Lecture 4: regulation of acid production and motility

Question 1

What are the** 3 phases **of gastric acid secretion?

Answer 1

1. cephalic phase
2. gastric phase
3. intestinal phase

Question 2

Describe the cephalic phase of gastric secretion

Answer 2

• this phase takes place b4 food reaches the stomach, it is initiated by the** sight, smell & taste of food **
• this phase involves a** long reflex pathway** involving the** vagus nerve **
• neurogenic signals originating in the cerebral cortex or **amygdala **or hypothalamus are relayed via efferent fibres whose cell bodies lie within the vagus nerve
• Ach released by vagus nerve which stimulates gastric acid production
• results in increase in HCL

Question 3

Describe the gastric phase of gastric acid secretion?

Answer 3

• 2 triggers for the gastric phase are distension of the stomach wall and the chemical contents of the food
• as food enters stomach, distension of the stomach activates mechanoreceptors and initiates **long loop vagal reflexes and local (short-loop) myenteric reflexes**
• results in increase in HCL

Question 4

Describe the** intestinal phase** of gastric acid secretion

Answer 4

• this phase involves a feedback inhibition of HCL production
• as acid causes the duodenum to distend, an enterogastric reflex occurs and gastric secretory activity is suppressed.
• several hormones contribute to this reflex eg secretin, CCK

Question 5

How does the feedback mechanism of the intestinal phase of gastric acid secretion work?

Answer 5

• secretin (inhibitor) is secreted by **S cells **in the duodenum in response to acid and chyme entering the SI
• secretin reaches the stomach via blood stream and prevents the release of gastrin
• also has a direct inhibitory affect on the parietal cells to reduce their sensitivity to gastrin

Question 6

What are examples of the triggers for gastric acid secretion?

Answer 6

• Ach (via vagus nerve)
• G cells release gastrin which acts on the parietal cell to produce HCL
• gastrin from G **cells also indirectly activates ECL like cells to produce histamine which also increases HCL production in parietal cells

Question 7

What are examples of inhibitors of gastric acid secretion?

Answer 7

• somatostatin
• secretin ( S cells in SI)
• prostaglandins
  *

Question 8

Describe 3 examples of drugs that can affect gastric acid secretion

Answer 8

• **NSAIDS **(ibuprofen) block effects of inhibitors (prostaglandins, secretin etc)
• **Anti- cholinergics **- block Ach action, and Ach is a stimulant of gastric acid production

Question 9

How is the stomach divided into 2 parts based on its motor function?

Answer 9

• the proximal motor unit consisting of the fundus and body of the stomach - storage functions
• the distal motor unit - pyloric regions - mixing of food and propulsion of food into duodenum

Question 10

what 3 muscles in the stomach co ordinate gastric motility?

Answer 10

1. outer longitudinal muscle
2. inner circular muscle
3. oblique muscle

Question 11

What is the basal electrical rhythm in the stomach?

Answer 11

• the spontaneous depolarisation and repolarisation of pacemaker cells in the** interstitial cells of Cajal** in the longitudinal muscle of the body of the stomach

Question 12

How are the pacemaker cells connected to the rest of the cells in the stomach?

Answer 12

• they are** electrically coupled** to the rest of the stomach muscle via **gap junctions **and their rhythm is transmitted to the entire muscularis

Question 13

what are the interstitial cells of cajal? what do they contain?

Answer 13

• nerve like cells at the end of motor neurons in organs innervated by the peripheral nervous system
• pacemaker cells
• they contain gap junctions with smooth muscle cells and are very close to nerve terminals

Question 14

How do ICC cells control contraction in the GIT?

Answer 14

• they generate slow waves of the GIT
• slow waves function to increase the probability of an action potential and therefore the opening of Ca2+ channels for contraction

Question 15

What does the rate of gastric emptying **depend **on?

Answer 15

***** the stomach empties at a rate proportional to gastric volume - ie the fuller the stomach, the quicker it empties
* the physcial and chemical nature of the gastric contents affect rate - carbohydrates empty first, then protein, then fat

Question 16

What is a key physiological factor for gastric emptying?

Answer 16

• gastric distension is key

Question 17

What is the enterogastric reflex?

Answer 17

• reflex triggered by vagal sensory fibres in the duodenum
• these fibres detect Ph, osmolality, fatty acids, and amino acids

Question 18

What 2 hormones decrease the gastric emptying rate?

Answer 18

1. secretin
2. CCK

Question 19

What is gastro oesophageal reflux disease? what may develop as a result of this disease?

Answer 19

• a disease in which stomach content regularly flows up the oesophagus
• heartburn is a key symptom
• barrets oesophagus may develop with untreated GORD

Question 20

What is vomiting?

Answer 20

• the sudden and forceful oral removal of the contents of the stomach and sometimes the duodenum

Question 21

Describe the** 2 phases** of vomiting?

Answer 21

1. prodromal phase - autonomic responses immediately b4 vomiting eg watery salivation, sweating, tears, pale
2. expulsive phase - the stomach and pyloric sphincter relax and contraction of the duodenum reverses the normal pressure gradient in the GIT, which allows SI content to enter stomach

Question 22

What are examples of inducers of vomiting?

Answer 22

• motion
• sickening smells and sights
• certain chemicals
• serotonin- neurons triggered by irritants/toxins