GI Physiology – Digestion, Motility, Neural & Endocrine Control

Vocabulary flashcards covering digestion, absorption, GI smooth-muscle structure and excitation, neural (intrinsic & extrinsic) regulation, major peptide hormones, and key nutrient monomers mentioned in the lecture notes.

Digestion

Chemical breakdown of ingested food into absorbable monomers inside the GI lumen.

Absorption

Transfer of nutrient monomers from the GI lumen across epithelium into blood or lymph.

Motility

Series of involuntary smooth-muscle contractions that move (peristalsis) and mix (segmentation) GI contents.

Peristalsis

Propulsive, wave-like contraction moving a bolus toward the anus.

Segmentation

Non-propulsive mixing contractions that churn luminal contents to enhance digestion and absorption.

Exocrine Secretion (GI)

Release of digestive enzymes, mucus or bicarbonate into the GI lumen via ducts—technically ‘outside’ the body.

Endocrine Secretion (GI)

Release of peptide hormones from mucosal cells into blood to act at distant targets.

Enteric Nervous System (ENS)

Intrinsic ‘brain of the gut’ comprising neurons in myenteric and submucosal plexuses that control motility, secretion and blood flow.

Myenteric Plexus

ENS plexus located between longitudinal and circular muscle layers; primarily regulates smooth-muscle activity.

Submucosal Plexus

ENS plexus in the submucosa; mainly controls secretion and local blood flow.

Muscularis Externa

GI wall layer containing outer longitudinal and inner circular smooth-muscle coats responsible for motility.

Longitudinal Muscle Layer

Outer smooth-muscle fibers running parallel to tract; shortening length during contraction.

Circular Muscle Layer

Inner smooth-muscle fibers encircling lumen; contraction narrows diameter.

Smooth Muscle Cell (GI)

Spindle-shaped cell lacking sarcomeres; contracts as a syncytium via gap junctions.

Slow Wave / Basal Electrical Rhythm

Spontaneous, rhythmic depolarization–repolarization cycles setting contraction frequency in GI smooth muscle.

Interstitial Cells of Cajal (ICC)

GI pacemaker cells that generate and propagate slow waves to adjacent smooth-muscle cells.

Calmodulin

Cytosolic Ca2+-binding protein that activates MLCK in smooth-muscle contraction.

Myosin Light-Chain Kinase (MLCK)

Enzyme phosphorylating myosin light chains to initiate cross-bridge cycling in smooth muscle.

Calponin & Caldesmon

Regulatory proteins that, when bound to Ca2+-calmodulin, reduce inhibition of actin-myosin interaction.

Protein Kinase A (PKA)

cAMP-activated kinase that phosphorylates targets to decrease GI smooth-muscle excitability and contraction.

Vasoactive Intestinal Peptide (VIP)

ENS inhibitory neurotransmitter causing smooth-muscle relaxation and stimulating intestinal & pancreatic secretion.

Nitric Oxide (GI)

Gaseous ENS messenger producing smooth-muscle relaxation via cGMP.

Parasympathetic Nervous System (PNS)

Extrinsic autonomic input (vagus, pelvic nerves) that generally excites GI activity via ACh release.

Sympathetic Nervous System (SNS)

Extrinsic autonomic input (postganglionic norepinephrine) that inhibits GI motility & secretion, constricts sphincters.

Intrinsic Primary Afferent Neuron (IPAN)

Enteric sensory neuron detecting luminal chemicals, stretch or osmolarity; initiates local reflexes.

Peristaltic Reflex

ENS circuit causing contraction oral to and relaxation aboral to a bolus, propelling contents.

Enteroendocrine Cell

Single mucosal epithelial cell that releases peptide hormone into blood in response to luminal or ECF stimuli.

Gastrin

Hormone from G cells (stomach antrum/body) stimulated by peptides, AAs & distension; increases gastric HCl and mucosal growth.

Cholecystokinin (CCK)

Hormone from I cells (duodenum/jejunum) stimulated by fats & peptides; contracts gallbladder, stimulates pancreatic enzyme secretion, slows gastric emptying.

Secretin

Hormone from S cells (duodenum) released by low pH; stimulates pancreatic & biliary HCO3−, inhibits gastric motility.

GIP (Glucose-Dependent Insulinotropic Peptide)

Hormone from K cells (proximal small intestine) stimulated by fats & protein; inhibits gastric activity and stimulates insulin release.

Motilin

Hormone from M cells (duodenum/jejunum) secreted during fasting; triggers migrating motor complex (MMC) contractions.

Somatostatin (GI)

Paracrine hormone from D cells; inhibits gastrin, HCl and histamine release in stomach.

Peptide YY (PYY)

Ileal/colonic hormone released post-meal; slows gastric and pancreatic secretion (ileal brake) and reduces appetite.

Histamine (Gastric)

Paracrine amine from enterochromaffin-like (ECL) cells; acts on parietal cells to enhance acid secretion.

Monosaccharides

Absorbable carbohydrate units—glucose, galactose, fructose—enter blood after digestion.

Amino Acids, Di- & Tripeptides

Absorbable protein digestion products transported across small-intestinal epithelium.

Free Fatty Acids

Absorbable lipid units generated from triglyceride and phospholipid digestion.

Triglyceride Digestion Products

Two free fatty acids + one 2-monoglyceride generated by lipase cleavage.

Phospholipids

Membrane lipids hydrolyzed to fatty acids and lysophospholipids for absorption.

Cholesterol Ester

Dietary cholesterol bound to fatty acid; must be de-esterified before absorption.

G-Protein-Coupled Receptor (GPCR)

Cell-surface receptor linking peptide hormones or neurotransmitters to second-messenger pathways (cAMP, IP3/DAG).

IP3 & DAG

Second messengers produced by PLC; IP3 releases Ca2+ from SR, DAG activates PKC.

Basal Electrical Rhythm Frequency

Typical slow-wave rates: stomach ~3 waves/min; small intestine 10-14 waves/min.

Migrating Motor Complex (MMC)

Periodic fasting motility pattern driven by motilin that sweeps residual contents toward colon.