7-Swallowing, gastric Emptying, intestinal motility

Peristalsis vs. Segmentation

Peristalsis

• Progressive movement

Segmentation

• Mixing

What detects stretch in the GI system? What NT’s are involved? How do interneurons come into play?

Sensory neurons

CGRP and ACh

Interneurons stimulate enteric motor neurons (ACh here too)

2 types of enteric motor neurons?

Excitatory EMN

• ACh or Substance P

Inhibitory EMN

• VIP or NO

What do stimulated enteric excitatory motor neurons result in? What about inhibitory?

Excitatory = Smooth muscle contraction (oral side)

Inhibitory = Smooth muscle relaxation (anal side)

What big things are involved in the swallowing reflex?

• Touch Receptors (pharynx opening)

• Integration center (medulla/pons)

• Effectors (pharyngeal/esophageal striated & smooth muscle)

What are the 3 phases of the swallowing reflex?

1. Oral

2. Pharyngeal

3. Esophageal

Oral Phase

• Voluntary

• Touch receptors of pharynx detect bolus

  • Reflex initiated!

Pharyngeal Phase

• Involuntary

• Propels food from pharynx → esophagus

• Epiglottis blocks tracheal entry!

• UES Relaxes → Peristalsis → UES Constricts (after bolus passes)

Esophageal Phase

• Involuntary

• UES closes →LES relaxes (opens)

• Primary peristaltic wave below UES → swallowing center initiates reflex → secondary wave (initiated by distention)

CNS and ENS

Input from esophageal sensory fibers to HERE modulates both primary and secondary peristalsis.

Dysphagia

Trouble swallowing

Can be:

• Structural

  • Anatomy

• Functional

  • Abnormal swallowing reflex

Achalasia

Failure of LES to relax (constantly contracted)

• Food is retained in esophagus and in LES

Complete lack of peristalsis within esophagus!

Incompetent LES

Failure of LES to contract (constantly relaxed)

Cause is unknown!

GERD

Gastroesophageal reflux disease, common

LES doesn’t work right

Stomach contents leak back into the esophagus

Decreased secretion of mucus and bicarb in saliva (decreased protection against acid)

Heartburn!!

Barrett’s Esophagus

Special type of GERD, a complication of it

• An adaptation to chronic acid exposure

AKA pre-cancerous lesion (Metaplasia)

Columnar cells replace squamous cells in mucosa of esophagus

Predisposition to esophageal cancer

Diffuse Esophageal Spasms

Irregular, uncoordinated esophageal contractions

Contractions can be powerful

Sx:

• dysphagia

• regurgitation

• chest pain

Corkscrew esophagus appearance!

Disorder of Esophageal Function

• Dysphagia

• Achalasia

• Incompetent LES

• GERD

• Barrett’s Esophagus

• Diffuse Esophageal Spasms

3 types of gastric motility:

Mixing

Peristalsis

Migrating Myoelectric Complex (MMC)

• Sweep undigested stuff out during fasting

Orad vs. Caudad Regions of Stomach

Orad = Fundus and proximal body (receives/stores food)

Caudad = Distal body and antrum (mixes/propels)

Receptive Relaxation

Allows for gastric accommodation.

Initiated by swallowing, causes stomach to relax.

Modulated by vagus n.

Gastric Accommodation

LES and stomach relax.

Ensures changes in volume don’t result in increased gastric pressure.

Modulated by vagus n.

A way to temporarily store ingested food before controlled release (gastric emptying) into the intestine occurs.

Antral Pump: Gastric Mixing

• Propulsion

• Grinding

• Retropulsion

• Trituration: reducing particle size right after a meal

Emptying

THIS is delayed until solids are mechanically broken down!

Slowed by byproducts of digestion and hypertonic chyme.

Rate of gastric emptying contents:

Liquids > Carbs > Proteins > Fats

Isotonic > Hyper/Hypotonic

Duodenal Contents

Delays gastric emptying.

Enterogastric Reflex

• Stimulated by:

  • Duodenum pH 3-4

  • Gastric pH 1.5

• Gastric motility inhibited

Pyloric Sphincter Hormonal & Neural Regulators

• Increased constriction:

  • CCK, GIP, Secretion, Gastric

  • Sympathetic

• Relaxation:

  • Parasympathetic

Fasting State

The only time MMC occurs!

Removes remaining ingested (but not digested) contents!

Vomiting (Emesis) (stimuli and steps?)

Stimuli:

• Gastric/duodenal distention

• Dizziness

• Drugs

• GU injury

Steps:

1. Reverse peristalsis (SI → pylorus)

2. Pyloric sphincter relaxes

3. Abdominal muscle contraction

4. Pylorus/Antrum Contract

5. LES relaxes

6. Gastric contents move up esophagus

7. UES relaxes

8. Glottis closes

9. Increased saliva

10. Expulsion

Segmentation

Postprandial period (alternating contractions of smooth m.)

Slow propulsion/retropulsion

Peristalsis

Coordinated propulsions of circular smooth m.

Contraction of longitudinal smooth m.

Intestinointestinal Reflex

Distention in one segment, relaxation in the rest of the small intestine

MMC (migrating myoelectric complex)

• Ileocecal sphincter is relaxed

• Correlated with high levels of motilin (SI hormone)

• Prevents backflow of bacteria from colon → ileum

Gastroileal Reflex

Increased gastric activity → increased ileal motility and relaxation of sphincter

Controls rate of chyme entering the colon so colon can effectively absorb everything!

Works with gastrocolic reflex to stimulate urge to defecate.

3 types of colonic motlity:

• Haustrations

• Long-duration contractions

• Mass movements (propulsion)

Gastrocolic Reflex

• Stomach distention results in generalized increase in colonic motility

• Can lead to ‘urge’ to defecate

• Stimulates mass movements

Regulators of colonic motility:

• Parasymp:

  • increased motility via excitatory EMN

  • Excessive = diarrhea

• Symp:

  • Inhibits motility via inhibitory EMN

  • Excessive = constipation

Internal Anal Sphincter

Smooth muscle, involuntary, most tone

External Anal Sphincter

Striated muscle, involuntary and voluntary!

Rectosphincteric Reflex

• For defecation

• Relaxes IAS (urge):

  • VIP and NO

Defecation Steps

• Voluntary relaxation of EAS

• Contract abdominal muscles

• Relax pelvic muscles

Hirschsprung’s Disease

• Congenital megacolon

• Congenital failure of ENS development

  • No PNS ganglion

• Aganglionic segment stays contracted

• Rectosphincteric reflex is non-functional

  • Colon proximal to obstruction is inflated!

Pacemaker Zone

Sets the rate of gastric peristalsis (body of the stomach)!

ACh and Gastrin

• Increases amplitude

• Increases contractility

NE

• Decreases contractility

Variables influencing gastric emptying

• Volume of chyme

• Degree of fluidity

• Presence of fat, acid, hypertonicity, or distension

• Emotion

• Intense pain