Gastrointestinal System-Introduction

The gastrointestinal (GI) tract is a complex system primarily responsible for digesting food and absorbing nutrients, while also maintaining homeostasis and regulating growth of its mucosa. Its functions are intricately controlled by an interplay of neural and hormonal influences.

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I. Overall Organization & Structure

• General Structure

    ◦ GI Tract/Alimentary Canal: A tube extending from the mouth to the anus.

        ▪ Layers of the Wall (from deepest to superficial):

            • Mucosa: Inner lining for protection, secretion, absorption. Contains mucosal membrane and mucosa-associated lymphoid tissue (MALT).

            • Submucosa: Areolar connective tissue layer with blood and lymphatic vessels, and nerves. Houses the submucosal (Meissner's) plexus.

            • Muscularis (Muscular Layer): Primarily smooth muscle (except in pharynx, upper esophagus, external anal sphincter which are striated).

                ◦ Inner Circular Layer: Contraction decreases lumen diameter.

                ◦ Outer Longitudinal Layer: Contraction shortens GI tract segment.

                ◦ Contains the myenteric (Auerbach's) plexus, a major nerve supply.

            • Serosa (Visceral Peritoneum): Most superficial layer (or adventitia in esophagus).

    ◦ Accessory Organs:

        ▪ Teeth, Tongue, Salivary Glands

        ▪ Liver, Gallbladder, Pancreas

II. Regulation of GI Motility

Motility is integrated by regulatory systems that monitor internal and external events.

• Neural Regulation:

    ◦ Autonomic Nervous System (ANS):

        ▪ Extrinsic Nervous System:

            • Parasympathetic Division: Generally excitatory.

                ◦ Vagus Nerve (Cranial Nerve X): Innervates esophagus, stomach, pancreas, and upper large intestine. Contains preganglionic fibers synapsing in enteric plexuses. About 75% of vagal fibers are afferent, providing sensory input.

                ◦ Pelvic Nerves: Innervate lower large intestine, rectum, and anus (from sacral region S2-S4).

                ◦ Vagovagal Reflexes: Both afferent and efferent pathways contained within the vagus nerve, important for regulating GI functions.

            • Sympathetic Division: Generally inhibitory.

                ◦ Fibers originate in spinal cord (T8-L2), synapse in prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric).

                ◦ Postganglionic fibers innervate enteric nervous system, blood vessels, and some smooth muscle cells.

        ▪ Intrinsic (Enteric) Nervous System (ENS): Can function as a self-contained system.

            • Composed of myenteric (Auerbach's) plexus (controls motility) and submucosal (Meissner's) plexus (controls secretion and blood flow).

            • Receives input from extrinsic nerves and GI tract receptors.

            • Coordinates activities through local reflexes.

            • Neurotransmitters include acetylcholine (ACh) (major excitatory), nitric oxide (NO) and vasoactive intestinal peptide (VIP) (major inhibitory), serotonin, and somatostatin.

• Hormonal Regulation:

    ◦ Gastrin: Released from G cells in antral and duodenal mucosa. Stimulated by peptides, amino acids, distention, vagal stimulation. Increases H+ secretion, stimulates growth of gastric mucosa.

    ◦ Cholecystokinin (CCK): Released from I cells in duodenum and jejunum by fat and protein digestion products. Stimulates gallbladder contraction and pancreatic enzyme secretion, increases ileal motility, relaxes ileocecal sphincter, inhibits gastric emptying and acid secretion. Also associated with satiety.

    ◦ Secretin: Released from S cells in duodenum and jejunum by H+ in the duodenum. Stimulates pancreatic bicarbonate and water secretion, inhibits gastric acid secretion and emptying.

    ◦ Gastric Inhibitory Peptide (GIP) (or Glucose-dependent insulinotropic peptide): Released from K cells in duodenum and jejunum by all three nutrient types (fat, protein, carbohydrate). Stimulates insulin release. Inhibits gastric acid secretion and emptying.

    ◦ Motilin: Released from M cells in duodenum and jejunum. Initiates Migrating Motor Complex (MMC) in stomach and small intestine during fasting.

    ◦ Somatostatin (SS): Widely distributed in gut nerves and D cells. Inhibits gastrin release, histamine release, and gastric H+ secretion.

    ◦ Ghrelin: Secreted by gastric cells; stimulates appetite.

• Myogenic/Intrinsic Smooth Muscle Properties:

    ◦ Slow Waves (Basic Electrical Rhythm): Rhythmic fluctuations in membrane potential that set the maximum frequency of contractions. Initiated by interstitial cells of Cajal (ICCs), which act as pacemakers.

    ◦ Spike Potentials: Rapid depolarizations superimposed on slow waves that trigger muscle contractions.

    ◦ Tonic Contractions: Sustained contractions (e.g., LES, orad stomach).

    ◦ Phasic Contractions: Periodic contractions followed by relaxation (e.g., esophagus, gastric antrum, small intestine).

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III. Motility & Reflexes of Specific GI Organs

A. Mouth & Pharynx

• Chewing (Mastication): Lubricates food with saliva, decreases particle size.

• Swallowing (Deglutition): Initiated voluntarily but proceeds as a coordinated involuntary reflex.

    ◦ Swallowing Center: In the medulla of the brainstem, coordinates activity of cranial nerves.

    ◦ Pharyngeal Phase: Rapid, less than 1 second. Nasopharynx closes, breathing inhibited, UES opens, pharyngeal muscles contract sequentially (peristalsis) [21, 22f, 297].

B. Esophagus

• Esophageal Peristalsis: Coordinated contractions propelling food to the stomach.

    ◦ Primary Peristalsis: Follows pharyngeal phase of swallowing.

    ◦ Secondary Peristalsis: Elicited by esophageal distension, clears remaining food.

    ◦ Coordinated by both central and intrinsic peripheral mechanisms.

• Upper Esophageal Sphincter (UES): Thickening of circular muscle. Relaxes during swallowing, then contracts to prevent reflux.

• Lower Esophageal Sphincter (LES): No separate anatomical muscle, but functions as a sphincter.

    ◦ Tonic Contraction at rest, preventing reflux from stomach.

    ◦ Receptive Relaxation: Occurs during swallowing (vagovagal reflex), allowing food entry. Mediated by enteric nerves releasing VIP and NO.

• Clinical Relevance: Gastroesophageal reflux (heartburn) if LES tone decreased; Achalasia if LES fails to relax.

C. Stomach

• Divisions for Motility:

    ◦ Orad Portion (Fundus and part of Body): Primarily for accommodation of ingested material. Exhibits low-amplitude, long-lasting tonic contractions.

    ◦ Caudad Portion (Distal Body and Antrum): Primarily for mixing and propulsion.

• Receptive Relaxation: Vagovagal reflex where orad stomach relaxes with LES during swallowing, accommodating large volumes with minimal pressure increase. CCK also increases gastric distensibility.

• Contractions of Caudad Stomach: Mostly peristaltic.

    ◦ Begin in midstomach and move towards gastroduodenal junction.

    ◦ Increase in force and velocity as they near the junction.

    ◦ Retropulsion: Most contents are propelled back into the stomach as the contraction overtakes them, aiding mixing and reducing solid particle size.

    ◦ Slow wave frequency is ~3 cycles/minute.

• Gastric Emptying: Coordinated activity of stomach, pylorus, and proximal small intestine.

    ◦ Liquids empty almost immediately; solids require a lag period for reduction to ~1mm3 particles.

    ◦ Rate affected by chemical composition (e.g., lipids, H+, non-isotonic solutions slow emptying).

    ◦ Regulated by feedback inhibition from duodenal receptors (stimulated by osmotic pressure, H+, fatty acids). This causes decreased orad tone, decreased caudad contractions, increased pyloric tone, and increased duodenal segmenting contractions.

• Migrating Motor Complex (MMC): During fasting, powerful peristaltic contractions sweep undigested contents from stomach through small intestine into colon. Cycle every ~90 minutes. Motilin is a key mediator.

D. Small Intestine

• Functions: Mixing, local circulation, and aboral propulsion of contents for digestion and absorption.

• Contraction Types:

    ◦ Segmentation Contractions: Main movement for mixing and local circulation, dividing bowel into segments. Do not result in net forward movement.

    ◦ Peristaltic Contractions: Highly coordinated, propel chyme distally. Seldom seen in long segments normally, but short segments (1-4 cm) occur.

• Peristaltic Reflex (Law of the Intestines): Contraction oral (proximal) and relaxation aboral (distal) to stimulation point, moving material aborally. Coordinated by the enteric nervous system. Serotonin (from enterochromaffin cells) initiates this reflex.

• Intestino-intestinal Reflex: Gross distention of one bowel segment inhibits contractile activity in the rest of the bowel. Depends on extrinsic neural connections.

• Slow Waves: Frequency decreases distally (12 cpm in duodenum to 8 cpm in ileum).

• Migrating Motor Complex (MMC): Continues from stomach, sweeps undigested contents to colon, and maintains low bacterial counts in upper intestine. Disappears during feeding.

• Gastroileal Reflex: Food in stomach increases peristalsis in ileum and relaxes ileocecal sphincter, facilitating flow to large intestine. Mediated by extrinsic ANS and potentially gastrin/CCK.

E. Large Intestine

• Functions: Optimal absorption of water and electrolytes, aboral movement, storage and orderly evacuation of feces.

• Anatomy: Longitudinal fibers concentrated into three bands called taeniae coli, forming sac-like segments called haustra.

• Contractions:

    ◦ Most are segmental (haustral churning), aiding water absorption.

    ◦ Mass Movements: Infrequent (1-3 times/day), powerful peristaltic contractions that propel colonic contents long distances distally. Usually occur during or after a meal.

• Gastrocolic Reflex: Presence of food in stomach increases colon motility and frequency of mass movements. Has rapid parasympathetic component (stomach stretch) and slower hormonal component (CCK, gastrin).

• Defecation & Rectosphincteric Reflex:

    ◦ When rectum is distended by feces, internal anal sphincter relaxes involuntarily (rectosphincteric reflex). This creates urge to defecate.

    ◦ Normally prevented by voluntary tonic contraction of external anal sphincter.

    ◦ Defecation involves relaxation of both sphincters, increased intra-abdominal pressure (Valsalva maneuver), and pelvic floor relaxation.

    ◦ Loss of voluntary control can occur with spinal cord injuries.

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IV. Secretions

• Saliva:

    ◦ Functions: Initial starch (α-amylase) and triglyceride (lingual lipase) digestion, lubrication, protection (dilution, buffering).

    ◦ Composition: Hypotonic, high K+ and HCO3-, low Na+ and Cl-.

    ◦ Regulation: Primarily neural (parasympathetic and sympathetic increase flow).

• Gastric Secretion:

    ◦ Parietal cells secrete HCl and intrinsic factor. Chief cells secrete pepsinogen. G cells secrete gastrin.

    ◦ Phases: Cephalic, Gastric, Intestinal.

• Pancreatic Secretion:

    ◦ Contains high HCO3- (neutralizes acidic chyme) and enzymes for protein, carbohydrate, and fat digestion.

    ◦ Regulation by secretin (bicarbonate) and CCK (enzymes).

• Bile Secretion & Gallbladder Function:

    ◦ Bile produced by hepatocytes. Contains bile salts (for fat emulsification and absorption), phospholipids, cholesterol, bile pigments (bilirubin).

    ◦ Stored and concentrated in gallbladder.

    ◦ Gallbladder contracts and sphincter of Oddi relaxes (expelling bile) in response to CCK (from fat/peptides in duodenum) and vagal stimulation. Recirculation of bile acids (enterohepatic circulation).

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V. Digestion & Absorption

• Overall: Occurs primarily in small intestine, aided by increased surface area (Kerckring's folds, villi, microvilli).

• Carbohydrates: Digestion begins in mouth (salivary α-amylase), completed in small intestine (pancreatic α-amylase, brush-border enzymes). Absorbed as monosaccharides (glucose, fructose, galactose).

• Proteins: Digestion begins in stomach (pepsin), completed in small intestine (pancreatic proteases, brush-border enzymes). Absorbed as amino acids, dipeptides, and tripeptides.

• Lipids: Minimal digestion in mouth/stomach (lingual lipase, gastric lipase), most in small intestine (pancreatic lipase, colipase, bile salts). Absorbed as fatty acids, glycerol, monoglycerides. Micelles crucial for solubilization.