Digestion In Stomach – Motility & Gastric Emptying

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. 
Proximal stomach – Storage unit:
  • 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).
Distal stomach:
  • 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.
Type 1 wave:
  • Pressure waves 10 cm/H2O mixing stomach contents.
Type 2 wave:
  • 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.
2. Inter-digestive phase:

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.

Gastrin: 

  • 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.
Exam Question
 

DIGESTION IN STOMACH (MOTILITY & GASTRIC EMPTYING 

Proximal stomach – Storage unit:
  • Proximal stomach serves primarily the function of storage (receptive area).
Distal stomach:
  • Pacemaker location – 
  • Along the greater curvature of an upper part of distal stomach (middle third of body) 
  • Generates BER of 3 to 4/min  

Gastric contraction:

Inter-digestive phase:

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

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

Factors affecting gastric emptying rate:

1. Increasing factors:

  • Type of food ingested.
  • 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 → Increasing gastric emptying.
  • Gastrin

2. Decreasing factors:

  • Duodenal distension.
  • Duodenal hyperosmolarity (Hyperosmolar chyme).
  • Presence of acid, fats, & end products of fat digestion.
  • Done by release of several intestinal hormones like secretin, cholecystokinin, VIP, & GIP.
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