Foundations 4.1

GI Physiology 4.1: The Digestive Process

Lecture Objectives

  • Describe the neural and hormonal control of the digestive system.

  • Explain forms of enteric motility involved in digestive functions; correlate with common pathologies.

  • Outline structures of the gastrointestinal tract and their functions.

  • Explain secretory functions of digestive organs and glands; correlate with common pathologies.

  • Describe liver structure and function; correlate with common pathologies.

Outline

A. GI Motility

  • Functional anatomy

  • Neural control

  • Hormonal control

  • Functional movements

B. GI Blood Flow
C. Propulsion of the GI Tract
D. Secretory Functions

A. GI Motility: Anatomy

  • Big Picture: The GI system encompasses the digestive system, GI tract, and accessory exocrine glands.

  • Functions:

    • Assimilation of nutrients.

    • Excretion of waste involves multiple sub-functions:

      • Motility: Movement from mouth to anus.

      • Secretion: Water for lubrication, enzymes for digestion.

      • Digestion: Breakdown of food to smaller molecules.

      • Absorption: Nutrient transfer into the blood.

GI Tract Components

  • Mouth

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

  • Rectum/Anus

GI Tract Walls (Inside to Out)

  1. Mucosal Layers:

    • Epithelial cells and connective tissue

    • Mucosal muscle

  2. Submucosa

  3. Muscularis Layers:

    • Circular muscle

    • Longitudinal muscle

  4. Serosa

Principles of Motility

Walls of the GI Tract

  • Comprised of smooth muscle that creates a syncytium.

  • Smooth muscle cells are interconnected via gap junctions allowing signals to propagate in all directions.

  • Continuous intrinsic electrical activity characterizes GI smooth muscle.

Electrical Activity

  1. Slow Waves:

    • Gradual changes in resting membrane potential (RMP), frequency varies along GI tract.

    • Related to pacemaker cells (interstitial cells of Cajal); no contraction.

  2. Spike Potentials:

    • Action potentials generating contractions triggered when slow waves reach -40mV.

    • Frequency increases with higher membrane potentials and last longer than skeletal muscle action potentials due to calcium-sodium channels.

  3. RMP Variability:

    • RMP can rise or fall; influenced by stretch, neurotransmitters, and autonomic nervous system (ANS) signals.

Neural Control of GI Motility

The Enteric Nervous System (ENS)

  • Components: Comprises two plexuses with distinct functions:

    • Myenteric Plexus (Auerbach's plexus): Responsible for GI movements.

    • Submucosal Plexus (Meissner's plexus): Regulates GI secretions and local blood flow.

  • Functions independently yet is modulated by sympathetic (SNS) and parasympathetic (PSNS) systems.

Myenteric Plexus Functions

  1. Controls movements from mouth to anus.

  2. Regulates sphincter tone:

    • Pyloric sphincter (controls release from stomach to duodenum).

    • Ileocecal valve (controls passage from small to large intestine).

Clinical Consideration

  • Hirschsprung Disease: Congenital absence of the myenteric plexus leading to lack of peristalsis and functional obstruction.

ANS Effects on GI Tract

PSNS Influence

  • Increases ENS activity.

  • Cranial: The vagus nerve (esophagus to middle transverse colon).

  • Sacral nerves S2-S4 control the distal large intestine to anus.

  • Acetylcholine (ACh) is the primary neurotransmitter, particularly around the mouth and anus.

SNS Influence

  • Decreases ENS activity.

  • Spinal nerves T5-L2 influence through celiac, superior, or inferior mesenteric ganglia.

  • Norepinephrine (NE) has two roles: inhibits ENS neurons and affects GI smooth muscle.

Sensory Neurons

  • Respond to stimuli such as mucosal irritation, gut wall distension, and specific chemicals.

GI Reflexes

  1. Local digestive actions such as mixing and secretion.

  2. Reflexes influencing distant GI parts (e.g., gastric stimuli triggering colonic activity).

  3. Complex activities involving brainstem or spinal cord reflexes influencing motility.

Hormonal Control of GI Motility

  • Hormones from the GI tract enter portal circulation to influence GI activities independently from neural control.

  • Stimuli for hormone release include nutrients (protein, fat) and GI tract distension.

  • Key hormones:

    • Gastrin

    • Cholecystokinin (CCK)

    • Secretin

    • Glucose-dependent insulinotropic peptide (GIP)

    • Motilin

Types of GI Movement

  1. Propulsion: Peristalsis (esophagus, stomach, small intestine).

  2. Mixing Movements:

    • Peristaltic contractions and segmentation contractions (small and large intestines).

Conclusion

  • Understanding the GI physiology involves a deep dive into the coordination of motility, hormonal control, and sensory feedback, which together facilitate effective digestion and nutrient absorption.