Physiology of the Gastrointestinal Tract

Physiology of the Gastrointestinal (GI) Tract

Overview of the GI Tract

  • The GI tract, along with the major digestive accessory glands (liver and pancreas), plays a crucial role in the digestive system.

  • Primary function: Supply the body with nutrients, electrolytes, and water.

  • Five main functions of the GI tract:

    • Motility

    • Secretion

    • Digestion

    • Absorption

    • Storage

  • Structural composition: A tube-like structure extending from the mouth to the anus, consisting of various organs:

    • Oral cavity

    • Esophagus

    • Stomach

    • Small intestines (divided into the duodenum, jejunum, ileum)

    • Large intestines (including cecum, colon, rectum)

    • Accessory glands: liver, pancreas, gallbladder

Digestive System Structure

  • Figure Reference: Digestive system illustrated with parts:

    • GI tract (esophagus, stomach, small intestine: duodenum, jejunum, ileum; large intestine: cecum, colon, rectum)

    • Accessory glands (liver, pancreas)

Histological Composition of the GI Tract

  • The GI tract consists of four main histological layers:

    • Mucosa

    • Composed of:

      1. Epithelial cells (e.g., enterocytes, endocrine cells, and others)

      2. Lamina propria

      3. Muscularis mucosae

    • Submucosa

    • Muscularis

    • Contains two muscle layers:

      • Inner thick circular layer

      • Outer thin longitudinal layer

    • Serosal layer

Nerve Plexuses in the GI Tract

  • Two main plexuses containing nerves are located between the musculature layers:

    • Submucosal Plexus (also known as Meissner's plexus)

    • Located between submucosa and inner circular muscle layers.

    • Myenteric Plexus (Auerbach's plexus)

    • Located between the inner circular muscle and the outer longitudinal muscle layers.

  • These plexuses form a part of the intrinsic control system of the GI tract, known as the Enteric Nervous System (ENS), which helps regulate:

    • Motility

    • Secretion

    • Digestion

    • Absorption

    • Storage

Control Systems of the GI Tract

Components of Control Systems

  • The control systems can be divided into two:

    1. Intrinsic control system (ENS)

    2. Extrinsic control system

  • Both systems consist of:

    1. Nerves

    2. Endocrine secretions

  • Key hormones involved include

    • CCK: Cholecystokinin

    • GIP: Gastric Inhibitory Peptide

Mechanisms of Action for Secretions

  • The secretions of the intrinsic and extrinsic control systems regulate GI tract activities; they:

    • Are not secreted into the lumen of the gut thus not digestive in nature.

    • Reach targets via four different mechanisms:

    • Endocrine Mechanism: Secretion deposited near blood vessels, where blood carries them to distant target cells/tissues.

    • Neurocrine Mechanism: Neuromodulators secreted by enteric neurons affect nearby muscle cells, glands, or blood vessels.

    • Paracrine Mechanism: Secretion of peptides diffuse through interstitial space to nearby cells.

    • Autocrine Mechanism: Secretion modifies or regulates the functions of the cell that produced it.

Cellular Composition of the GI Tract

  • Endocrine and paracrine cells are columnar in shape:

    • Characteristics:

    • Wide base and narrow apex.

    • Apex is exposed to the lumen of the gut and contains microvilli that sense luminal contents, responding by secreting hormones/regulatory substances.

    • Base contains secretory granules with hormones and paracrine substances, enabling a wider spread of their secretions.

  • Other present cells:

    • Immune cells and immune mediators interact with the intrinsic control system to regulate GI functions, specifically motility and secretion.

GI Endocrine System

  • Specialized endocrine cells are dispersed amongst the epithelial cells of the gut, classified as:

    • Enterocytes: Absorptive function.

    • Enterochromaffin Cells: Secretory function.

    • Secretes peptides or hormones regulating gut motility, digestion, and absorption of nutrients.

Enteric Nervous System (ENS)

Characteristics and Function

  • The ENS is part of the autonomic nervous system (ANS), functioning independently from the central nervous system (CNS).

  • Composed of:

    • Submucosal Plexus (Meissner's plexus): Between submucosa and inner circular muscles.

    • Myenteric Plexus (Auerbach's plexus): Between inner circular muscles and outer longitudinal muscles.

  • Both plexuses communicate:

    • Through interneurons with each other.

    • With the CNS through vagal, pelvic, and splanchnic nerves.

Types of Enteric Neurons

  • Neuronal Classification:

    • Sensory (Afferent) Neurons: Receive signals from mechanoreceptors (detecting distension) and chemoreceptors (monitoring chemical conditions).

    • Interneurons: Intermediate neurons that communicate between sensory and motor neurons.

    • Motor (Efferent) Neurons: Innervate blood vessels, gut muscles, and glands.

    • Can act in a stimulatory or inhibitory manner.

Functional Mechanism of Enteric Neurons

  • Enteric neurons release neuromodulators from vesicles called varicosities, located along axon branches:

    • Varicosities permit activation of broader areas around axons.

    • Regulatory peptides termed neurocrines are secreted in response to action potentials affecting smooth muscle or glandular cell activities.

    • Varying species may have up to 100 million enteric neurons, sometimes exceeding spinal cord neuron counts.

Classification of Enteric Neurons Based on Criteria

  1. Morphology:

    • Dogiel Type I: Small, irregular cell bodies with multiple short dendrites.

    • Dogiel Type II: Large, with oval-shaped cell bodies and one or two long dendrites.

    • Dogiel Type III: Large, variable-shaped cell bodies with multiple dendrites.

  2. Chemical Coding (based on neurotransmitters):

    • Cholinergic neurons (e.g., Acetylcholine)

    • Adrenergic neurons (e.g., Epinephrine)

  3. Electrophysiological Properties:

    • S-type (fast action potential)

    • AH neurons (long action potentials with an After Hyperpolarization phase)

  4. Function:

    • Excitatory Neurons: Increase secretion or cause muscle contraction.

    • Inhibitory Neurons: Decrease secretion or promote muscle relaxation.

    • Sensory Neurons: Detect luminal pH, pressure, or temperature within the gut wall.

    • Motor Neurons: Directly innervate muscles and sphincters, inducing contraction or relaxation.

Extrinsic Control Systems of the GI Tract

Overview

  • Two extrinsic systems also regulate gut function via:

    1. Nerves: Efferent and afferent pathways connecting the CNS and GI tract.

    2. Endocrine Secretions: Hormonal control featuring limited hormonal involvement.

Vagus Nerve Functionality

  • The vagus nerve consists of:

    • Parasympathetic Efferent Nerve Fibers: Stimulatory, increases gut blood flow, motility, and glandular secretion.

    • Vagal Afferent Nerve Fibers: Communicate information from the gut to the brain.

  • Innervates the GI tract through:

    • Branches:

    • Left vagus: Celiac and left gastric nerves.

    • Right vagus: Hepatic, right gastric, and accessory celiac nerves.

Splanchnic Nerve Functionality

  • The splanchnic nerve provides:

    • Sympathetic efferent and spinal afferent innervation to the GI tract.

  • Transmits signals regarding:

    • Distension of gut wall

    • Inflammation

    • Presence of noxious substances related to colic and abdominal pain.

  • Painful stimuli evoke sympathetic responses resulting in:

    • Inhibition of gut motility

    • Increased glandular secretions.

Extrinsic Hormonal Control

  • The only hormone significantly participating in gastrointestinal regulation is Aldosterone:

    • Source: Secreted by the zona glomerulosa of the adrenal cortex.

    • Stimuli for Secretion: Low-sodium diet, angiotensin, adrenocorticotropic hormone (ACTH), or high potassium levels.

    • Function in the GI tract: Stimulates sodium and water reabsorption from the gut and salivary glands, exchanging sodium for potassium ions.