#Gastrointestinal motility

#Describe gastric emptying

Gastric emptying is the coordinated delivery of gastric contents (chyme) into the duodenum.

#Gastric motility when fasted

Characterised by migrating motor complexes (MMCs) which generate slow, intermittent peristaltic waves that propagate from the fundus to the pylorus.

Functions to clear residual gastric contents between meals.

#Gastric emptying after feeding

1. Storage

• Food ingestion triggers receptive relaxation of the proximal stomach.
• Allows accommodation of a meal with minimal rise in intragastric pressure.

2. Mixing

• Gastric electrical activity originates in the fundus via spontaneous membrane depolarisation.
• These generate slow waves known as basal electrical rhythm, pushing food forwards (propulsion).
• When the waves reach the antrum, it contracts against a relatively closed pylorus causes retropulsion.
• Repeated cycles mechanically break down food particles (trituration).

3. Emptying

• The pyloric sphincter intermittently relaxes to allow particles < 2mm to pass into the duodenum.
• Powerful peristaltic contractions of the distal stomach drives chyme forward, known as the pyloric pump.
• Liquids pass easily:
  • The emptying rate is exponentia and proportionate to antral-duodenal pressure gradient. The half-time (t_1/2) for liquids leaving the stomach is approximately 20min.
• Solids pass more slowly:
  • Food particles must be broken down.
  • The emptying rate is initially zero, then linear (t_1/2 ≈ 2h).

#Describe the physiological factors that influence gastric emptying

#Myenteric reflex

In the stomach, gastric distension activates stimulatory myenteric reflexes which amplify the pyloric pump and relax the pylorus.

The duodenum exerts powerful inhibitory myenteric reflexes that slow gastric emptying when it receives chyme from the stomach. Specifically, the duodenum responds to certain properties of the chyme:

• Volume / distension: allows time for processing
• Acidity: protects duodenal mucosa
• High or low osmolality: prevents rapid shifts in serum osmolality associated with absorption of food contents
• Protein and fat content: allows additional time for pancreatic enzymes to work

#Neurohormonal control

#Stimulators

Gastrin

• Secreted by gastric G cells
• Stimulated by vagal (ACh) input, gastric distension, and protein content
• Enhances the pyloric pump

Motilin

• Promotes gastric motility

Vagal stimulation

• Stimulates the myenteric plexus

#Inhibitors

Cholecystokinin

• Secreted by jejunum in response to a fat or protein load
• Inhibits the pyloric pump
• Increases bile and pancreatic enzyme secretion

Secretin

• Secreted by duodenum in response to acidic chyme
• Inhibits gastric emptying
• Stimulates pancreatic bicarbonate secretion

Gastric inhibitory peptide (GIP)

• Released in response to glucose
• Slows gastric emptying

Sympathetic nervous system

• Inhibits the myenteric plexus

#Explain the physiological factors that prevent gastro-oesophageal reflux

Gastro-oesophageal reflux is prevented by the coordinated action of intrinsic sphincter tone, extrinsic muscular support, and anatomical valve mechanisms at the gastro-oesophageal junction (GOJ).

#Lower Oesophageal Sphincter – Intrinsic Sphincter

The lower oesophageal sphincter (LES or LOS) is tonically constricted, maintaining a pressure approximately 20 mmHg higher than intragastric pressure, thereby preventing retrograde flow. It forms the intrinsic oesophageal sphincter.

Receptive relaxation occurs transiently during swallowing, mediated by vagal inhibitory pathways and nitric oxide.

Factors increasing LES tone:

• Gastrin
• Motilin
• Catecholamines (α₁-adrenergic stimulation)

Factors reducing LES tone:

• Secretin
• Cholecystokinin (CCK)
• Gastric inhibitory peptide (GIP)

#Diaphragmatic Crura – Extrinsic Sphincter

The diaphragmatic crura surround the distal oesophagus, forming the extrinsic oesophageal sphincter.

During inspiration and increases in intra-abdominal pressure, the crura constrict the oesophagus, augmenting LES pressure and providing a dynamic external sphincter effect.

#Gastro-Oesophageal Junction Valve Mechanism

A short segment of oesophagus extends obliquely into the stomach, forming the gastro-oesophageal junction (GOJ). This segment is below the diaphragm and is hence within the abdomen.

This configuration forms a flap-valve mechanism:

• Increased intra-abdominal pressure compresses this distal, abdominal segment of the oesophagus.
• The segment collapses, acting as a valve and preventing reflux.