Digestion In Stomach – Motility & Gastric Emptying

DIGESTION IN STOMACH (MOTILITY & GASTRIC EMPTYING)

Divisions of stomach:

1. Anatomically divided,

• Fundus
• Body (corpus)
• Antrum 
• Pylorus. 

2. Functionally,

• Proximal stomach – Fundus & proximal l/3rd of body
• Distal end – Distal 2/3rd of the body & antrum. 

• Vasovagal reflex triggered by swallowing food bolus.
• Results in opening of lower esophageal sphincter.
• Dilates proximal stomach (receptive relaxation) to accommodate swallowed food.
• Proximal stomach serves primarily storage function (receptive area).

• Displays pacesetter electric potentials & peristaltic motor activity.
• Associated with rised intragastric pressure.

Pacemaker location – 

Along greater curvature of an upper part of distal stomach (middle third of body)

• Pacemaker generates BER in stomach 
• Duration 3-4 /minute
• BER depolarization crosses excitation threshold, action potential develops & gastric contraction occurs.

Gastric contraction:

2 phases – 

• Digestive phase
• Interdigestive phase

1. Digestive phase:

• Food is ingested
• 2 contraction wave.

• Pressure waves 10 cm/H2O mixing stomach contents.

• Increasing strength & intensity of contraction with time.
• Pressure waves – About 60 cm/H2O.
• Propelling contents towards pylorus. 
• Pyloric sphincter allows only small fraction of propelled contents to enter duodenum. 
• Larger fraction squirted back into stomach, resulting in agitation & mixing. 
• Stronger waves achieve both propulsion & mixing.

Contractions observed: 

2 types, 

• Migratory Motor Complexes.
• Hunger Contractions.

Migratory Motor Complexes (MMC/Inter-digestive myoelectric complexes)

• Gastric antrum shows bursts of propulsive (peristalitic) muscular activity every 90 minutes.
• Initiated by motilin.
• Purpose – To clear stomach & small intestine of luminal contents in preparation for next meal.
• Activity conducted along entire small intestine length, from stomach to distal ileum at rate of about 5 cm/min.
• On reaching terminal ileum, new wave begins in stomach.

Hunger contractions:

• Associated with hunger.
• Pressure level reaches as high as 100 cm/H2O.
• Both MMCs & hunger contractions are abolished immediately by food ingestion.

Gastric Emptying:

• Rate of stomach emptying its contents into duodenum.

Factors affecting rate:

1. Increasing factors:

Rate of gastric emptying depends on, 

• Quantity

Type of food ingested

• Carbohydrate-rich foods leave stomach in a few hours.
• Protein-rich food leaves more slowly.
• Emptying is slowest after fat-rich meal.

Distension of stomach

• Triggers long (vagally mediated) & short (intrinsic neural plexus mediated) reflexes
• Leads to strong peristaltic waves
• Increased gastric emptying.

• Increases rate of gastric emptying.

2. Decreasing factors:

• Duodenal signals – More important regulatory control.
• Distension of duodenum
• Duodenal hyperosmolarity (Hyperosmolar chyme)
• Presence of acid, fats, & end products of fat digestion 

Mechanism of decreasing effect:

1. Hormonal mechanism:

• By secretin, cholecystokinin, VIP, & GIP.
• Inhibit both gastric emptying & gastric acid secretion.

2. Enterogastric reflex:

• Inhibit gastric emptying through intrinsic enteric neural plexus.