6. Gastric Secretion

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Questions and Answers

What is the primary source of vitamin B12 for adult strict herbivores?

Ingested foods

Microbial production in the large intestine

Microbial production in the rumen

Cobalt supplementation (correct)

Which of the following cell types is responsible for secreting pepsinogen in humans?

Enteroendocrine cells

G cells

Chief cells (correct)

Parietal cells

What is the main function of intrinsic factor?

Activation of pepsinogen

Stimulation of gastrin release

Secretion of HCl

Absorption of vitamin B12 (correct)

Which of the following is NOT a stimulatory mediator of HCl secretion from parietal cells?

Somatostatin (D) Signup and view all the answers

What is the primary function of rennin in neonatal ruminants?

Milk clotting (C) Signup and view all the answers

What is the primary mechanism by which food buffering stimulates gastrin release?

Food buffering neutralizes gastric acidity, reducing inhibition of G cell gastrin release. (B) Signup and view all the answers

Which of the following represents a neural mechanism that stimulates gastric acid secretion?

Activation of cholinergic neurons by mechanoreceptors in the stomach. (C) Signup and view all the answers

How does acetylcholine stimulate gastrin release?

Acetylcholine inhibits the release of somatostatin, thus promoting gastrin release. (B) Signup and view all the answers

Which of the following is NOT a physiological inhibitor of gastric acid secretion?

Release of gastrin from G cells in the antrum. (B) Signup and view all the answers

What is the main role of pepsinogen in gastric acid secretion?

Pepsinogen is the precursor to pepsin, activated by HCl and pepsin itself. (D) Signup and view all the answers

What is the primary mechanism by which H2 receptor antagonists reduce gastric acid secretion?

By blocking the action of histamine on parietal cells. (B) Signup and view all the answers

Which of the following surgical interventions is specifically designed to decrease gastric acid secretion?

Vagotomy (D) Signup and view all the answers

Which of the following pharmacological treatments acts by directly inhibiting the proton pump in parietal cells?

H+-K+ ATPase inhibitors (A) Signup and view all the answers

Which component of gastric juice is responsible for activating pepsinogen?

HCl (C) Signup and view all the answers

Which epithelial cell type in the oxyntic gland secretes HCl?

Parietal cells (D) Signup and view all the answers

In the gastric mucosa, which glands produce gastrin?

Pyloric glands (B) Signup and view all the answers

What is the primary function of ghrelin in the stomach?

Stimulate appetite (C) Signup and view all the answers

Which substance in gastric juice contributes to the optimal pH for pepsin activity?

Hydrochloric acid (A) Signup and view all the answers

Which type of cell in the gastric glands secretes somatostatin?

D cells (C) Signup and view all the answers

Which is a correct statement about the gastric pit?

Gastric pits contain various types of cells and glands. (A) Signup and view all the answers

How does HCl affect protein digestion in the stomach?

Denatures proteins for easier digestion (D) Signup and view all the answers

What is the primary stimulus for gastric secretion during the gastric phase?

Presence of food in the stomach (C) Signup and view all the answers

Which neurotransmitter is involved in stimulating parietal cells during the cephalic phase?

Acetylcholine (D) Signup and view all the answers

What percentage of gastric secretion occurs during the cephalic phase?

15% (B) Signup and view all the answers

Which cell type is responsible for the release of gastrin?

G cells (A) Signup and view all the answers

What is the role of mechano- and chemoreceptors during the gastric phase?

To send afferent signals locally and centrally (A) Signup and view all the answers

During which phase of gastric secretion is the vagus nerve particularly influential?

Cephalic phase (C) Signup and view all the answers

Which factor does not directly stimulate HCl secretion from parietal cells?

Somatostatin (A) Signup and view all the answers

What is the cumulative influence of the gastric phase on total gastric secretion?

70% to 80% (B) Signup and view all the answers

What process occurs primarily through the H+-K+ ATPase in parietal cells?

Secretion of HCl (D) Signup and view all the answers

Which of the following is a secretagogue that stimulates HCl secretion via paracrine signaling?

Histamine (C) Signup and view all the answers

What effect does somatostatin have on ECL cells?

Inhibits histamine release (D) Signup and view all the answers

Which of the following substances primarily stimulates the release of histamine from ECL cells?

Gastrin (C) Signup and view all the answers

What is the main consequence of increased cAMP in parietal cells due to histamine stimulation?

Direct stimulation of H-K ATPase (B) Signup and view all the answers

Which type of cells are primarily responsible for releasing histamine in the stomach?

ECL cells (D) Signup and view all the answers

What is the role of acetylcholine in the secretion of HCl from parietal cells?

Increasing intracellular Ca2+ levels (A) Signup and view all the answers

What is the primary source of gastrin in the stomach?

G cells (C) Signup and view all the answers

What is the alkaline tide associated with the secretion of HCl in the stomach?

Increase in blood bicarbonate levels (C) Signup and view all the answers

Which of the following is NOT a stimulus for gastrin release?

Histamine release from ECL cells (D) Signup and view all the answers

Which of the following correctly describes the indirect action of gastrin?

It stimulates the release of histamine from ECL cells. (C) Signup and view all the answers

What is the role of somatostatin in the stomach?

Inhibits the release of histamine and gastrin. (C) Signup and view all the answers

Which receptor does gastrin bind to on parietal cells to exert its effects?

CCK-B receptor (A) Signup and view all the answers

What is the result of acetylcholine acting on muscarinic receptors on G cells?

Increased gastrin secretion. (B) Signup and view all the answers

Which ionic change is stimulated by gastrin in parietal cells?

Increased calcium concentration (B) Signup and view all the answers

Study Notes

Gastric Secretion (Simple Stomach)

Gastric juice is composed of:
- HCl: Isotonic (150mM H+, 150 mM Cr) + Abomasum
- Water
- Pepsin (secreted as pepsinogen)
- Intrinsic factor
- Mucus
- Proteolytic enzymes (secreted as pepsinogen) from pancreatic acini in dogs and cats. Primates and pigs secrete these from pancreatic acini.

Functional Anatomy of Gastric Mucosa

The gastric mucosa is lined with simple columnar epithelium dotted with numerous gastric pits (up to 3 million).
Each pit leads to 1-3 gastric glands.
Gastric glands are composed of several cell types, and those regions differ across species (see Figure 1 of Section 1).

Regions of Stomach by Species (Figure 1)

Various species show different regional stomach anatomical structures. (e.g., stratified squamous, nonglandular, cardiac glands, proper gastric/fundic, pyloric/antrum in different animals).

Gastric Gland Cell Types and Products (Figure 2)

Oxyntic Gland (Proper Gastric):
Parietal Cells: Secrete HCl and intrinsic factor.
Chief Cells: Secrete pepsinogen.
Mucous Cells: Secrete mucus and pepsinogen.
Enteroendocrine Cells: Release histamine, somatostatin, ghrelin, and gastrin. ECL cells (Enterochromaffin-like cells) specifically secrete histamine.
Pyloric Gland (Antral):
Mucous Cells: Secrete mucus and pepsinogen.
Enteroendocrine Cells: Secrete gastrin, somatostatin, and ghrelin

Ghrelin (Hunger Hormone)

Found mainly in oxyntic glands of stomach.
Released during fasting or negative energy balance.
Agonists of ghrelin are used to stimulate appetite in dogs and cats.

Functions of Gastric HCl

Activates pepsinogen to pepsin.
Creates optimal pH (2-4) for pepsin function.
Denatures proteins.

Mechanism of Gastric Acid Secretion

HCl secretion occurs via H+-K+ ATPase (proton pump) in parietal cells.
Requires energy (ATP).
Secretion against a concentration gradient.
Processes CO2 and H2O in the parietal cell.

Mediators Affecting Parietal Cell HCl Secretion

Histamine: A paracrine mediator that stimulates HCl secretion by binding to H2 receptors, increasing cAMP, directly stimulating H-K ATPase, and potentiating the effect of other stimuli on H-K ATPase. ECL cells are the main source, and stimuli include gastrin and acetylcholine. Somatostatin inhibits ECL cell function and histamine release.
Acetylcholine: A neurotransmitter from parasympathetic nerves that stimulates HCl secretion via muscarinic receptors. Indirect stimulation can result in gastrin and histamine release which further stimulate parietal cell activity.
Gastrin: A hormone that stimulates HCl secretion, mainly by increasing histamine release. Gastrin binds to gastrin (CCKB) receptors and increases intracellular Ca2+, further stimulating the H-K ATPase activity.

Inhibitor: Somatostatin

Paracrine inhibitor produced by D cells in oxyntic and antral stomach.
- Acts on parietal cells, ECL cells, and G cells to inhibit HCl secretion, histamine release, and gastrin release, respectively.
- Stimuli for release include increased luminal H+ (low pH).

Three Phases of Gastric Secretion

Cephalic Phase: (15% of total gastric secretion) - Triggered by anticipation or smell of food; mediated by long-chain reflexes. Relays via central nervous system (CNS) to vagus nerve. Releases gastrin, histamine, and stimulates parietal cells.
Gastric Phase: (70-80% of total gastric secretion) - Triggered by presence of food in the stomach. Stimuli include mechanical stretching, chemical stimulation (amino acids and peptides). Includes both neural mechanisms (mechanical and/or chemical stimuli) and direct hormonal stimulation.
Intestinal Phase: (5% of total gastric secretion) - Triggered by presence of chyme in the small intestine, mediated by feedback mechanisms from the intestines.

Physiological Inhibitors of Acid Secretion

Local feedback control: As gastric acidity rises, it inhibits gastrin release and stimulates somatostatin in the stomach.
Feedback control: Duodenum mechanisms like enterogastric reflexes use CCK and GLP-1 to regulate gastric function and output.

Other Gastric Juice Constituents

Pepsin: A protease secreted as pepsinogen. Pepsinogen becomes pepsin via HCl activation. It has optimal activity at pH 2-4.
Intrinsic factor: A glycoprotein secreted by parietal cells. Essential for vitamin B12 absorption in the small intestine.
Rennin: A milk-clotting enzyme found in neonatal ruminants (and some other species).

Clinical Relevance

Gastroduodenal ulcers linked to Helicobacter pylori. Treatments involve antibacterial agents, proton pump inhibitors, and H2 receptor antagonists to address the acid production issue.

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Description

Test your knowledge on gastrointestinal physiology with this quiz covering essential topics such as gastric acid secretion, hormones, and cellular functions. Questions address the roles of various cell types, stimulation mechanisms, and the physiological processes involved in digestion and absorption. Perfect for students in anatomy or physiology courses.