GI Motility

1. Chewing, Swallowing, and Esophageal Peristalsis

Chewing (mastication) grinds food, mixes it with saliva, and reduces particle size. Swallowing (deglutition) has three phases: the oral (voluntary) phase pushes the bolus backward; the pharyngeal phase (involuntary, coordinated by the medullary swallowing center) closes the airway, briefly stops breathing, and opens the upper esophageal sphincter; the esophageal phase carries the bolus to the stomach. Primary peristalsis is the swallow-initiated wave; secondary peristalsis is a distension-triggered wave that clears residue. The lower esophageal sphincter (LES) relaxes to admit the bolus, then closes to prevent reflux.

2. Gastric Mixing and Emptying

The proximal stomach relaxes to store food (receptive relaxation, vagally mediated). In the distal stomach, slow-wave contractions mix food with gastric juice; because the pylorus stays nearly closed, most contents are forced back (retropulsion) for further churning, and only small particles pass into the duodenum. Gastric emptying is regulated so the intestine receives a manageable load: duodenal receptors sensing acid, fat, hypertonicity, and distension reflexively slow emptying via enterogastric reflexes and hormones (CCK, secretin, GIP). Liquids empty faster than solids; fat and acid empty most slowly.

3. Small-Intestine Motility

Segmentation consists of localized ring-like contractions that chop and mix chyme with digestive juices and press nutrients against the mucosa — ideal for absorption, with little net propulsion. Peristalsis is a coordinated wave (contraction behind, relaxation ahead) that propels chyme slowly aborally. Between meals, a migrating motor complex (MMC) — an intense peristaltic wave from the stomach sweeping the whole small intestine every 90–120 minutes, driven by motilin — clears residual food and bacteria into the colon.

4. Large-Intestine Motility

The colon mixes and stores rather than rapidly propels. Haustrations are slow segmenting contractions that form the pouch-like "haustra" and move contents back and forth, maximizing water and electrolyte absorption. Mass movements are strong peristaltic waves, a few times a day (often after meals, via the gastrocolic reflex), that push contents long distances toward the rectum. The defecation reflex is triggered by rectal distension: the intrinsic reflex relaxes the internal anal sphincter, and voluntary relaxation of the external anal sphincter plus increased intra-abdominal pressure (Valsalva) completes evacuation.

How it works

1. Chewing reduces food size and mixes it with saliva.
2. Swallowing (oral → pharyngeal → esophageal) moves the bolus past the airway by primary peristalsis.
3. The stomach stores, mixes (retropulsion), and slowly empties; duodenal feedback slows emptying.
4. The small intestine mixes via segmentation and propels via peristalsis; the MMC cleans it between meals.
5. The colon mixes via haustrations and periodically propels via mass movements.
6. Rectal distension triggers the defecation reflex; voluntary external-sphincter relaxation completes evacuation.

Common confusions

Memory aids

"Chew-Swallow-Churn-Sweep-Haul-Squat." Chewing breaks it down, Swallowing protects the airway, the stomach Churns and slowly empties, the small intestine Sweeps (MMC) between meals, mass movements Haul contents through the colon, and you Squat to complete defecation.

Quick review

Topic Recap

Motility proceeds region by region: chewing and swallowing deliver food to the stomach; the stomach stores, mixes (retropulsion), and empties chyme under duodenal feedback; the small intestine mixes via segmentation and propels via peristalsis, cleaned between meals by the motilin-driven MMC; the colon mixes via haustrations and propels via mass movements, culminating in the defecation reflex that separates involuntary readiness (internal sphincter) from voluntary timing (external sphincter).

Knowledge Check

1. Which phase of swallowing is involuntary, and why is it important?
2. What is the strongest brake on gastric emptying, and through which mediators?
3. What is the functional difference between segmentation and peristalsis?
4. When does the migrating motor complex occur, and what triggers it?
5. Which anal sphincter is under voluntary control, and what type of muscle is it?

Answers and Rationales

1. The pharyngeal phase; it is reflex-coordinated by the medullary swallowing center to close the airway and prevent aspiration.
2. Duodenal fat (and acid), acting via CCK and secretin plus enterogastric reflexes (GIP also slows emptying).
3. Segmentation mixes chyme and presses it against the mucosa with no net forward movement; peristalsis produces net aboral propulsion.
4. During the interdigestive (between-meal) period, triggered by the hormone motilin.
5. The external anal sphincter, which is skeletal (striated) muscle; the internal sphincter is involuntary smooth muscle.