GIT Motility & Defaecation

Gastrointestinal motility is the coordinated activity of smooth muscle, the enteric nervous system (ENS) and pacemaker cells that propels luminal contents from mouth to anus while mixing them for digestion. This topic links pure physiology (slow waves, peristalsis, the migrating motor complex) directly to high-yield clinical disorders such as achalasia and Hirschsprung disease.

Electrical basis of GI smooth muscle

GI smooth muscle behaves as a functional syncytium because cells are joined by gap junctions, allowing electrical activity to spread cell-to-cell. Two basic waves of electrical activity exist:

• Slow waves — rhythmic undulating changes in resting membrane potential. They are not action potentials and do not by themselves cause contraction; they set the frequency (rhythm) of contraction.
• Spike potentials (action potentials) — true action potentials fired when slow waves cross threshold (about −40 mV). These trigger Ca²⁺ entry and actual contraction. Their amplitude/frequency determines force.

The pacemaker for slow waves is the Interstitial Cells of Cajal (ICC) — the GI pacemaker that sits between longitudinal and circular muscle layers.

High-yield: Interstitial Cells of Cajal generate the basic electrical rhythm (slow waves). Loss of ICC is implicated in gastroparesis and several dysmotility states.

Slow-wave frequency is region-specific and a favourite MCQ:

Region	Slow-wave frequency (per minute)
Stomach (body/antrum)	~3
Duodenum	~12
Terminal ileum	~8–9
Colon	~3–8 (variable)

High-yield: Gastric slow-wave frequency is 3/min; duodenum is the fastest at ~12/min. The intestine shows a proximal-to-distal frequency gradient (decreasing aborally), which favours net aboral movement.

The depolarising spread of GI smooth muscle is mediated mainly by Ca²⁺ influx through L-type calcium channels (unlike skeletal muscle which uses Na⁺). Contraction couples through Ca²⁺–calmodulin → myosin light chain kinase.

The enteric nervous system (the "little brain")

The ENS can function autonomously and contains as many neurons as the spinal cord. Two plexuses:

Plexus	Other name	Location	Chief function
Myenteric	Auerbach's	Between longitudinal & circular muscle	Controls motility (tone, intensity, velocity of peristalsis)
Submucosal	Meissner's	Submucosa	Controls secretion, local blood flow, absorption

Excitatory transmitters → contraction: acetylcholine and substance P. Inhibitory transmitters → relaxation: nitric oxide (NO) and vasoactive intestinal peptide (VIP) — the key mediators of receptive relaxation and sphincter opening.

High-yield: NO and VIP are the principal inhibitory (relaxing) neurotransmitters of the ENS. Their absence is central to achalasia and Hirschsprung disease.

Autonomic modulation: parasympathetic (vagus, pelvic nerves) generally increases motility; sympathetic generally inhibits it (and contracts sphincters).

Basic patterns of motility

1. Propulsive (peristalsis): A contractile ring moves aborally. The stimulus is distension; the response is the "law of the gut" (myenteric/peristaltic reflex) → contraction behind the bolus (ACh, substance P) and relaxation ahead of it (NO, VIP). Net movement is always oral → aboral.
2. Mixing (segmentation): Local constrictions chop and mix chyme without net forward movement — dominant in the small intestine.

Oesophageal motility

Swallowing has three phases — oral (voluntary) → pharyngeal → oesophageal.

• Primary peristalsis: A continuation of the pharyngeal swallowing wave; travels down the oesophagus, coordinated by the swallowing centre in the medulla via the vagus. Takes ~8–10 seconds to reach the stomach.
• Secondary peristalsis: Triggered by distension of the oesophagus by retained food/refluxed material; mediated by both the intrinsic myenteric plexus and vagal reflexes. It occurs independently of swallowing and clears residual bolus.

Sphincters:

• Upper oesophageal sphincter (UES) — striated muscle (cricopharyngeus), under voluntary/reflex control.
• Lower oesophageal sphincter (LES) — smooth muscle, tonically contracted at rest. Resting tone is maintained largely intrinsically by the ENS (myenteric plexus), assisted by the crural diaphragm. With each swallow, receptive relaxation of the LES (mediated by NO/VIP, vagally driven) allows the bolus through.

High-yield: LES resting tone is maintained mainly by the enteric nervous system; LES relaxation during swallowing is mediated by NO and VIP. This single fact explains achalasia.

Gastric motility & emptying

Functions: storage, mixing (forming chyme), and regulated emptying.

• Receptive relaxation: When food enters, the fundus/body relax (vagovagal reflex, NO/VIP-mediated) to accommodate volume with little rise in pressure.
• Mixing & retropulsion: Antral peristaltic waves (3/min) push chyme toward the pylorus; most is squirted back (retropulsion) for grinding. Only small, liquefied particles (<1–2 mm) pass.

Determinants of gastric emptying:

Promote emptying → increased gastric volume/distension, gastrin (weakly), liquids empty faster than solids.

Delay emptying (the enterogastric reflex + duodenal hormones):

Duodenal stimulus	Mediator / hormone	Effect
Fat, fatty acids	CCK (most potent)	Strongly delays emptying
Acid (low pH)	Secretin, neural reflex	Delays emptying
Hyperosmolar chyme	Osmoreceptor reflex	Delays emptying
Distension of duodenum	Enterogastric (vagal/enteric) reflex	Delays emptying

High-yield: CCK is the most important hormonal inhibitor of gastric emptying — slows emptying so the duodenum is not overwhelmed by fat. Liquids empty faster than solids; carbohydrates fastest, fats slowest.

Order of emptying by content: carbohydrate > protein > fat (carbs quickest, fat slowest).

The Migrating Motor Complex (MMC)

Between meals (the interdigestive or fasting state), the gut shows the MMC — the "intestinal housekeeper."

• Recurs every 90–120 minutes during fasting.
• Sweeps undigested residue, debris and bacteria from stomach → terminal ileum, limiting bacterial overgrowth.
• The key hormonal trigger is MOTILIN (from duodenal/jejunal M cells).
• Abolished by feeding (eating switches off the MMC).

High-yield: Motilin initiates the MMC. The macrolide erythromycin is a motilin-receptor agonist → used as a prokinetic (e.g., in diabetic gastroparesis, to clear stomach before endoscopy).

MMC phases: Phase I (quiescence) → Phase II (irregular contractions) → Phase III (intense regular contractions — the actual sweeping wave) → back to Phase I.

Small-intestinal motility

• Segmentation contractions dominate — concentric, ring-like, spaced constrictions that mix chyme with digestive juices and maximise mucosal contact for absorption. They produce little net propulsion; the slight aboral drift comes from the frequency gradient.
• Peristaltic waves are weak and short, moving chyme slowly (≈1 cm/min); transit through the small bowel takes 3–5 hours.
• Gastroenteric reflex (stomach distension → increased small-bowel motility) and the gastroileal reflex (gastric activity → opening of ileocaecal valve, emptying ileum into caecum).

The ileocaecal valve/sphincter resists backflow from caecum to ileum; caecal distension and irritation (e.g., appendicitis) reflexly contract it and inhibit ileal emptying.

Colonic motility & the defaecation reflex

Functions: absorption of water/electrolytes and storage of faeces. Movements:

• Haustral contractions / haustral shuttling: Segmentation-like, slow, churning movements forming haustra — maximise water absorption; little propulsion.
• Mass movements: 1–3 times/day, a modified intense peristalsis moving contents over long segments toward the rectum; often triggered after meals via the gastrocolic reflex (largest after breakfast — explains post-prandial urge).

High-yield: The gastrocolic reflex (gastric distension → colonic mass movement) drives the post-meal urge to defaecate; it is partly mediated by gastrin and CCK plus neural pathways.

Defaecation reflex — stepwise

Distension of the rectum (when it fills to ~25% or rises above the threshold) initiates defaecation:

Rectal filling/distension → stretch receptors fire → reflex contraction of rectum + reflex relaxation of the internal anal sphincter (IAS) → "call to stool"

1. Internal anal sphincter (IAS): smooth muscle, involuntary, relaxes via the rectosphincteric (recto-anal inhibitory) reflex — an intrinsic myenteric reflex carried by the ENS, augmented by the parasympathetic (pelvic) spinal/parasympathetic defaecation reflex (S2–S4).
2. External anal sphincter (EAS): striated muscle, voluntary (pudendal nerve, S2–S4) — keeps continence; can override the urge until socially appropriate.
3. When voluntarily relaxed → with help of the Valsalva manoeuvre (closed glottis, abdominal wall + diaphragm contraction raising intra-abdominal pressure) and relaxation of puborectalis (straightening the anorectal angle), evacuation occurs.

High-yield: The recto-anal inhibitory reflex (RAIR) — rectal distension relaxing the IAS — is ABSENT in Hirschsprung disease (anorectal manometry). This is a classic diagnostic point.

The intrinsic (myenteric) reflex alone is weak; it is greatly reinforced by the parasympathetic (pelvic nerve, S2–S4) reflex. Spinal-cord injury above the sacral cord can leave this sacral reflex intact (reflex/automatic bowel), whereas sacral cord/cauda equina lesions abolish it.

Clinically integrated dysmotility

Achalasia cardia

• Defect: Degeneration/loss of inhibitory (NO/VIP) myenteric neurons in the distal oesophagus and LES.
• Physiology consequence: (1) Failure of LES relaxation on swallowing + raised resting LES pressure, and (2) aperistalsis of the oesophageal body.
• Features: Dysphagia to both solids and liquids from the outset (vs mechanical obstruction → solids first), regurgitation, chest pain, weight loss.
• Investigation of choice: Oesophageal manometry (gold standard) — shows incomplete LES relaxation + absent peristalsis; integrated relaxation pressure (IRP) raised.
• Barium swallow: "bird-beak" tapering with proximal dilatation.
• Management: Pneumatic dilatation, Heller's myotomy / POEM; pharmacological (CCBs, nitrates) for poor surgical candidates; botulinum toxin (blocks ACh).

Hirschsprung disease (congenital aganglionic megacolon)

• Defect: Failure of neural-crest cell migration → absence of ganglion cells (aganglionosis) in both myenteric (Auerbach) and submucosal (Meissner) plexuses, starting at the rectum and extending proximally for a variable length.
• Physiology: Lack of inhibitory enteric neurons → the aganglionic segment is tonically contracted/spastic → functional obstruction; proximal normal bowel dilates (megacolon).
• Features: Neonatal failure to pass meconium within 48 hours, abdominal distension, bilious vomiting; later chronic constipation.
• Diagnosis: Rectal biopsy = gold standard (absent ganglia, hypertrophied nerve trunks, raised acetylcholinesterase). Anorectal manometry → absent RAIR. Contrast enema → transition zone.
• Management: Surgical resection of aganglionic segment (Swenson/Soave/Duhamel pull-through).

Feature	Achalasia	Hirschsprung
Site	LES / oesophageal body	Rectum/colon (aganglionic)
Lost cells	Inhibitory myenteric neurons	All ganglion cells (both plexuses)
Lost mediator effect	NO/VIP relaxation of LES	NO/VIP relaxation of IAS/colon
Key test	Oesophageal manometry	Rectal biopsy; RAIR absent
Classic sign	Bird-beak barium swallow	No meconium <48 h; transition zone

High-yield: Both diseases share the same physiological lesion — loss of inhibitory NO/VIP enteric neurons → a segment that cannot relax → functional obstruction. This unifying concept is heavily tested.

Mnemonics & quick memory aids

• "3-12-8-3" — slow-wave frequency: stomach 3, duodenum 12, ileum ~8, colon ~3. Duodenum fastest.
• ENS inhibitory pair = "NO VIP" — NO and VIP relax sphincters.
• MMC = "Motilin Makes it Move" (when fasting); Meals stop the MMC.
• Auerbach = "A" for Action (motility); Meissner = "M" for Mucosal secretion.
• Gastric emptying delayers — "Fat Acid Osmolar Distension" all slow it; CCK is king.

Recently asked / exam angle

• Pacemaker of GI tract → Interstitial Cells of Cajal (slow waves).
• Slow-wave frequency of stomach → 3/min; fastest segment → duodenum (~12/min).
• Hormone initiating the MMC → Motilin; drug acting on motilin receptors → erythromycin (prokinetic).
• Inhibitory neurotransmitters of ENS → NO and VIP.
• Most potent inhibitor of gastric emptying → CCK (fat-induced).
• Reflex absent in Hirschsprung disease → recto-anal inhibitory reflex (RAIR) on anorectal manometry.
• Plexus controlling gut motility → myenteric (Auerbach's); secretion → submucosal (Meissner's).
• Investigation of choice for achalasia → oesophageal manometry.
• Type of secondary oesophageal peristalsis stimulus → distension; mediated by intrinsic + vagal reflexes.
• Nerve roots of defaecation parasympathetic reflex / EAS → S2–S4 (pelvic & pudendal nerves).
• Dysphagia to both solids and liquids from onset → motility disorder (achalasia), not mechanical obstruction.

Rapid revision

1. ICC = GI pacemaker → generate slow waves; spikes (Ca²⁺ via L-type channels) cause actual contraction.
2. Slow-wave rate: stomach 3, duodenum 12 (fastest), ileum 8, colon 3 — proximal→distal gradient drives aboral flow.
3. Auerbach (myenteric) = motility; Meissner (submucosal) = secretion.
4. ENS excitatory = ACh + substance P; inhibitory = NO + VIP.
5. Law of the gut: distension → contraction behind, relaxation ahead → net aboral movement.
6. Primary oesophageal peristalsis = swallow-initiated (vagal); secondary = distension-triggered, intrinsic + vagal, swallow-independent.
7. LES resting tone maintained by ENS; relaxes on swallowing via NO/VIP.
8. Gastric emptying: liquids > solids; carbs > protein > fat; CCK is the chief delayer (enterogastric reflex).
9. MMC every 90–120 min in fasting, swept by motilin; erythromycin = motilin agonist prokinetic; feeding abolishes it.
10. Small bowel = segmentation (mixing); colon = haustral shuttling + mass movements; gastrocolic reflex gives post-meal urge.
11. Defaecation: rectal distension → IAS relaxes (RAIR, involuntary); EAS voluntary (pudendal, S2–S4); Valsalva aids evacuation.
12. Achalasia (lost NO/VIP in LES → failed relaxation; manometry, bird-beak) and Hirschsprung (aganglionosis, RAIR absent, rectal biopsy) both = loss of inhibitory enteric neurons.