Chapter 63 — General Principles of Gastrointestinal Function: Motility, Nervous and Hormonal Control, Blood Circulation, and Microbiota

Overview

1. General Principles of Gastrointestinal Motility

Physiologic Anatomy of the GI Wall

1. Serosa
2. Longitudinal smooth muscle — runs along the length of the gut
3. Circular smooth muscle — wraps around the gut
4. Submucosa
5. Mucosa (with a thin inner layer of mucosal muscle)

Electrical Activity of GI Smooth Muscle

• Not true action potentials; they are oscillations in resting membrane potential (roughly −50 to −65 mV baseline).
• Generated by interstitial cells of Cajal (ICC), which act as pacemakers for GI smooth muscle.
• Frequency varies by region: ~3/min in the stomach, ~12/min in the duodenum, ~8–9/min in the terminal ileum.
• Slow waves alone do not cause muscle contraction, but they set the rhythm.

• Occur when resting membrane potential rises above approximately −40 mV (threshold).
• Triggered when slow waves peak high enough, or when the muscle is stretched/stimulated by parasympathetics or certain hormones.
• Each spike causes a phasic contraction of the muscle.
• Multiple spikes = stronger, more sustained contraction.

2. Neural Control — The Enteric Nervous System (ENS)

• Excitatory neurons: acetylcholine, substance P, glutamate
• Inhibitory neurons: vasoactive intestinal polypeptide (VIP), nitric oxide (NO), ATP — especially important for relaxing sphincters (pyloric sphincter, ileocecal valve)

3. Autonomic Control of the GI Tract

• Vagus nerve supplies the esophagus, stomach, and most of the small and large intestine to the splenic flexure.
• Pelvic nerves (sacral parasympathetics, S2–S4) supply the descending colon, sigmoid, rectum, and anus.
• Postganglionic neurons are the ENS neurons themselves; the vagus and pelvic nerves primarily modulate the ENS rather than directly innervating muscle.
• Effect: increases GI motility and secretion.

• Originates from T5–L2.
• Sympathetic postganglionic fibers release norepinephrine, which inhibits smooth muscle and decreases GI secretion.
• Strong sympathetic stimulation can almost completely block GI movement.

4. Gastrointestinal Reflexes

1. Enteric-only reflexes (entirely within the ENS): local reflexes for secretion, peristalsis, mixing contractions, sphincter relaxation
2. Prevertebral ganglia reflexes (gut → sympathetic ganglia → gut): e.g., the gastrocolic reflex (filling of stomach → increased colonic motility), the enterogastric reflex (distension of intestine → inhibits stomach)
3. Spinal cord and brain stem reflexes: e.g., pain, nausea/vomiting signals traveling via extrinsic pathways; also the defecation reflexes

5. Hormonal Control of GI Motility

6. Functional Movements in the GI Tract

Propulsive Movements — Peristalsis

• Peristalsis is the basic propulsive movement: a contractile ring appears in the circular muscle and moves forward (aborally), pushing contents ahead of it.
• The usual stimulus is distension of the gut wall (also chemical/physical irritation, strong parasympathetic signals).
• Mechanism: distension → ENS activation → contraction orad to the bolus + relaxation aborally ("receptive relaxation" or "Law of the Gut").
• Peristalsis travels only toward the anus (anally directed) because the myenteric plexus is "polarized" in that direction.
• Absent or blocked when: the myenteric plexus is absent (congenital aganglionosis) or when atropine is given (blocks cholinergic ENS terminals).

Mixing Movements — Segmentation Contractions

• Rhythmic, ring-like contractions that appear at intervals along the intestine, then relax and appear elsewhere.
• Do not propel food forward; instead, chop and mix chyme with digestive juices.
• Frequency determined by the slow-wave rhythm of that segment.

7. Gastrointestinal Blood Flow (Splanchnic Circulation)

• When the gut is active (digesting/absorbing), local metabolites (CO₂, adenosine, decreased O₂) cause vasodilation → "functional hyperemia" — blood flow can increase 8-fold during peak absorption.
• The intestinal villi have a special countercurrent exchange arrangement of arterioles and venules that can allow shunting of O₂ directly from arterioles to venules at the villus tip, potentially making the villus tip hypoxic — particularly important in conditions of low flow.

• Parasympathetic stimulation → mild increase in blood flow (secondary to increased secretory activity)
• Sympathetic stimulation → intense vasoconstriction of arterioles → sharply decreased blood flow. Over time, "autoregulatory escape" returns flow toward normal via local metabolic mechanisms.
• In hemorrhagic shock or heavy exercise, sympathetic vasoconstriction of the GI vessels can divert 200–400 mL of blood to the systemic circulation (since the large mesenteric veins constrict and act as a blood reservoir).

8. Gastrointestinal Microbiota

• The GI tract is colonized by trillions of microorganisms (bacteria, archaea, fungi, viruses) — the microbiota. The microbiome includes the collective genomes of these organisms.
• The gut is essentially sterile at birth and colonizes rapidly thereafter, shaped by: birth modality (vaginal vs. C-section), diet (breast milk vs. formula), and antibiotic use.
• Microbial concentration increases along the tract:
  • Stomach/duodenum: 10¹–10³ per gram
  • Jejunum/ileum: 10⁴–10⁷ per gram
  • Colon: 10¹¹–10¹² per gram
• ~90% of gut bacteria belong to Bacteroidetes and Firmicutes.

1. Protection against pathogens — competition for nutrients and mucosal attachment sites; production of antimicrobial compounds
2. Digestion of dietary fiber — anaerobic fermentation produces short-chain fatty acids (SCFAs) (acetate, propionate, butyrate), a key energy source for colonocytes
3. Vitamin synthesis — especially vitamin K and some B vitamins
4. Immune system development and regulation — microbiota educates the immune system; dysbiosis is linked to inflammatory bowel disease, allergies, obesity, and metabolic disorders
5. Gut-brain axis — microbiota influences behavior and mood via neural, endocrine, and immune pathways

Key Takeaways