The Digestive System - Study Notes

The Digestive System - Study Notes

Lecturer: Dr. R. Ahangari
University of Central Florida, Orlando
Reference: Human Physiology by Linda S. Constanzo, WebMed

Physiology of the Gastrointestinal (GI) Tract

  • The GI tract is designed for nutrition provision through multiple activities including:
    • Movement of Food: Transport of food through the GI tract.
    • Secretion of Digestive Juices: Involves digestion of food.
    • Absorption: Absorption of digestive products, water, and electrolytes.
    • Circulation: Circulation of blood through the GI organs to remove absorbed substances.
    • Control Mechanisms: Regulation of these functions through nervous and hormonal systems.

Alimentary Tract Overview

  • Oral Cavity: Initial breakdown of food begins here.
  • Esophagus: Serves as a passage for food to the stomach.
  • Stomach: Functions for storage and performs a second breakdown of food.
  • Accessory Organs: Includes liver, pancreas, and gall bladder aiding digestion.
  • Intestines: Comprises small and large intestines, responsible for further digestion and absorption.

Principles of Gastrointestinal Motility

  • GI Wall Components:
    • Composed of major and minor muscle layers and valves.
    • Electrical Activity: Responsible for coordinating muscle contractions in the GI tract.

Structures and Innervation of the GI Tract

  1. Mucous Membrane: Specialized epithelial cells for secretion or absorption.
  2. Muscularis Mucosa: Layer of muscle fibers beneath lamina propria that affects surface area for secretion/absorption.
  3. Muscle Layer: Consists of:
    • Inner Circular Layer:
      • Function: Contraction decreases the diameter of the GI lumen.
    • Outer Longitudinal Layer:
      • Function: Causes shortening of a segment of the GI tract.
  4. Serosa (Adventitia): The external peritoneal covering layer.

Intrinsic (Enteric) Innervation of the Digestive Tract

  • Plexuses: The digestive system is supplied by two different plexuses:
    1. Submucosal Plexus of Meissner
    2. Myenteric Plexus of Auerbach
  • Function: These plexuses integrate and coordinate the motility as well as secretory and endocrine functions of the GI tract.
  • Sympathetic Fibers: Interspersed between these two plexuses.

Extrinsic Innervation of the GI Tract

  • Efferent Fibers: Carry information from the brainstem and spinal cord to the GI tract.
  • Afferent Fibers: Carry sensory information (via chemoreceptors and mechanoreceptors) from the GI tract back to the brainstem and spinal cord.
  • Parasympathetic Innervation:
    • Includes Vagus nerve (CN X) and pelvic splanchnic nerve (S2-4).
    • Effect: Excitatory on GI functions, synapse in myenteric and submucosal plexuses.
    • Vagus Nerve: Supplies the esophagus, stomach, pancreas, and intestines to the upper parts of the large intestine.
    • Pelvic Splanchnic Nerve: Supplies lower parts of the large intestine and pelvic organs.
  • Sympathetic Innervation:
    • Originates from the spinal cord through abdominal splanchnic nerves (T5-L2).
    • Preganglionic cholinergic fibers synapse in prevertebral ganglia.
    • Postganglionic adrenergic fibers inhibit peristalsis and gastric secretion, causing pyloric contraction and conveying pain signals from the stomach.

Gastrointestinal Motility

  • Contractile Tissue: The predominant type in the GI tract is unitary smooth muscle.
  • Exceptions: Pharynx, upper third of the esophagus, and the external anal sphincter are composed of striated muscle.
  • Muscle Contraction Types:
    • Circular Muscle Contraction: Reduces the diameter of the GI segment.
    • Longitudinal Muscle Contraction: Reduces the length of the GI segment.
  • Types of Contractions:
    • Phasic Contraction: Found in the esophagus, gastric antrum, and small intestine; these contract and relax periodically.
    • Tonic Contraction: Located in lower esophageal sphincter, the orad stomach, ileocecal, and internal anal sphincters.

Membrane Potentials in Intestinal Smooth Muscle

  • Slow Waves:
    • Description:
    • Oscillating membrane potentials inherent to smooth muscle cells of some GI parts.
    • Occur spontaneously, originating in interstitial cells of Cajal (the pacemaker for GI smooth muscle).
    • Not action potentials but influence the pattern of action potentials.
    • Mechanism: Slow wave production cycle involves the activation and deactivation of cell membrane. Depolarization (Calcium ions inward) increases the probability of action potentials occurring, which leads to muscle contraction.
    • Repolarization Phase: Caused by potassium (K+) ions outward.

Frequency of Slow Waves

  • Varies along the GI tract but remains constant within segments:
    • Stomach: Lowest frequency (3 cycles/min).
    • Duodenum: Highest frequency (12 cycles/min).
    • Ileum: Medium frequency (9 cycles/min).
  • The action potentials on top of the slow waves are influenced by neural and hormonal inputs.

Spike Potentials and Action Potentials

  • Spike Potentials: Represent action potentials triggered at -40 mV.
  • Resting Membrane Potential: Ranges between -50 to -60 mV.
  • Factors Influencing Spike Count: The number of spikes is proportional to the threshold rise and duration of time above threshold (1-10 per burst).
  • Duration: Action potential duration in GI muscles is 10-40 times longer than in large nerve fibers (approximately 10-20 ms) due to calcium-sodium channel activity.

Resting Membrane Voltage Characteristics

  • Normal Values: Average resting membrane voltage is about -56 mV.
  • Depolarization Factors:
    • Stretching of the muscle.
    • Acetylcholine release.
    • Stimulation from the parasympathetic nervous system.
    • Specific gastrointestinal hormones.
  • Hyperpolarization Factors:
    • Norepinephrine/epinephrine release.
    • Stimulation from the sympathetic nervous system.

Calcium Ions and Muscle Contraction

  • Mechanism of Action: In GI smooth muscle, calcium works through calmodulin rather than troponin C as the calcium regulator, coordinating the binding of myosin and actin filaments.

Tonic Contraction of GI Smooth Muscle

  • Definition: Even in the absence of action potentials, sub-threshold slow waves result in weak contractions.
  • Characteristics:
    • GI smooth muscle does not completely relax, exhibiting a basal level of contraction (tonic contraction).
    • Controlled by:
    • Nervous input (not related to slow waves).
    • Hormonal influence.
    • Potentially by calcium entry influenced by varying resting membrane potential.
  • Peristalsis: This occurs in relation to the underlying tone of the smooth muscle.