Esophageal Motility and Stomach Epithelial Cells

Motility of the Esophagus

  • Primary Peristalsis (~9s):

    • Unidirectional movement of the bolus through the esophagus.

  • Secondary Peristalsis:

    • Initiated if a bolus gets stuck in the esophagus.

  • Digestion:

    • Carbohydrates: Digested by salivary amylase.

    • Proteins: Digested by pepsin.

    • Lipids: Digested by lingual lipase and gastric lipase.

  • Esophageal Peristalsis:

    • Initiated by the opening of the Upper Esophageal Sphincter (UES).

    • Longitudinal muscles play a minor role.

    • Under central and peripheral neural control.

Epithelial Cells of the Stomach

  • Exocrine (Fundus and Body):

    • Mucus Neck Cells: Secrete mucus.

    • Chief Cells: Secrete pepsinogen, gastric lipase (for lipid breakdown).

    • Parietal Cells: Secrete intrinsic factor, H+H^+, Cl−Cl^-.

  • Endocrine (Antrum):

    • G Cells: Secrete gastrin.

    • D Cells: Secrete somatostatin.

Parietal Cell Activity

  • Inactive Parietal Cell:

    • Apical side (facing gut lumen) is smooth.

    • Many tubulovesicles close to the apical membrane.

  • Active Parietal Cell:

    • Tubulovesicles fuse with the apical side to form a canaliculus, increasing surface area.

    • Numerous proton pumps are present to pump acid, requiring significant energy supplied by mitochondria.

  • Acid Production:

    • Produced by carbonic anhydrase. This process also generates bicarbonate (related to the bicarbonate equation).

  • Proton Pumping:

    • Protons are pumped out via H+/K+H^+/K^+-ATPase (proton out, potassium in), which requires ATP.

  • Potassium Regulation:

    • Potassium leaky channels on the apical side eliminate excess potassium by returning it to the gut lumen.

  • Bicarbonate Removal:

    • Bicarbonate is removed from the parietal cell via a chloride/bicarbonate exchanger on the basal side, leading to chloride buildup.

  • Chloride Transport:

    • Chloride leaky channels on the apical side allow chloride to move into the lumen, following the positive charge of the protons.

Parietal Cells and Acid Release

  • Parietal cells, located in the body of the stomach, are responsible for releasing acid.

  • These cells exist in rested and activated phases, with the activated stage leading to acid secretion.

Chief Cells and Protein Digestion

Chief cells, also found in the gastric glands, play a crucial role in protein digestion by secreting pepsinogen, an inactive precursor of the enzyme pepsin.

  • They produce enzymes that break down proteins, resulting in amino acids and small peptides.

G Cells and Gastrin

  • Amino acids and small peptides stimulate G cells (endocrine cells) to secrete gastrin.

  • Gastrin acts on its receptors (CCKb), which are expressed by parietal cells, to stimulate acid secretion.

Enterochromaffin-like Cells (ECL Cells) and Histamine

  • Gastrin stimulates neighboring ECL cells, which are located below the epithelial cell layer.

  • ECL cells release histamine, which acts on histamine receptors (H2 receptors) on parietal cells, further promoting acid secretion.

Neural Control of Acid Secretion

  • Vagal nerves innervate ECL cells and parietal cells directly (releasing ach on both)

  • Vagal nerve stimulation of ECL cells leads to histamine release.

  • Vagal nerve release Ach onto receptors (m3 receptors) on the surface of parietal cells, prompting them to secrete hydrochloric acid, which is crucial for digestion.

Gastrin Releasing Peptide (GRP)

  • Nerves also release GRP, which stimulates G cells to release more gastrin, amplifying acid secretion.

D Cells and Somatostatin

  • D cells release somatostatin, which inhibits gastrin release from G cells, providing a regulatory mechanism.

Regulation of Stomach Acid

  • The regulation of stomach acid secretion is tightly controlled due to its importance.

  • Multiple inputs regulate this function.

Protein Digestion in the Stomach

  • Protein digestion begins in the stomach.

  • Acid released by parietal cells activates the enzyme pepsinogen, which breaks down proteins.

Pepsinogen and Pepsin

  • Pepsinogen is released in an inactive form (zymogen) by chief cells.

  • It is not activated until it reaches the stomach lumen.

Activation of Pepsinogen

  • Pepsinogen is activated in the gut lumen to prevent it from breaking down proteins lining the stomach cells.

  • Mucus protects the stomach layer, and the protein to be digested is located in the lumen.

  • Acid secreted into the lumen activates pepsinogen, which auto-catalyzes to become pepsin.

  • Activated pepsin can also cleave pepsinogen to become more pepsin.

Functions of Stomach Acid

  • Activates pepsin from pepsinogen for protein breakdown.

  • Inactivates carbohydrate-digesting enzymes (salivary amylase) and activates lipid-digesting enzymes (gastric and lingual lipas)

  • Destroys bacteria, serving as a defense mechanism (Defense Mechanism\text{Defense Mechanism}

  • Denatures proteins, aiding in digestion and microbial destruction.

  • Stimulates hormone secretion from other endocrine cells.

Gastroesophageal Reflux Disease (GERD) Case Study

  • GERD is a condition where stomach contents reflux into the esophagus.

  • Prevalence: 20-25% 

Symptoms of GERD:

  • Heartburn (due to acid irritating the esophagus).

  • Pain.

  • Regurgitation of food.

  • Vomiting.

Causes of GERD:

  • Overproduction of stomach acid, leading to overfilling and irritation of the lower esophageal sphincter.

  • Medications that affect and slightly open the sphincter, allowing acid to escape.

Treatment Targets:

  • The mechanisms of parietal cell activation and acid release are targeted to prevent or reduce GERD occurrence.