Gastro-Intestinal System

Question 1

List the layers of the GI tract

Answer 1

Serosa

Muscularis Externa

- Longitudinal plexus
- Myenteric plexus
- Circular muscle

Submucosa

Mucosa

- Eptihelium
- Lamina propria
- Muscularis mucosa

Question 2

List the three divisions of autonomic neural systems (NS) that regulate GI functions and their functions

Answer 2

PSNS - parasympathetic:

• S2-S4
• stimulates GI secretions and motility

SNS - sympathetic:

• T5-T12, L1-L3
• decreases GI secretions and motility

ENS - enteric:

• myenteric plexus - regulates intestinal smooth muscle
• submucosal plexus - regulates intestinal secretions and local absorptive environment

Question 3

Give two examples of two non-neuronal signalling molecules and give examples of each

Answer 3

Hormones - endocrine cells; eg.: CCK, gastrin, secretin

Paracrines - prostaglandins and somatostatin

Question 4

What is prostaglandin responsible for?

Answer 4

Increases blood flow, mucus, and HCO3- secretions

Question 5

What is somatostatin responsible for?

Answer 5

Inhibits peptide hormones and gastric acid secretions

Question 6

What are the three digestive phases?

Describe them and their responses/secreted molecules

Answer 6

Cephalic phase

• triggered by thought of food
• releases ACh and VIP
• responsible for 25% of overall secretions by salivary glands, stomach, pancreas, intestines

Gastric phase

• begins when food and oral secretions enter stomach
• responsible for 10% of overall secretions
• causes neural, hormonal, paracrine GI responses

Intestinal phase

• begins when stomach content reach duodenum
• causes mainly hormonal, but paracrine and neural responses too

Question 7

Talk about ENS reflexes and neurotransmitters

Answer 7

Reflexes:
- Neural circuits that involve mechanoreceptor or chemoreceptor stimulation in the mucosa

Neurotransmitters:

• Enkaphalins: constrict circular muscle around sphincters
• VIP, substance P, ACh, nitric oxide, serotonin

Question 8

Give 5 examples of GI hormones and explain their function

Answer 8

Cholecystokinin (CCK) - increases enzyme secretions

Gastrin - increases gastric acid secretions

Glucose-dependent insulinotropic peptide - releases insulin; inhibits acid secretions

Motilin - increases contractions and migrating motor complexes

Secretin - releases HCO3- and pepsin

Question 9

Give 3 examples of GI paracrines and explain their function

Answer 9

Histamine - increases gastric acid secretions

Prostaglandins - increases blood flow, mucus, and HCO3- secretion; decreases gastric acid secretions

Somatostatin - inhibits peptide and gastric acid secretions

Question 10

Describe the roles of the mouth and its salivary glands in the GI system

Answer 10

Mouth:

• mechanical and chemical breakdown
• tongue repositions food

Salivary glands:

• Sublingual gland
• Submandibular gland
• Parotid gland

Question 11

Describe the role of the oesophagus

Answer 11

Swallowing - parasympathetic process:

• Afferent feedback to swallowing centre
• Efferent responses through other nuclei (causing muscle contraction)

Peristalsis

• Series of co-ordinated muscle contractions/relaxations that create a wave that pushes food down
• Sphincter relaxes to allow food enter the stomach

Question 12

Describe the functions, and locations and contents of secretions of the stomach

Answer 12

Functions:

• Accept and store food
• Mix food with secretion
• Digest food
• Deliver food to small intestine

Secretions:

• secretion occurs at gastric vaginations/pits, which are devided into the neck and base
• they contain different types of cells, eg.: Mucous neck cells, G cells, D cells, Chief cells, Parietal cells
• Pits near sphinctors have more mucus cell for protective secretions

Secretions include:

• ions and water;
• mucus from mucous neck cells;
• pepsinogen from chief cells;
• intrinsic factor and H+ from parietal cells

Question 13

What molecules increase and decrease gastric acid secretion?

Answer 13

Increases:

• ACh (from nerves)
• Gastrin (from G cells)
• Histamine (from enterochromaffin-like cells)

Decreases:

• Somatostatin (from D cells)
• Prostaglandin

Question 14

Describe the small intestine and list the three sections of the small intestine

Answer 14

• Most macronutient, vitamin and mineral absorption occurs here
• Facilitated by large surface area due to villi and microvilli
• Mixing via segmentation (can slow movement of chyme), and propulsion via peristalsis

Parts:
Duodenum - C-shape around pancreas; function is to neutralise gastric acid and continue digestion (especially fat digestion via bile salts)

Jejunum - suspended on mesentries;

Ileum - suspended on mesentries; enters the first part of the colon (cecum) in the right iliac fossa;

Question 15

Describe the parts of the large intestine

Answer 15

Cecum (ileocecal sphincter)
Ascending, transverse, descending, sigmoid colon
Rectum
Anus

Question 16

Function of the ileocecal sphincter

Answer 16

Regulates the amount if chyme entering the large intestine