Gastric Motility and Secretion Flashcards
(21 cards)
*What are the 4 layers of the GI tract and what are thier nerve plexuses?
•From the lumen outwards, the four layers are the:
- Mucosa
- Submucosa
- Muscularis
- Serosa
Enteric neurons in the submucosa (submuscosal plexus)
Enteric neurons in the muscularis ( muscularis plexus?)
What are the fucntions of the stomach?
- To store food
- To kill bacteria
- To break down food into smaller parts
- To regulate release of stomach contents into the duodenum
- To secrete IF
Gastric motility allows the stomach to:
Serve as a reservoir for a large volume of food ( allows food to enter through receptive relaxation)
Fragment the food into smaller particles and mix it with gastric secretions
Empty gastric contents into the duodenum at a controlled rate
How can the stomach intake 1.5 L of substance when it only has a capacity of 0.5 L at rest?
(In the empty state the stomach is contracted and its mucosa and submucosa are highly folded into rugae.)
What is receptive relaxation?
Through receptive relaxation
- In the empty state the stomach is contracted and its mucosa and submucosa are highly folded into rugae.
*What is the innervation of the GI tract?
What is receptive relaxation of the stomach and how is it controlled?
When food is swallowed, the smooth muscle in the wall of the orad region of the stomach relaxes enabling the stomach to increase in volume to as much as 1.5L
Mediated by the vagus nerve
Acts on stomach’s enteric nerve plexuses.
The enteric neurons release NO and serotonin = relaxation of the smooth muscle cells
- The predominant motor activity of the orad region is the accommodation of ingested food.*
- Because the muscle layers are thin, contractions of the orad region are normally weak.*
- Gastric contents often remain in relatively undisturbed layers for an hour or more after eating.*
How does mixing occur in the stomach?
- Gastric contractions (peristaltic waves) begin in the body of the stomach and moves towards the antrum.
- As the wave approaches the antrum it causes a powerful contraction. The pyloric sphincter closes.
- Most of the contents of the antrum are forced backwards into the body of the stomach. This is termed retropulsion.
How do we regulate stomach emptying?
coordinated contractile activity of the stomach, pylorus and proximal small intestine keep the stomach closed and limit emptying.
High: fat, acid, hypertonicity decrease gastric emptying rate
Sensed by receptors for each in stomach wall causing the release of intestinal hormones (IH)
IH: Î pyloric sphyncter contraction , inhibit antral contractions
Describe the compostition of the pyloric sphincter
Name 2 of its functions.
Made of smooth muscle and connective tissues
Allows the regulated emptying of gastric contents at a rate consistent with the ability of the duodenum to process chyme.
Prevents the regurgitation of duodenal contents back into the stomach.
Describe the control of the Pyloric Sphincter
By the autonomic nervous system:
- Sympathetic nerve fibres increase the constriction of the sphincter.
- Vagal fibres are either excitatory or inhibitory to the sphincter’s smooth muscle.
- By hormones:
Gastrin, cholecystokinin (CCK), gastric inhibitory peptide (GIP) and secretin all elicit constriction of the sphincter.
Normal gastrointestinal motility results from coordinated contractions of smooth muscle.
The contractions derive from 2 basic patterns of electrical activity, name them.
slow waves and action potentials
What is basic electrical rhythm of the GI tract?
When smooth muscle cells undergo spontaneous cycles of depolarization and repolarization
What are slow waves and action potentials?
SW: fluctuations in membrane potential spreading to adjacent sections of muscle.
3 x /min
rhthym generated by pacemaker zone
Coordinate contractions by controlling the appearance of action potentials ( they do not elicit them)
APs: Excitatory neurotransmitters and hormones further depolarize the membrane, thereby increasing the amplitude of the slow wave to induce APs.
- If the peak of a slow wave rises above a certain threshold, the cell fires one or more action potentials - muscle contraction
- The number of action potentials occurring at the peak of a slow wave determines the strength of the muscle contraction.
What are the contents of stomach juice?
What are the 4 major types of epilelial cells in the stomach?
- hydrochloric acid (HCl)
- mucus
- enzymes: pepsinogens, gastric lipase
- intrinsic factor
Cheif cells ( secrete pepsinogen)
Parietal cells ( secrete protons and IF) (located in the proximal 80 % of the stomach)
G cells ( produce Gastrin, found in antrum)
Mucus cells - mucus and bicarb
What are the Gastric Hormones and what do they do?
What is Mucus, and what is it’s function in the stomach?
What is Pepsin + function?
Mucus is a mixture of glycoproteins and glycopolysaccharides
Secreted by mucus cells of stomach
forms a gel (up to 200 mm thick) on the luminal surface of the gastric mucosa w. alkaline secretions entrapped within it constitute a “gastric mucosal barrier”
This protects the stomach against acid, proteolytic enzymes and mechanical damage
Pepsin is activated by low stomach PH from its precursor pepsinogen ( released from cheif cells), breaks down polypeptides
What is intrinsic factor and what is it’s function ?
glycoprotein secreted by parietal cells
essential B12 absorption
How do parietal cells produce and secrete acid?
How is acid secretion regulated ( hormones, nervous system and other factors)?
What are the drugs that act and where?
How the stomach is protected from autodigestion?
Gastric epithelial cells secrete HCO3- , mucus keeps it in the outer layer - can keep the epithelial cells at a neutral PH
Due to damage, the mucosal layer must be renewed constantly ( the cells have a high rate of cell division)
Local irritation to the mucosa stimulates the production of prostaglandins (PGs), which increase production of mucus and HCO3- and inhibit acid secretion. They also promote healing of local damage.
non-steroidal anti-inflammatory drugs (NSAIDS) interfere with this.
What is peristalisis?
- Waves of alternating contractions and relaxations of smooth muscle layers that mix and squeeze the contents through hollow tubes.
- Move a bolus along the length of the GI tract.
What is segmentation?
- Cycles of contraction that mix contents but do not push them in any one direction.
- In the GI tract, churn and fragment the bolus. Mix contents with intestinal secretions.
