Lecture 4: regulation of acid production and motility Flashcards
What are the** 3 phases **of gastric acid secretion?
- cephalic phase
- gastric phase
- intestinal phase
Describe the cephalic phase of gastric secretion
- 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
Describe the gastric phase of gastric acid secretion?
- 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
Describe the** intestinal phase** of gastric acid secretion
- 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
How does the feedback mechanism of the intestinal phase of gastric acid secretion work?
- 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
What are examples of the triggers for gastric acid secretion?
- 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
What are examples of inhibitors of gastric acid secretion?
- somatostatin
- secretin ( S cells in SI)
- prostaglandins
*
Describe 3 examples of drugs that can affect gastric acid secretion
- **NSAIDS **(ibuprofen) block effects of inhibitors (prostaglandins, secretin etc)
- **Anti- cholinergics **- block Ach action, and Ach is a stimulant of gastric acid production
How is the stomach divided into 2 parts based on its motor function?
- 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
what 3 muscles in the stomach co ordinate gastric motility?
- outer longitudinal muscle
- inner circular muscle
- oblique muscle
What is the basal electrical rhythm in the stomach?
- the spontaneous depolarisation and repolarisation of pacemaker cells in the** interstitial cells of Cajal** in the longitudinal muscle of the body of the stomach
How are the pacemaker cells connected to the rest of the cells in the stomach?
- they are** electrically coupled** to the rest of the stomach muscle via **gap junctions **and their rhythm is transmitted to the entire muscularis
what are the interstitial cells of cajal? what do they contain?
- 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
How do ICC cells control contraction in the GIT?
- 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
What does the rate of gastric emptying **depend **on?
***** 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
