Lecture 2 - GI Anatomy + Control of the Gut

Question 1

What are the 3 muscles of the anterolateral abdominal wall and what do they all have?

Answer 1

1) External Oblique (outermost)
2) Internal Oblique (middle)
3) Transverse Abdominus (innermost)

Aponeurosis (thin sheet like tendon)

Question 2

What are the origins, insertion + actions of the 3 anterolateral abdominal wall muscles?

Answer 2

1) External oblique:
- 5th to 12th rib
- ASIS , anterior half of iliac crest
- Flexes, laterally flexes and rotates trunk + compresses

2) Internal oblique:
- Inguinal ligament/thoracocolumnar fascia
- Inferior margin of 10th to 12th ribs + pubic crest
- Flexes, laterally flexes, rotates trunk + compresses

3) Transverse abdominus:
- Costal cartilage of 7th to 12th ribs
- Linea alba, pubic crest
- Compresses core and helps with core stability

Question 3

What are the 3 anterolateral abdominal wall muscles innervated by?

Answer 3

Anterior rami of 7th to 12th thoracic spinal nerves (T7 to T12)

• Internal oblique and transverse abdominus also take L1

Question 4

What is the rectus sheath formed from?

Where is the arcuate line?

Answer 4

• Fibrous covering surrounding rectus abdominus, formed from aponeurosis of the 3 main anterolateral abdominal wall muscles
• Halfway between umbilicus and pubic crest, marks lower limit of posterior layer of rectus sheath

Question 5

What are the 4 distinct layers of the gut from in to out?

Answer 5

1) Mucosa
2) Submucosa
3) External muscle layers
4) Serosa

Question 6

What are the prominent features of the first 2 gut layers?

Answer 6

1) Mucosa
- Has lamina propria which has lots of lymphoid nodules & macrophages, produces antibodies (mainly IgA).
- Has epithelial layer which is selectively permeable for transport/digestion of food & promotes absorption

2) Submucosa
- Dense connective tissue containing blood vessels, glands and lymphoid tissue

Question 7

What are the prominent features of the final 2 gut layers?

Answer 7

3) Muscularis mucosae
- Inner circular muscle layer and outer longitudinal muscle layer that aid in peristalsis down GIT

4) Serosa
- Continous with mesenteries, containing blood vessels, lymph vessels and adipose tissue.

Question 8

What are the 2 types of epithelial cells found in the gut & where are they found in the GIT?

Answer 8

1) Stratified squamous (non-keratinised) - in the oesophagus & distal anus (i.e.: where there is mechanical stress)
2) Simple columnar - stomach to proximal anus

Question 9

Where are surface mucous cells found?

What is their role?

Answer 9

• Simple columnar cells in stomach form gastric pits, which are connected to gastric glands at the base. Lining the gastric pits are mucous cells.
• Secrete mucus/HCO3- that form barrier + protection to stomach acid.

Question 10

What is the predominant cell in the small intestine?

What adaptations does it have for the primary role of the small intestine?

Answer 10

• Enterocyte

- Microvilli on apical surface forms brush border, increasing SA for absorption.

Question 11

Where are goblet cells found & why are they names this?

What are the roles of goblet cells?

Answer 11

• Scattered between enterocytes, increasing in number of duodenum to colon. Narrow base + larger apical surface so look like a goblet.
• Produces mucus, protects epithelia from friction, chemical damage (acidic environment), and provides physical barrier against bacterial infection.

Question 12

What 3 major cell types are found in intestinal crypts?

What are their roles|?

Answer 12

1) Enteroendocrine cells - found deeper in crypts, secrete hormones for control of gut function (gastrin, CCK & secretin)
2) Stem cells - constantly divide to replace epithelia every 2-4 days
3) Paneth cells - located at the base, secrete antibacterial proteins to protect stem cells

Question 13

Describe the major similarities and differences between the sympathetic and parasympathetic branch of the ANS in relation to the gut.

Answer 13

• Long pre-ganglionic fibres on PNS vs short on SNS
• Ganglion close to or within target organ on PNS
• PNS carried by vagus (CN10) or pelvic splanchnic nerves (S2-S4) SNS carried by thoraco-lumbar nerves (T5-L2)
• PNS postganglionic fibres cholinergic + peptidergic, SNS postganglionic fibres release NA
• PNS stimulates GI function, SNS generally inhibits GI function

Question 14

What is the enteric nervous system?

Answer 14

Independently operating nervous system with extensive connection to ANS, existing from oesophagus to anus with 2 main plexuses:

1) Submucosal (Meissener’s) - in submucosa
2) Myenteric (Auerbach’s) - in-between circular + longitudinal muscle

Question 15

What are the 3 modes of hormonal communication within the GIT?

Answer 15

1) Endocrine - peptide hormones from endocrine cells released into blood stream that can act long distance.
2) Paracrine - peptide hormones released by endocrine cells which diffuse short distances acting locally
3) Neurocrine - released after AP by neurones in the GIT, e.g.: GRP

Question 16

Where is gastrin and cholecystokinin (CCK) released from and what are their roles?

Answer 16

Gastrin:
- Released from G-cells in antrum of stomach, causes parietal cells to released gastric acid. Stimulated by stretch + breakdown of proteins (e.g.: presence of AA’s)

CCK:
- Released from I cells in duodenum and jejunum. Contracts gall bladder, increases pancreatic enzyme secretions + relaxes sphincter of Oddi. Stimulated by fat + proteins.

Question 17

Where is secretin, gastric inhibitory polypeptide (GIP) and somatostatin released from and what are their roles?

Answer 17

Secretin:
- S-cells in duodenum, stimulated by H+ (lowered pH) and fatty acids, increases HCO3- from pancreas and decreases gastric acid secretions

GIP:
- Cells in duodenum and jejunum, stimulated by sugars, AAs and fatty acids. Increases insulin and decreases gastric acid secretion.

Somatostatin:
- Released in response to a low pH, paracrine hormone that switches off G-cells producing gastrin/gastric acid.