Normal Bowel Sensation
Healthy gut nerves stay quiet during ordinary digestion.
- 70–90%: Distension pain threshold (before conscious awareness)
- ~100M: Visceral afferent neurons (gut sensory nerve fibers)
- ~0%: Healthy pain reports (during normal peristalsis)
- constant: Gut-brain signals (mostly stay subconscious)
Silent gut-brain signaling
Most digestive signals never reach conscious perception.
Mechanoreceptor function
Stretch receptors fire in proportion to bowel wall tension.
High pain threshold
Distension must be extreme before nerves signal pain.
Healthy volunteers rarely notice normal gas or peristalsis.
Sensitization Trigger
Infection, inflammation, or stress prime gut nerves to overreact.
- ~10%: Post-infectious IBS risk (after acute gastroenteritis)
- increased: Mast cell activation (near nerve endings)
- released: Inflammatory mediators (histamine, serotonin, cytokines)
- elevated: Stress-axis activation (cortisol and CRF signaling)
Post-infectious priming
A gut infection can sensitize nerves for months.
Inflammatory mediators
Mast cells release chemicals that excite nearby nerves.
Stress amplifies signaling
Chronic stress lowers the gut's tolerance to stretch.
Roughly one in ten gastroenteritis cases triggers lasting IBS.
Lowered Pain Threshold
Sensitized nerves now fire at normal, non-harmful distension.
- ~30–40%: New pain threshold (down from ~80% baseline)
- 2–3×: Nerve excitability (higher firing rate)
- increased: TRPV1 channel density (heat / stretch receptor)
- earlier: Symptom onset (with less bowel filling)
Peripheral sensitization
Nerve endings become abnormally excitable after injury.
Ion channel changes
More TRPV1 and ASIC channels lower firing thresholds.
Everyday triggers hurt
Ordinary gas or meals now provoke pain signals.
A 20% threshold drop can make routine digestion painful.
Central Amplification
Spinal cord and brain circuits boost the already-sensitized signal.
- up to 3×: Spinal signal gain (dorsal horn wind-up)
- impaired: Descending inhibition (less natural pain relief)
- ACC, insula: Brain regions activated (seen on fMRI scans)
- beyond stimulus: Pain persistence (wind-up phenomenon)
Spinal wind-up
Repeated signals make spinal neurons amplify further.
Weakened descending control
The brain's natural pain brakes work less effectively.
Cortical amplification
Pain-processing brain regions show heightened, prolonged activity.
fMRI shows IBS patients recruit more brain area per stimulus.
Visceral Hyperalgesia
Normal gas and distension now register as significant pain.
- ~10%: IBS prevalence (of the global population)
- up to 60%: Threshold drop vs healthy (on balloon distension testing)
- meals, stress, gas: Common symptom triggers (everyday digestive events)
- significant: Quality-of-life impact (chronic, unpredictable pain)
A vicious cycle
Pain itself raises stress, further sensitizing the gut.
Diagnostic marker
Lowered rectal balloon thresholds help define IBS in labs.
Treatment targets
Therapies aim to raise thresholds and calm nerve signaling.
Balloon distension testing remains the clinical gold-standard measure.