Phases of Digestion – Key Vocabulary

Big-Picture Roadmap

• Digestive activities occur in three sequential temporal windows:

  1. Cephalic Phase ("head‐related")

  2. Gastric Phase (food inside the stomach)

  3. Gastrointestinal / Intestinal Phase (chyme has reached the duodenum)

• Each phase is defined by:

  • Primary stimulus (sight, stretch, pH, etc.)

  • Dominant effectors (nerves, local plexuses, hormones)

  • Ultimate goal (prime, intensify, or modulate stomach activity)

• The body deliberately uses redundant control pathways—neural and hormonal—to ensure vital digestive shifts occur reliably.

Phase 1 – Cephalic (“Head”) Phase

• Name origin: cephalic = “head” (Latin).

• Triggers (all are pre-ingestion or oral events):

  • Thought, sight, smell, or taste of food.

  • Tactile sensation of food in the mouth.

• Central integration:

  • Sensory input → cerebral cortex & hypothalamus → autonomic centers in the medulla oblongata.

  • Medulla ramps up parasympathetic (vagus) outflow to digestive organs.

• Vagus-mediated stomach responses:

  • ↑ Secretion of gastric juice (HCl + pepsinogen + other enzymes).

  • Initiation of segmentation (mixing) contractions → “stomach rumbling” before food arrives.

• Hormonal reinforcement:

  • Vagus stimulation ⟹ G-cells (endocrine) secrete gastrin → enters blood → feeds back to stomach.

  • Gastrin further ↑ HCl secretion & motility (positive feedback/“priming”).

• Physiological theme: Prepare the battlefield before the army (food) arrives.

Phase 2 – Gastric Phase

• Entry criterion: Food has been swallowed; stomach begins to fill.

• Primary stimulus: Distension of the stomach wall.

  • Stretch receptors work analogously to arterial baroreceptors—membrane deformation encodes pressure/volume.

• Neural loop:

  • Stretch receptor afferents travel via vagus nerve → medulla.

  • Medulla sends stronger vagal efferents back → enhanced gastric activity.

• Resultant changes (intensifying Phase 1 actions):

  • Further ↑ gastric juice secretion.

  • Stronger segmentation contractions.

• Progression to chyme handling:

  • Ongoing mixing + HCl ↓ pH & liquefies bolus → chyme.

  • Stomach begins peristaltic waves propelling chyme toward the pylorus & duodenum.

Phase 3 – Gastrointestinal (Intestinal) Phase

Strategic Goal

• Protect the intestine by neutralizing gastric acid quickly.

• Achieve this by allowing only small acidic batches of chyme into the duodenum at a time.

  • Cycle: “Push a bit → neutralize → push next bit… until stomach empty.”

Detection Mechanism in Duodenum

• pH receptors (chemically identical concept to H⁺-sensitive chemoreceptors) monitor luminal acidity.

  • Normal duodenal pH ≈ \text{pH}\approx7 (neutral).

  • Arrival of gastric chyme drops it to acidic values (e.g., \text{pH}<3) → triggers response.

Three Parallel Communication Pathways (all inhibitory to stomach)

1. Extrinsic Neural (long vagal reflex)

   • Acid detected → vagal afferents → medulla.

   • Medulla reduces parasympathetic vagal tone to stomach.

   • Less vagus activity = ↓ secretions & ↓ motility.

2. Intrinsic / Local Neural Reflex

   • Enteric nerve plexuses of duodenum directly synapse with those of stomach.

   • Local inhibitory signals further ↓ gastric secretions & contractions (CNS not required).

3. Hormonal Pathway

   • Duodenal endocrine cells release:

     • Secretin

     • Cholecystokinin (CCK)

   • Via bloodstream these hormones reach the stomach → inhibit HCl production & motility.

   • (They also act on pancreas, liver, gallbladder—covered in later material.)

Cyclic Feedback Logic

• Acidic batch arrives → 3 pathways inhibit stomach → duodenum neutralizes acid (via pancreatic bicarbonate, etc.).

• Once duodenal pH returns to ~neutral, pH receptors fall silent.

• Medulla lifts inhibition → vagus re-activates stomach → next peristaltic pulse of chyme.

• Cycle repeats until stomach is completely empty.

Key Definitions & Concepts (Quick-Reference)

• Gastric juice: aqueous mix of \text{HCl}, pepsinogen/pepsin, intrinsic factor, mucus.

• Segmentation: Non-propulsive mixing contractions that churn contents.

• Peristalsis: Wave-like propulsion moving material distally.

• Chyme: Semi-liquid stomach output (food + gastric secretions).

• Secretin: Hormone that stimulates pancreatic bicarbonate, inhibits stomach.

• CCK (Cholecystokinin): Hormone that stimulates gallbladder contraction, pancreatic enzyme secretion, inhibits stomach.

Integration & Redundancy Themes

• The digestive system relies on both fast neural signals (vagus, enteric plexus) and slower hormonal signals (gastrin, secretin, CCK) for the same task → provides fail-safe control.

• Similar strategy recurs across physiology (e.g., blood pressure control uses baro-reflex + renin-angiotensin system).

Everyday Manifestations & Clinical Relevance

• “Mouth-watering” and stomach growling before eating = cephalic vagal activation.

• Overeating can swamp the duodenum’s ability to neutralize acid, potentially irritating intestinal mucosa.

• Drugs that block acid (e.g., PPIs) or block vagus (rarely used) can alter these phases.

Study Tips & Next Steps

• Sketch a flow chart linking stimuli → pathway → stomach response for each phase.

• Practice labeling where each hormone is produced and what organ it targets.

• Complete the “Phases of Digestion” lab worksheet to solidify cyclic feedback understanding.