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Chapter 63 - General Principles of Gastrointestinal Function: Motility, Nervous Control, and Blood Circulation

1. General Organization of the GIT

1. Mucosa - innermost layer; responsible for secretion and absorption
2. Submucosa - connective tissue with nerves (Meissner's/submucosal plexus) and blood vessels
3. Muscularis externa - two smooth muscle layers:
   • Inner circular layer (contracts = narrows lumen)
   • Outer longitudinal layer (contracts = shortens gut)
4. Serosa - outermost covering

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2. Smooth Muscle Electrical Activity (VERY HIGH-YIELD)

A. Slow Waves (Basic Electrical Rhythm - BER)

• Not true action potentials - just slow oscillations in resting membrane potential (5-15 mV)
• Generated by interstitial cells of Cajal (ICC) - act as pacemakers, forming a network between muscle layers
• Frequency varies by location:
  • Stomach body: ~3/min
  • Duodenum: ~12-13/min
  • Terminal ileum: ~8-9/min
• Slow waves SET THE MAXIMUM frequency of contraction but do not by themselves cause contraction

B. Spike Potentials (True Action Potentials)

• True action potentials that ride on top of slow waves
• Occur when slow wave membrane potential rises above threshold (~-40 mV)
• Each spike → muscle contraction
• Triggered by: stretch, hormones (gastrin, CCK), ACh, parasympathetic stimulation
• Inhibited by: norepinephrine, sympathetic stimulation, secretin

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3. Functional Types of Movement in the GIT

Peristalsis - The Basic Mechanism

• A contractile ring forms behind a bolus of food
• The muscle in front relaxes ("law of the gut" or Cannon's law)
• The nerve circuits responsible: myenteric plexus (Auerbach's plexus)
• Peristalsis is an intrinsic property of the gut - occurs even after cutting all extrinsic nerves

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4. Enteric Nervous System (ENS) - "The Second Brain"

Neurotransmitters in the ENS

• Excitatory: ACh, substance P
• Inhibitory: NO (nitric oxide), VIP (vasoactive intestinal peptide)
• Stimulation of the myenteric plexus → increases gut tone, movement velocity, and intensity of rhythmic contractions

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5. Autonomic Control of the GIT

Parasympathetic (Cholinergic - ACh)

• Cranial division: vagus nerve (X) - innervates esophagus, stomach, pancreas, small intestine, proximal colon
• Sacral division: pelvic nerves - innervates distal colon, rectum, anus
• Effect: stimulates GI activity (motility and secretion)
• Postganglionic neurons are located IN the ENS

Sympathetic (Adrenergic - NE)

• Originates from T1-L2 spinal cord segments
• Effect: inhibits GI activity; contracts sphincters
• Strong stimulation can block all food movement through the GIT

GI Reflexes (3 Types)

1. Purely intrinsic (within ENS) - control secretion, peristalsis, mixing, local inhibition
2. Gut → prevertebral sympathetic ganglia → gut - gastrocolic reflex, enterogastric reflexes, colonoileal reflex
3. Gut → spinal cord/brain stem → gut - gastric motor/secretory control via vagus, pain reflexes, defecation reflexes

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6. Gastrointestinal Hormones (Table 63.1 - HIGH-YIELD)

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7. GI Blood Flow ("Splanchnic Circulation")

• Most of the GI organs receive blood from the celiac artery and superior and inferior mesenteric arteries
• Most blood passes through the portal vein → liver before entering systemic circulation
• Blood flow directly proportional to metabolic activity of gut tissues
• Autoregulation: when food is present in the gut, local metabolic factors (CO2, adenosine, decreased O2) cause local vasodilation → increase blood flow

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Chapter 64 - Propulsion and Mixing of Food in the Alimentary Tract

1. Mastication (Chewing)

• Voluntary initially, then involuntary rhythmic chewing reflex
• Reduces food particle size → increases surface area for enzyme action
• Mixes food with saliva

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2. Swallowing (Deglutition)

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3. Esophageal Motility

• Primary peristalsis: continuation of swallowing peristaltic wave from pharynx - travels entire esophageal length in 8-10 sec
• Secondary peristalsis: initiated by distention of esophagus by food remaining after primary wave; not initiated by swallowing reflex
• Lower Esophageal Sphincter (LES) / Gastroesophageal Sphincter: normally constricted (25-35 mmHg above gastric pressure); relaxes with each swallow (via VIP/NO); prevents reflux

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4. Stomach Motility

Stomach as Reservoir

• Stomach can hold ~800-1500 mL with minimal pressure rise due to receptive relaxation (vagal reflex, mediated by VIP/NO)

Gastric Contractions - Two Types

1. Tonic contractions: slow, sustained; mainly in fundus - squeeze food toward antrum
2. Peristaltic (antral pump) contractions: rhythmic waves begin in mid-stomach; increase in intensity toward antrum; ~3 waves/min (set by BER)

Pyloric Pump

• Intense antral contractions push chyme toward the pylorus
• Most chyme is forced back (retropulsion) - serves to mix and grind food
• Small squirts exit through the pylorus into the duodenum
• The pyloric sphincter is nearly (but not fully) closed during digestion

Stomach Emptying Regulation

• Increased food volume → stretch → activates myenteric reflexes → stimulates pyloric pump

• Enterogastric nervous reflexes (via ENS, sympathetic ganglia, vagus): triggered by:
  1. Duodenal distention
  2. Mucosal irritation
  3. Acidity (pH < 3.5-4)
  4. Osmolality (hypo- or hyperosmolar chyme)
  5. Protein/fat breakdown products
• Hormonal feedback: CCK, secretin, GIP → slow gastric emptying

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5. Small Intestine Movements

A. Mixing Contractions (Segmentation Contractions)

• Localized concentric contractions spaced along intestine
• Divide intestine into "sausage-like" segments (Fig. 64.3)
• Frequency: up to 12/min in duodenum/jejunum; 8-9/min in terminal ileum (set by BER)
• Require background excitation from myenteric plexus (blocked by atropine)

B. Propulsive Movements - Peristalsis in Small Intestine

• Weak waves; travel only 3-5 cm before dying out (rarely >10 cm)
• Velocity: 0.5-2.0 cm/sec (faster proximally)
• Net movement: ~1 cm/min → takes 3-5 hours for chyme to travel from pylorus to ileocecal valve
• Peristaltic rush: intense peristalsis triggered by extreme irritation of mucosa (e.g., cholera toxin) - sweeps contents rapidly into colon

C. Migrating Motor Complex (MMC) - "Housekeeping Wave"

• Occurs during fasting (interdigestive period)
• Sweeps intestinal contents (bacteria, undigested residue) toward colon every ~90 min
• Initiated by motilin; abolished when eating resumes

D. Ileocecal Valve Function

• Normally closed; prevents backflow from colon to ileum
• Opens reflexly when peristaltic wave arrives from the ileum
• Gastroileal reflex: distention of stomach → relaxes ileocecal valve → allows flow into cecum

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6. Large Intestine Movements

Haustrations (Mixing Movements)

• Circular muscle contracts in segments creating "haustral pouches"
• Very slow (turn over every ~30 min)
• Content slowly advances toward anus while water/electrolytes are absorbed

Mass Movements (Propulsive)

• Large constrictive ring forms, then colon distally to it loses its haustrations and contracts en masse
• Drives fecal matter 20+ cm forward in seconds
• Occurs 1-3 times/day, mostly after meals
• Triggered by gastrocolic reflex (stomach distention → colon activity) and duodenocolic reflex

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7. Defecation

Anal Sphincters

• Internal anal sphincter: smooth muscle, involuntary, tonically contracted
• External anal sphincter: skeletal muscle, voluntary (pudendal nerve)

Defecation Reflexes

1. Intrinsic myenteric reflex: distention of rectum → peristalsis in sigmoid/rectum + relaxation of internal sphincter - WEAK alone
2. Parasympathetic defecation reflex (sacral cord - pelvic nerves): afferents → S2-S4 → back via pelvic nerves → greatly intensifies peristalsis and further relaxes internal sphincter - POWERFUL
3. Valsalva maneuver supplements: deep breath → glottis closes → abdominal muscles contract → push feces out

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Key Summary Tables for Exam Use

Chapter 63 - Fast Facts

Chapter 64 - Fast Facts

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