Physiology chapters
Gastrointestinal motility
How food is chewed and swallowed, how the oesophagus and stomach move it on, what controls gastric emptying, how the small intestine mixes and propels contents, the migrating motor complex between meals, and the defecation reflex.
- Swallowing has a voluntary oral phase and involuntary pharyngeal and oesophageal phases, coordinated by the swallowing centre in the medulla; the lower oesophageal sphincter relaxes ahead of the food.
- Gastric emptying is speeded by stomach distension and gastrin, and slowed by fat, acid and hypertonic contents in the duodenum (CCK, secretin, enterogastric reflex); liquids leave faster than solids.
- The small intestine mixes (segmentation) and propels (peristalsis); between meals the migrating motor complex (motilin) sweeps it clean; defecation is a rectal reflex controlled by the external sphincter.
On this page
Overview
Food must be moved along the gut at the right speed: slowly enough to be digested and absorbed, but steadily enough not to stagnate.
Movement happens through chewing, swallowing, peristalsis and mixing contractions, controlled by the enteric nervous system, autonomic nerves and hormones (see GI organisation and hormones).
Mastication (chewing)
Chewing is partly voluntary and partly reflex (the chewing reflex: food in the mouth relaxes the jaw muscles, then the stretch makes them contract again).
It breaks food into small pieces, mixes it with saliva (amylase, mucus) and increases the surface area for digestion. Chewing muscles are supplied by the mandibular nerve (V3).
Swallowing (deglutition)
| Phase | Control | What happens |
|---|---|---|
| Oral (voluntary) | Voluntary | The tongue pushes the bolus back into the pharynx |
| Pharyngeal | Involuntary reflex | Soft palate rises (closes the nasopharynx); larynx rises and the epiglottis covers it; vocal cords close; breathing stops briefly; the upper oesophageal sphincter relaxes; pharyngeal muscles push the bolus into the oesophagus. Takes under 1–2 seconds |
| Oesophageal | Involuntary | Peristalsis carries the bolus to the stomach |
The swallowing centre in the medulla and lower pons coordinates the reflex. Sensory signals travel in the trigeminal and glossopharyngeal nerves; motor signals go out through cranial nerves V, IX, X and XII.
Only the oral phase is voluntary. Once food touches the pharyngeal receptors, swallowing becomes an automatic reflex.
Oesophageal motility
- Primary peristalsis: continues the wave started in the pharynx; it takes about 8–10 seconds to reach the stomach.
- Secondary peristalsis: started by food left behind distending the oesophagus (local enteric reflex and vagal reflexes).
The upper third of the oesophagus is skeletal muscle; the lower two thirds smooth muscle.
Lower oesophageal sphincter (LES)
- Normally tonically contracted, preventing reflux of stomach acid.
- Relaxes ahead of the peristaltic wave (inhibitory neurons: VIP and nitric oxide).
- Helped by the right crus of the diaphragm and the angle at which the oesophagus enters the stomach.
Gastric motility
Storage: receptive relaxation
When food enters, the body and fundus relax so the stomach can hold up to about 1–1.5 L without much rise in pressure. This receptive relaxation is a vagal (vagovagal) reflex.
Mixing and the pyloric pump
- Weak mixing waves start in the upper stomach about 3 times a minute (following slow waves).
- Stronger peristaltic waves in the antrum push the food towards the pylorus. Much of it is squirted back (retropulsion), which grinds and mixes it into chyme.
- Only a small amount passes the pylorus with each wave.
Gastric emptying
| Speeds emptying (from the stomach) | Slows emptying (from the duodenum) |
|---|---|
| Increased volume (stretch) of the stomach | Fat in the duodenum (via CCK): the strongest |
| Gastrin | Acid in the duodenum (secretin, enterogastric reflex) |
| Liquid and finely broken-down food | Hypertonic or hypotonic chyme |
| Distension and irritation of the duodenum (enterogastric reflex) | |
| Breakdown products of protein |
Emptying is controlled mainly by feedback from the duodenum, so chyme does not arrive faster than it can be digested. Fat slows it most, through CCK.
Liquids empty much faster than solids, and carbohydrates faster than proteins, with fats slowest.
Small intestinal motility
- Segmentation (mixing) contractions: rings of contraction appear and relax in alternating segments, chopping and mixing the chyme with digestive juices and bringing it into contact with the mucosa. Their maximum frequency is set by slow waves (about 12/min in the duodenum).
- Peristalsis: waves that move chyme slowly towards the colon, about 1 cm per minute; chyme takes about 3–5 hours to pass from pylorus to ileocaecal valve.
- Peristaltic reflex (law of the gut): distension causes contraction above and relaxation below, moving contents onwards. It needs the myenteric plexus.
Ileocaecal valve
Prevents backflow from the colon. Distension of the ileum relaxes it; distension of the caecum contracts it.
The migrating motor complex (MMC)
- Occurs during fasting (between meals), about every 90–120 minutes.
- Strong bursts of contraction that start in the stomach and move along the small intestine to the ileum.
- Sweeps undigested material, mucus and bacteria into the colon (the "intestinal housekeeper").
- Started by the hormone motilin; stopped by eating.
The MMC occurs in the fasting state, is triggered by motilin, and stops when food is eaten. Erythromycin acts on motilin receptors and can speed gastric emptying.
Colonic motility
- Haustrations: mixing contractions of circular muscle and taeniae coli, exposing contents to the mucosa to absorb water and electrolytes.
- Mass movements: strong waves that push faeces into the rectum, a few times a day, often after meals.
- Gastrocolic reflex: food entering the stomach triggers mass movements in the colon (the urge to pass stool after meals, especially in infants).
The defecation reflex
- A mass movement fills the rectum; stretch receptors are stimulated.
- Intrinsic (enteric) reflex: peristalsis in the descending colon, sigmoid and rectum; the internal anal sphincter relaxes.
- Parasympathetic reflex (pelvic nerves, S2–S4): greatly strengthens the contractions and relaxation.
- The external anal sphincter is under voluntary control (pudendal nerve). If the time is convenient, it relaxes and abdominal muscles contract to help push (Valsalva). If not, it stays contracted and the urge passes until the next mass movement.
| Sphincter | Muscle | Control |
|---|---|---|
| Internal anal | Smooth | Involuntary (relaxes reflexly) |
| External anal | Skeletal | Voluntary (pudendal nerve, S2–S4) |
Clinical relevance
Gastro-oesophageal reflux disease (GORD)
How it happens: the LES is weak or relaxes too often, letting acid into the oesophagus; worse with obesity, pregnancy and hiatus hernia.
What you see: heartburn and regurgitation.
Achalasia
How it happens: loss of inhibitory neurons in the lower oesophagus; the LES fails to relax and peristalsis is lost.
What you see: difficulty swallowing solids and liquids; a dilated oesophagus with a "bird's beak" narrowing on barium swallow.
Spinal cord injury
How it happens: damage above the sacral segments removes voluntary control while the rectal reflex survives.
What you see: reflex, uncontrolled bowel emptying; a lesion of the sacral cord (S2–S4) or cauda equina abolishes the reflex.
Gastroparesis
How it happens: damage to the vagus or enteric nerves, often from diabetes.
What you see: delayed gastric emptying, fullness, nausea and vomiting.
Expect: which phase of swallowing is voluntary, events of the pharyngeal phase, the swallowing centre, primary vs secondary peristalsis, LES control, receptive relaxation, factors speeding and slowing gastric emptying (fat and CCK), segmentation vs peristalsis, the MMC and motilin, and the defecation reflex with internal vs external sphincters.
Quick revision
- Swallowing: oral voluntary; pharyngeal and oesophageal reflex; centre in medulla and lower pons.
- LES relaxes ahead of the bolus (VIP, NO).
- Receptive relaxation: vagal reflex.
- Gastric emptying slowed by duodenal fat (CCK), acid, hypertonic chyme; liquids fastest.
- Segmentation mixes; peristalsis propels.
- MMC every 90–120 minutes when fasting; motilin; stopped by food.
- Defecation: rectal stretch; internal sphincter involuntary; external voluntary (pudendal).
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
Which phase of swallowing is voluntary?
Only the oral phase; the pharyngeal and oesophageal phases are reflex.
What slows gastric emptying?
Fat, acid and hypertonic contents in the duodenum, acting through CCK, secretin and the enterogastric reflex.
What is the migrating motor complex?
A cycle of strong contractions that sweeps the stomach and small intestine about every 90–120 minutes during fasting, started by motilin.
Which anal sphincter is voluntary?
The external anal sphincter, supplied by the pudendal nerve.