisation of the peritoneal ‘ligaments’

Answer 28

Greater omentum – consists of four layers of visceral peritoneum
-descends from the greater curvature of the stomach & proximal part of the duodenum, then folds back up to attach to the anterior surface of the transverse colon
Lesser omentum – double layer of visceral peritoneum
Falciform ligament – separates the left and right subphrenic spaces
Gastrocolic ligament – stretches from the greater curvature of the stomach to the transverse colon (portion of the greater omentum)
Gastrosplenic ligament – connects the greater curvature of stomach with the hilum of the spleen (portion of the greater omentum)