Principles of Human Physiology - Gastrointestinal System

Principles of Human Physiology: Gastrointestinal System

Define key concepts:
- Peristalsis: Wave-like muscle contractions that move food through the digestive tract.
- Segmentation: Rhythmic contraction of intestinal muscles to mix food and enhance nutrient absorption.
- Migrating Motility Complex (MMC): A pattern of peristalsis that occurs in the small intestine between meals to clear residual food.
- Haustration: Segmentation contractions occurring in the colon to mix colonic content.
- Mass Movement: Intense muscular contractions in the colon that propel contents towards the rectum.
- Basic Electrical Rhythm (BER): Electrical patterns in smooth muscle that indicate contractile activity.

GI Tract Motility:
- Movements of the GI tract walls due to contractions of the muscularis externa.
- Function: To mix and propel the contents within the GI tract.
- Structure involved: Blood vessels, lymph nodes, mesentery.

Mucosa: Inner lining of the GI tract.
- Enterocytes: Absorptive cells in the mucosa.
- Lamina Propria: Connective tissue layer beneath the mucosal epithelium.
- Muscularis Mucosae: Thin layer of muscle beneath the lamina propria.
Submucosa: Layer containing blood vessels, lymphatics, and the submucosal plexus.
Muscularis Externa: Comprises two layers of muscle, circular and longitudinal.
- Myenteric Plexus: Nerve net in between muscle layers, part of the enteric nervous system.
Serosa: Outermost layer of the GI tract walls.

Pacemaker Activity:
- Exhibits spontaneous slow waves of depolarization.
- Amplitude can change based on neural and hormonal inputs:
- Parasympathetic stimulation leads to excitation and increased amplitude.
- Sympathetic stimulation leads to inhibition and decreased amplitude.
- Slow waves may or may not reach threshold to generate action potentials.

Events of Peristalsis:
- A wave of peristalsis facilitates movement throughout the esophagus:
- Upper esophageal sphincter closes to prevent backflow.
- Lower esophageal sphincter opens to allow food into the stomach.
- Contracting muscles propel bolus forward; relaxing muscles accommodate the esophagus.
- Time for bolus to travel to stomach: approximately 9 seconds.

Gastric Motility Patterns:
- Occur at 3 waves per minute, increasing in intensity as they approach the pylorus.
- Function:
- Mixing of chyme
- Pyloric sphincter remains closed to promote backpropagation of chyme for mixing.
Migrating Motility Complex (MMC):
- Intense contraction waves travel short distances between stomach and intestines.
- Occurs between meals to clear the stomach and prevent bacterial growth.

Promoting Contractions:
- Activation of the stomach enhances motility through hormones:
- Gastrin increases the force of contractions.
Inhibiting Contractions:
- Activation of the small intestine reduces contractions in the stomach through:
- CCK (Cholecystokinin)
- Secretin
- GIP (Gastric Inhibitory Peptide)
- These hormones decrease the force of contractions.

Motility Reflexes: Controlled by CNS:
- Colonocolonic Reflex: Distension in one part of the colon triggers relaxation in other areas.
- Gastrocolic Reflex: Presence of food in the stomach increases colonic motility, promoting greater frequency of mass movements.

Understanding the principles of gastrointestinal motility is crucial for grasping how food is processed, absorbed, and eliminated from the human body, illustrating the refined orchestration of muscular actions and regulatory mechanisms that maintain digestive health.