Physiology of the Gastrointestinal Tract
Physiology of the Gastrointestinal (GI) Tract
Main Functions of the GI Tract: The GI tract performs five primary functions to supply the body with nutrients, electrolytes, and water:
Motility
Secretion
Digestion
Absorption
Storage
Structure: The GI tract is a tube-like structure extending from the mouth to the anus, comprising:
Oral cavity
Esophagus
Stomach
Small intestines
Large intestines
GI Motility Patterns
Intrinsic Control: The first level of control in GI motility arises from the intrinsic electrical properties of smooth muscle mass.
Electrical Activity:
Characterized by spontaneously undulating waves of partial depolarization across the gut smooth muscle.
Originates from specialized smooth muscle cells known as Interstitial Cells of Cajal (ICC).
ICC form an interconnected lattice surrounding the circular and longitudinal muscle layers throughout the gut.
Pacemaker Cells:
ICC exhibit rhythmical oscillations in their transmembrane electrical potentials, similar to heart pacemaker cells.
They are interconnected by tight junctions or nexuses, allowing ion flow between cells.
Role of Interstitial Cells of Cajal (ICC)
Electrical Rhythmicity: The spontaneous electrical rhythmicity and connections to smooth muscle give ICC their role as electrical pacemakers of the gut.
Membrane Potential:
Baseline membrane potential in GI smooth muscle cells is usually between -70 to -60 millivolts (mV).
Under ICC influence, membrane potential fluctuates by 20 to 30 mV, resulting in partial depolarization, but does not reach 0 mV.
Propagation of Slow Waves:
Membrane potentials initiated by ICC travel from the oral end to the aboral end of the gut.
These waves are termed slow waves or basic electrical rhythm of the gut.
In dogs, slow waves occur approximately 20 times/minute in the small intestine and about 5 times/minute in the stomach and colon.
Modulation of Slow Waves:
The amplitude and frequency of slow waves can be modulated by the Enteric Nervous System (ENS).
Nervous, endocrine, and paracrine factors control the link between slow waves and muscle contractions.
Smooth Muscle Contraction
Action Potentials: Slow waves constantly travel through the GI tract but do not directly stimulate smooth muscle contraction.
Requirements for Contraction:
Smooth muscle cells contract due to action potentials characterized by complete depolarization, which occur in association with slow waves.
Presence of slow waves is necessary but not sufficient for contraction.
Modulation by ENS:
Excitatory substances elevate the baseline potential (toward zero) while inhibitory substances lower it (make it more negative).
When the membrane potential approaches zero, action potentials occur, leading to muscle contraction.
Synchronization of Muscle Contractions
Integration of Systems: Integrated actions of slow waves, ENS, and the endocrine/paracrine system synchronize contractions of GI muscle mass.
Efficiency of Contractions: For efficient performance, many muscle cells in one layer of a gut segment must contract simultaneously.
Frequency Limitations: Muscle contractions cannot exceed the frequency of slow waves; for example, in the dog's stomach, the upper limit is 5 contractions per minute based on slow wave frequency.
Food Habits
Different organisms have varied diets and can be classified into types based on their feeding habits:
Herbivores: Animals that eat only plants. They possess:
Sharp cutting teeth (incisors) in front.
Flat grinding teeth (molars) at the back.
Examples include cows and deer.
Carnivores: Animals that consume the flesh of other animals.
Examples include snakes and eagles.
Omnivores: Animals that ingest both plants and animals.
Examples include humans and crows.
Scavengers: Animals that eat dead bodies, contributing to environmental cleanup.
Parasites: Organisms that depend on other living hosts for food.
Jaw Types and Their Adaptation for Feeding
Jaw Structure and Motion:
Carnivores:
Angle not expanded.
Shearing motion with minimal side-to-side movement.
Large temporalis muscle; pointed canines.
Reduced facial muscles to allow a wide mouth gape.
Omnivores:
Well-developed jaw with an expanded angle.
Short and pointed incisors and molars with chewing capabilities.
Moderate-sized masseter and pterygoids for chewing.
Herbivores:
Well-developed jaw structure with an expanded angle.
Flat grinding molars for extensive chewing.
Digestion Process Initiation
Prehension: Before digestion begins, food must be directed into the GI tract.
Quadruped animals grasp food using lips, teeth, or tongues with coordinated muscle actions.
Species variations: horses use lips extensively; cattle utilize tongues for grasping.
Involvement of CNS: The process is controlled by the Central Nervous System (CNS) and involves:
Facial nerve
Glossopharyngeal nerve
Trigeminal nerve (motor branch)
Mastication:
Involves jaws, tongue, and cheeks to break down food into smaller particles.
Food is moistened and lubricated, mixing with saliva, which aids in digestion.
Deglutition (Swallowing)
Stages of Deglutition: Involves both voluntary and involuntary actions.
Voluntary Stage:
Tongue molds food into a bolus and pushes it back into the pharynx.
Pharynx serves as a pathway for both air (trachea) and food (esophagus).
Involuntary Reflex:
Initiated when food enters the pharynx, involving sensory nerve detection.
Involuntary Swallow Reflex
Coordination in Swallowing: The involuntary portion involves complex actions:
Brief cessation of breathing.
Elevation of the soft palate to close the nasopharynx.
Tongue presses against the hard palate, closing the oral opening of the pharynx.
Hyoid bone and larynx move forward, blocking the larynx's opening under the epiglottis.
Arytenoid cartilages constrict to further close the laryngeal opening.
Follow-through: This sequence ensures food is directed into the GI tract and away from the respiratory pathway.
Neural Control: Deglutition controlled by lower motor neurons in various brainstem centers, utilizing:
Efferent facial nerve
Vagus nerve
Hypoglossal nerve
Glossopharyngeal nerve
Trigeminal nerve
Esophagus Structure and Function
Muscle Composition: The esophagus contains an outer longitudinal and inner circular muscle layer but is unique in its muscular wall composition:
Much of it comprises striated skeletal muscle fibers with smooth muscle at the distal end (noted in horses, primates, and cats).
Control Mechanisms:
Striated muscle under somatic motor neuron control via vagus nerve.
Smooth muscle under ENS and autonomic nervous system control.
Myenteric Plexus: Present throughout the esophagus, likely provides sensory function coordinating movements between striated and smooth muscle segments.
Anatomy of the Esophagus
Sphincters: The esophagus consists of:
An upper sphincter (cricopharyngeal muscle)
A body
A lower sphincter
Function During Ingestion:
The upper sphincter closes tightly against the larynx when there is no ingestion.
Body facilitates the movement of food from the pharynx to the stomach through peristalsis.
Peristalsis: Defined as a moving constriction in the tubular organ walls reducing the esophageal lumen, pushing the bolus ahead.
Longitudinal muscles contract ahead of constriction to accommodate the advancing bolus.
Passage into the Stomach
Actions During Peristalsis: As the food bolus reaches the esophagus distal end, the lower sphincter (cardiac sphincter) relaxes, allowing the bolus into the stomach.
Secondary Peristaltic Movements: Occur if food remains in the esophagus for clearance purposes.
Resting State: In situations where deglutition is absent, the esophageal body relaxes while both sphincters remain constricted to prevent aspiration and reflux.
Structural Assistance: Anatomical configurations aid in the maintenance of the lower sphincter closure and opening, especially in horses but can risk rupture under high intra-gastric pressure before vomiting or reflux occurs.