0204 - Physiology of the GIT

Question 1

What are the two types of plexus (two layers of ENS neurons) found in the GIT?

Answer 1

Myenteric - Superficial, throughout the GIT, and provides motor control (Myenteric = Motor)
Submucosal - Deep, only in small and large intestines, and provides secretory control (Submucosal = Secretory)

Question 2

Outline the Sympathetic innervation of the GIT. What does SY Activation achieve?

Answer 2

3 Primary Ganglia - Coeliac (to transverse colon), superior mesenteric (tranverse and descending), and inferior mesenteric (rectum).
Reduces blood flow, activates sphincters, and inhibits non-sphincters.

Question 3

Outline the Parasympathetic innervation of the GIT. What does parasympathetic activation achieve?

Answer 3

Dorsal motor nucleus of Vagus (medulla), via Vagus to Left colic flexure. S2-4 via pelvic nerves distal third of colon.
Increases secretion and motility (rest and digest).

Question 4

What are the layers of the GIT (superficial to deep)?

Answer 4

Serosa (Epithelium and areola CT)
Muscularis (longitudinal and circular)
Submucosa
Mucosa (Muscularis mucosae, lamina propria, epithelium).

Question 5

What are the 3 functions of motor activity in the GIT? What type of muscle achieves them?

Answer 5

1 - Peristaltic contractions (propulsive)
2 - Segmental contractions (mix food without propulsion)
3 - Sphincter control (retain contents in stomach and large intestine)
All achieved by smooth muscle

Question 6

What are the two types of GIT contraction?

Answer 6

Tonic - Sustained contractions.

Rhythmic - Alternate contraction and relaxation (mixing contractions) - pacemaker provided by slow wave activity.

Question 7

Describe the process of a GIT contraction/action potential (7 steps)

Answer 7

1 - Voltage gated Ca++ channels open due to Vm excitement (stimulation by stretch, ACh, or VA).
2 - AP spikes.
3 - Influx of Ca++ leads to rise in internal Ca++ concentration
4 - High Ca++ opens Ca++-dependent K+ channels
5 - Slow hyperpolarisation due to K+ channels.
6 - Voltage-gated Ca++ channels close, lowering internal Ca++ concentration.
7 - Low Ca++ concentration closes Ca++-dependent K+ channels.

Question 8

What two forms of peristalsis occur after swallowing? How are they triggered?

Answer 8

Primary peristalsis - triggered by act of swallowing.
Secondary peristalsis - triggered by distension.
On average, bolus takes 10s to reach stomach.

Question 9

Describe the two Oesophageal Sphincters

Answer 9

Upper - Striated muscle, highest resting pressure of all GI sphincters, and under control of swallowing centre in medulla.
Lower - Smooth muscle, tonically active (prevent reflux), but relaxes upon swallowing. Under control of Vagus (contraction) and normal smooth muscle (release of NO or VIP causes relaxation).

Question 10

What happens to each oesophageal sphincter during the act of swallowing?

Answer 10

Both relax simultaneously due to act of swallowing. However, UES contracts before LES, while bolus is still in the cervical oesophagus.

Question 11

What are the motor functions of the stomach?

Answer 11

Regulate food intake via contraction/relaxation.
Mix food with secretions
Reduce food particle size
Empty chyme into duodenum (liquids - proximal part, solids, antral part).
VA innervation over entire organ.

Question 12

What is the purpose of stomach relaxation? What are the three types of stomach relaxation that allow gastric filling? How are they regulated?

Answer 12

Relaxation allows an increase in intragastric volume without an increase in pressure.
Receptive - triggered by swallowing
Adaptive - triggered by distension/filling
Feedback - triggered by nutrients in small intestine.
All regulated primarily by ENS with VA modulating.

Question 13

In what order are stomach contents released during gastric emptying?

Answer 13

1 - Saline and water.
2 - Acidic and caloric fluids.
3 - Fatty materials
4 - Solids (must be reduced to less than 2mm)

Question 14

What are the three phases of gastric emptying?

Answer 14

1 - Propulsion - initiated by gastric pacemaker, contractions propel food towards antrum.
2 - Grinding - Chyme is churned in the antrum by propulsion.
3 - Retropropulsion - pyloric valve is opened, some chyme released but most of this is returned to stomach via duodenal sphincter.

Question 15

How is the rate of gastric emptying determined?

Answer 15

Volume of liquid in stomach is primary determinant.

Composition of chyme is major determinant - duodenal and small bowel chemoreceptors provide feedback.

Question 16

What qualities of chyme can lead to a delayed gastric emptying? how does this occur?

Answer 16

Low pH, high caloric content, lipid, some AAs, and high osmolarity.
Occurs by fundic relaxation, decreasing antral motor activity, contracting pyloric sphincter and slowing intestinal motor activity.
Neural and hormonal signals - Vagus nerve, secretin, cholecystokinin, and gastric inhibitory peptide.

Question 17

What are three major hormones that control motility?

Answer 17

Secretin, Cholcystokinin (CKK), Gastric inhibitory peptide (GIP)

Question 18

Outline secretin

Answer 18

Secreted by duodenal mucosal S cells in response to low pH
Inhibit most of GI tract
Stimulate release of HCO3- from pancreas and liver (neutralise the acidity)

Question 19

Outline cholecystokinin (CCK)

Answer 19

Secreted by I cells in duodenal and jejeunal mucosa, in response to broken-down lipids.
Moderately inhibit stomach motility.
Potently increase contraction of gallbladder (release bile, break down the fats).

Question 20

Outline Gastric inhibitory peptide (GIP)

Answer 20

Secreted by K cells in mucosa of proximal small intestine, in response to fatty acids, AAs, and carbohydrates.
Moderately inhibit stomach motility
Purpose is to prevent overloading of small intestine.