Physiology of Gastrointestinal System Flashcards

Physiology of the Gastrointestinal System

Digestive Organ Motility

• Mouth and Salivary Glands: Chewing
• Pharynx and Esophagus: Swallowing
• Stomach: Receptive relaxation, peristalsis
• Small Intestine: Segmentation, migrating motility complex
• Large Intestine: Haustral contractions, mass movements
• Exocrine Pancreas: Not applicable
• Liver: Not applicable

Structure of the Digestive System

• Components: Includes the mouth, salivary glands, pharynx, esophagus, stomach, liver, gallbladder, pancreas, duodenum, jejunum, ileum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.
• Sphincters: Pharyngoesophageal and gastroesophageal sphincters are crucial.

Basic Digestive Processes

• Digestion: Biochemical breakdown into smaller molecules for absorption.
• Motility: Muscular contractions of the digestive tract.
• Secretion: Release of water, electrolytes, and organic constituents (enzymes, bile salts, mucus).
• Absorption: Transfer of digested molecules, water, vitamins, and electrolytes into the blood or lymph.

Motility Details

• Propulsive Movements (Peristaltic): Push contents forward.
• Mixing Movements (Segmentation): Mix food with digestive juices and expose contents to absorbing surfaces.

Secretion Details

• Exocrine and Endocrine Secretions: Digestive system produces both.
• Composition: Water, electrolytes, and specific organic constituents.
• Endocrine Role: Digestive system is the largest endocrine organ, producing GI hormones or peptides.

Digestion Details

• Biochemical Categories: Carbohydrates, proteins, and fats.
• Purpose: To break down complex foodstuffs into absorbable units.

Carbohydrates, Proteins, and Fats

• Carbohydrates: Broken down into monosaccharides like glucose, fructose, and galactose.
• Proteins: Composed of amino acids linked by peptide bonds.
• Fats: Primarily triglycerides, digested into monoglycerides and free fatty acids.

Enzymatic Hydrolysis

• Process: Enzymes break bonds by adding water (H_2O) at the bond site.

Absorption Details

• Location: Mostly occurs in the small intestine after digestion is completed.
• Process: Transfer of small absorbable units, water, vitamins, and electrolytes into blood or lymph.

Digestive Tract Wall Layers

• Structure: Consistent throughout, with regional variations.
• Layers (Innermost to Outermost):
  • Mucosa
  • Submucosa
  • Muscularis Externa
  • Serosa

Regulation of Digestive Function

• Factors Involved:
  • Autonomous smooth muscle function
  • Intrinsic nerve plexus
  • Extrinsic nerves
  • GI hormones

Autonomous Smooth Muscle Function

• Interstitial Cells of Cajal: Pacemaker cells in the muscularis externa.
• Slow Wave Potentials: Generated by pacemakers but do not directly induce muscle contraction.

Intrinsic Nerve Plexuses

• Networks: Submucosal plexus and myenteric plexus.
• Enteric Nervous System: Collective term for these plexuses.

Extrinsic Nerves

• Autonomic Nervous System: Nerve fibers from both branches.
• Sympathetic System: Inhibits digestive tract contraction and secretion.
• Parasympathetic System: Increases smooth muscle motility and promotes secretion of digestive enzymes and hormones via the vagus nerve.

Gastrointestinal Hormones

• Function: Exert excitatory or inhibitory influences on digestive smooth muscle and exocrine gland cells.

Sensory Receptors in the Digestive Tract Wall

• (1) Chemoreceptors: Sensitive to chemical components.
• (2) Mechanoreceptors (Pressure Receptors): Sensitive to stretch or tension.
• (3) Osmoreceptors: Sensitive to osmolarity.

Digestion in the Mouth

• Motility (Mastication): Chewing involves slicing, tearing, grinding, and mixing food.

Secretion in the Mouth

• Saliva: Produced by three major pairs of salivary glands.
• Composition: 99.5% water, 0.5% electrolytes and protein.
• Salivary NaCl Concentration: About one-seventh of plasma concentration.
• Importance: Enhances perception of salty and sweet tastes.

Salivary Proteins

• Amylase: Begins carbohydrate digestion by breaking down polysaccharides into maltose.
• Mucus: Facilitates swallowing by moistening food particles and providing lubrication.
• Lysozyme: Antibacterial enzyme.

Salivary Reflexes

• Simple Salivary Reflex: Triggered by chemoreceptors and pressure receptors in the oral cavity.
• Conditioned (Acquired) Salivary Reflex: Salivation occurs without oral stimulation.

Regulation of Saliva Production

• Parasympathetic Stimulation: Produces abundant, watery saliva rich in enzymes.
• Sympathetic Stimulation: Produces a smaller volume of thick saliva rich in mucus.

Taste Perception

• Taste Receptors: Located on the tongue, pharynx, epiglottis, and upper esophagus.
• Five Basic Tastes: Salty, sweet, umami, bitter, and sour.

Taste Transduction Mechanisms

• Salty: Na+ diffuses across tight junctions, causing depolarization via ENaC (epithelial Na+ channel).
• Sweet, Umami, and Bitter: Tastants bind to GPCRs on type II receptor cells, causing PLC-mediated Ca2+ release.
• Sour: H^+ diffuses across tight junctions and permeates cells via ENaC, inhibiting K^+ channels and causing depolarization.

Transmission of Taste Receptor Signals to CNS

• Pathways: Taste receptor signals transmit to the nucleus of the solitary tract, then to the ventral posterior medial nucleus of the thalamus, and finally to the gustatory cortex.

Pharynx and Esophagus

• Motility (Swallowing): Moving food from the mouth through the esophagus to the stomach.
• Initiation: Bolus (chewed food) is voluntarily forced to the rear of the mouth.

Swallowing Reflex

• Stimulation: Pressure of the bolus stimulates pharyngeal pressure receptors.
• Swallowing Center: Located in the medulla of the brain stem.
• Complexity: Most complex reflex; initiated voluntarily but then becomes involuntary.

Esophageal Secretion

• Protective Function: Entirely mucus, which lubricates food passage and protects against damage from sharp edges and gastric reflux.
• Transit Time: Takes 6-10 seconds for food to reach the stomach.

Digestion in the Stomach

• Main Functions:
  • Storage of ingested food.
  • Secretion of HCl and enzymes for protein digestion.
  • Mixing food with gastric secretions to form chyme.

Gastric Motility

• (1) Filling (Receptive Relaxation): Stomach relaxes with each mouthful.
• (2) Storage: Occurs in the body of the stomach.
• (3) Mixing: Antral peristaltic contractions mix food with gastric secretions.
• (4) Emptying: Regulated by gastric and duodenal factors.

Gastric Emptying Regulation

• Gastric Factors: Amount and fluidity of chyme in the stomach influence contraction strength and emptying rate.
• Duodenal Factors: Fat, acid, hypertonicity, and distension inhibit antral contractions.

Neural and Hormonal Responses

• Enterogastric Reflex: Neural response mediated through intrinsic nerve plexuses (short reflex) and autonomic nerves (long reflex).
• Enterogastrones: Hormonal response involving secretin and cholecystokinin (CCK) released from the small intestine mucosa.

Secretin and CCK

• Secretion: Secretin is produced by S cells, and CCK by I cells in the duodenal and jejunal mucosa.
• Function: Inhibit antral contractions to reduce gastric emptying.

Gastric Secretion

• Volume: Stomach secretes about 2 liters of gastric juice per day.
• Gastric Mucosa: Divided into oxyntic mucosa (body and fundus) and pyloric gland area (antrum).

Gastric Exocrine Secretory Cells

• Mucous Cells: Secrete thin, watery mucus.
• Chief Cells: Secrete pepsinogen.
• Parietal (Oxyntic) Cells: Secrete HCl and intrinsic factor.

Pepsinogen and Pepsin

• Pepsinogen: Inactive enzyme precursor secreted by chief cells.
• Conversion: HCl cleaves pepsinogen into active pepsin.
• Function of Pepsin: Initiates protein digestion by splitting amino acid linkages.

Intrinsic Factor

• Secretion: Secreted by parietal cells.
• Function: Important for the absorption of vitamin B12.

Endocrine and Paracrine Cells

• G Cells: Secrete gastrin into the blood.
• Enterochromaffin-like (ECL) Cells: Secrete histamine.
• D Cells: Secrete somatostatin.

Regulation of Gastric Secretion and Motility

• Inhibition:
  • Removal of protein and stomach distension
• Stimulation
  *Accumulation of acid to duodenum increases somatostatin production.
  *Fat, Acid, hypertonicity and distension in the duodenum inhibits gastric secretion and motility

Absorption in the Stomach

• Limited Absorption: No food or water is absorbed.
• Exceptions: Ethyl alcohol and aspirin are absorbed directly by the stomach.