9.1 GI Motility, Secretory Functions, Digestion, and Nutrient Absorption

9.1 GI Motility, Secretory Functions, Digestion, and Nutrient Absorption

Gastrointestinal Electrophysiology & Motility Patterns

Gastrointestinal (GI) motility is driven by intrinsic electrical rhythms generated by specialized pacemaker cells embedded within the muscularis externa known as the Interstitial Cells of Cajal (ICC). These cells generate continuous, rhythmic membrane potential oscillations called slow waves (or basal electrical rhythm). Slow waves themselves do not trigger muscular contraction; rather, when slow wave depolarization reaches a critical threshold (typically influenced by neural or hormonal stimuli), voltage-gated calcium channels open, eliciting spike potentials (action potentials) that drive smooth muscle contraction.

The intrinsic slow wave frequency varies systematically along the GI tract:

• Stomach: ~3 waves per minute
• Duodenum: ~12 waves per minute
• Ileum: ~9 waves per minute

GI motility manifests in distinct physiological patterns:

1. Deglutition (Swallowing): Coordinated by the medullary swallowing center, progressing from a voluntary oral phase to involuntary pharyngeal and esophageal peristaltic waves. The lower esophageal sphincter (LES) relaxes via VIP and nitric oxide (NO) release.
2. Peristalsis: A propulsive contraction pattern governed by the "law of the gut." Luminal distension triggers sensory interneurons, inducing upstream contraction (mediated by acetylcholine and substance P) and downstream relaxation (mediated by NO and VIP).
3. Segmentation: Primary non-propulsive motility pattern of the small intestine during the digestive state. Alternating contractions and relaxations mix chyme with digestive secretions and maximize mucosal contact time.
4. Migrating Motor Complex (MMC): Distinct interdigestive (fasting) motility pattern regulated by the hormone motilin. The MMC cycles every 90\u2013120 minutes from the stomach to the distal ileum, sweeping residual undigested debris, bacteria, and desquamated cells to prevent bacterial overgrowth.

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Gastrointestinal Secretory Functions & Hormonal Regulation

The digestive tract relies on a tightly integrated endocrine and paracrine signaling cascade to coordinate luminal secretions with motility.

Gastric Acid Secretion

Gastric acid (HCl) is secreted by parietal cells in the oxyntic glands of the stomach body and fundus. Parietal cells utilize an apical H+/K+ ATPase pump (proton pump) to secrete H+ against a steep concentration gradient in exchange for K+.

Acid secretion is stimulated by three convergent pathways:

• Histamine: Released by Enterochromaffin-like (ECL) cells, binding to H2 receptors (Gs-cAMP pathway). This is the most potent stimulatory pathway.
• Acetylcholine: Released by vagal postganglionic fibers, binding to M3 receptors (Gq-IP3/Ca2+ pathway).
• Gastrin: Secreted by G cells, binding to CCKB receptors (Gq-IP3/Ca2+ pathway) on parietal cells and ECL cells.

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Enzymatic Digestion & Transepithelial Nutrient Absorption

Carbohydrate Digestion and Absorption

Dietary carbohydrates consist of starches, glycogen, sucrose, and lactose. Digestion begins in the mouth with salivary alpha-amylase and continues in the small intestine via pancreatic alpha-amylase, which cleaves internal alpha-1,4 glycosidic bonds to yield disaccharides (maltose, lactose, sucrose) and limit dextrins.

Brush border disaccharidases perform final hydrolysis at the microvillar membrane:

• Maltase: Cleaves maltose into two glucose molecules.
• Lactase: Cleaves lactose into glucose and galactose.
• Sucrase: Cleaves sucrose into glucose and fructose.

Apical Transport Mechanics:

• Glucose and Galactose: Co-transported into enterocytes across the apical membrane via SGLT1 (Sodium-Glucose Cotransporter 1), an active secondary transporter driven by the basolateral Na+/K+ ATPase gradient.
• Fructose: Absorbed across the apical membrane via facilitated diffusion through GLUT5.

Basolateral Exit: Glucose, galactose, and fructose all exit the enterocyte into the portal circulation via facilitated diffusion through GLUT2.

Protein Digestion and Absorption

Protein digestion initiates in the stomach where parietal HCl denatures tertiary structures and converts inactive pepsinogen (secreted by chief cells) into active pepsin at optimal pH 1.5\u20132.0.

Upon entering the duodenum, acidic chyme triggers pancreatic zymogen release. Crucially, the duodenal brush border enzyme enteropeptidase (enterokinase) cleaves inactive trypsinogen into active trypsin. Trypsin then autocatalytically activates all other pancreatic zymogens:

• Endopeptidases (cleave internal peptide bonds): Trypsin, Chymotrypsin, Elastase.
• Exopeptidases (cleave C-terminal amino acids): Carboxypeptidase A and Carboxypeptidase B.

Small peptides (di- and tripeptides) are transported across the apical enterocyte membrane via PepT1 (coupled to H+ gradient), while free amino acids use Na+-dependent amino acid transporters. Intracellular peptidases hydrolyze oligopeptides into free amino acids prior to basolateral exit into portal capillary blood.

Lipid Digestion and Enterohepatic Circulation of Bile Salts

Lipids are insoluble in aqueous luminal fluid and require mechanical emulsification by bile salts and phospholipids synthesized in the liver and concentrated in the gallbladder.

1. Emulsification & Hydrolysis: Bile salts break large lipid droplets into microemulsions. Pancreatic lipase (requiring colipase to anchor it to the lipid-water interface) hydrolyzes triglycerides into free fatty acids (FFAs) and 2-monoacylglycerol (2-MAG).
2. Micelle Formation: FFAs, 2-MAG, cholesterol, and fat-soluble vitamins (A, D, E, K) combine with amphipathic bile salts to form mixed micelles. Micelles shuttle nonpolar lipids across the unstirred water layer to the enterocyte apical membrane.
3. Re-esterification & Chylomicron Assembly: Monoglycerides and fatty acids diffuse across the apical membrane. Inside the smooth endoplasmic reticulum, they are re-esterified into triglycerides and packaged into chylomicrons containing Apolipoprotein B-48.
4. Lymphatic Transport: Chylomicrons exit the basolateral membrane via exocytosis and enter central lacteals (lymphatic capillaries) because they are too large to penetrate vascular endothelium.
5. Enterohepatic Circulation: Approximately 95% of bile salts escape passive proximal absorption and are actively reabsorbed in the terminal ileum via the ASBT (Apical Sodium-Dependent Bile Acid Transporter). Reabsorbed bile salts travel through the portal vein back to the liver for re-secretion.