Motility & Regulation
Peristalsis & Hormones
Explore how smooth muscle contractions and hormonal signals coordinate the movement of food through the digestive tract.
Anatomy & Physiology
What Is GI Motility?
Gastrointestinal (GI) motility refers to the coordinated contractions of smooth muscle in the walls of the digestive tract that propel, mix, and break down food as it travels from the esophagus to the rectum.
Essential Vocabulary
Peristalsis
Wave-like contractions that push food forward through the GI tract
Segmentation
Rhythmic contractions that mix food with digestive juices without forward movement
Bolus
A soft, rounded mass of chewed food that travels from the mouth through the esophagus
Why Regulation Matters
The GI tract must coordinate mechanical and chemical digestion with nutrient absorption. This requires a dual control system:
Neural Control
The enteric nervous system (ENS): the "second brain" with over 100 million neurons embedded in the gut wall
Hormonal Control
GI hormones released into the bloodstream that regulate secretion, motility, and appetite
The Peristalsis Mechanism
Peristalsis vs Segmentation
| Peristalsis | Segmentation | |
|---|---|---|
| Purpose | Propel food forward | Mix food with enzymes |
| Location | Entire GI tract | Mainly small intestine |
| Pattern | Sequential wave | Alternating contractions |
| Net movement | Yes (aboral) | Minimal |
Muscle Layers of the GI Wall
The Enteric Nervous System
Often called the "second brain", the ENS operates largely independently of the CNS. It contains two major nerve networks:
1
Myenteric (Auerbach's) plexus: Located between the circular and longitudinal muscle layers. Controls motility: the speed and strength of contractions.
2
Submucosal (Meissner's) plexus: Located in the submucosa. Controls secretion and blood flow to the mucosa.
Key GI Hormones
Gastrin
Released by G-cells in the stomach. Stimulates HCl secretion and gastric motility.
Secretin
Released by S-cells in the duodenum. Stimulates bicarbonate secretion from the pancreas. Inhibits gastric motility.
CCK
Released by I-cells in the duodenum. Stimulates gallbladder contraction, pancreatic enzyme release. Inhibits gastric emptying.
Additional Hormonal Regulators
GIP
Gastric inhibitory peptide: Released by K-cells. Inhibits gastric motility and acid. Stimulates insulin release.
Motilin
Released by M-cells. Stimulates the migrating motor complex (MMC): the "housekeeper" contractions between meals.
GLP-1
Glucagon-like peptide-1: Released by L-cells. Slows gastric emptying, enhances insulin, promotes satiety.
How Hormonal Signaling Works
How to Trace a Motility Event
1
Identify the stimulus: What triggers the event? (Food entering, distension, pH change.)
2
Determine the receptor: Which cells detect the stimulus? (Mechanoreceptors, chemoreceptors.)
3
Trace the signal: Is it neural (ENS, vagus nerve) or hormonal (gastrin, CCK, secretin)?
4
Identify the effector: Which muscle layer or gland responds? (Circular muscle, parietal cells.)
5
State the outcome: Increased motility? Secretion? Sphincter relaxation?
Example: Swallowing the Bolus
Example: Gastric Motility
Example: Duodenal Hormone Cascade
When acidic, fatty chyme enters the duodenum, a coordinated hormonal response unfolds:
1
Low pH detected: S-cells release secretin into the blood.
2
Fats and proteins detected: I-cells release CCK into the blood.
3
Secretin acts: Pancreas releases bicarbonate to neutralize acid. Gastric motility decreases.
4
CCK acts: Gallbladder contracts (bile release). Pancreas releases enzymes. Gastric emptying slows.
Common Mistakes
✕ Confusing Peristalsis with Segmentation
Peristalsis propels food forward. Segmentation mixes food in place without net forward movement. Both are smooth muscle contractions, but their patterns differ.
✕ Mixing Up Secretin and CCK
Secretin is triggered by acid (low pH) and targets bicarbonate release. CCK is triggered by fats/proteins and targets enzyme and bile release.
✕ Thinking the GI Tract Needs the Brain
The enteric nervous system can operate independently. While the CNS modulates GI function, peristalsis and local reflexes work without brain input.
Summary: Hormones at a Glance
| Hormone | Source | Stimulus | Key Actions |
|---|---|---|---|
| Gastrin | G-cells (stomach) | Peptides, distension | HCl secretion, motility |
| Secretin | S-cells (duodenum) | Acidic chyme | Bicarbonate, slows stomach |
| CCK | I-cells (duodenum) | Fats, proteins | Bile, enzymes, slows stomach |
| GIP | K-cells (duodenum) | Glucose, fats | Insulin, inhibits acid |
| Motilin | M-cells (duodenum) | Fasting state | MMC "housekeeper" waves |
Question 1 of 5
During peristalsis, which muscle layer contracts behind the bolus to push it forward?
The circular muscle layer contracts behind the bolus, narrowing the lumen and pushing the bolus forward. Ahead of the bolus, the circular muscle relaxes while the longitudinal muscle contracts.
Question 2 of 5
Which hormone is released in response to acidic chyme entering the duodenum and stimulates bicarbonate release from the pancreas?
Secretin is released by S-cells in the duodenum when pH drops. Its primary action is stimulating the pancreas to release bicarbonate, which neutralizes the acid. CCK responds to fats and proteins instead.
Question 3 of 5
The myenteric (Auerbach's) plexus is located between which two layers and primarily controls what function?
The myenteric plexus lies between the circular and longitudinal muscle layers. It primarily regulates motility: the speed and strength of GI contractions. The submucosal plexus handles secretion.
Question 4 of 5
Which type of GI movement is characterized by alternating contractions that mix food with digestive juices without significant forward movement?
Segmentation consists of alternating contractions that mix chyme with digestive juices, primarily in the small intestine. Unlike peristalsis, it produces little net forward movement.
Question 5 of 5
Motilin is unique among GI hormones because it is released during which state?
Motilin is released during the fasting state and stimulates the migrating motor complex (MMC): cyclical waves that sweep undigested material and bacteria from the stomach and small intestine between meals.