Pylorus: Functions as a sphincter, controlling the rate of ingested material exiting the stomach.
Contractions
Phasic contractions:
Occur in the distal stomach.
Duration: Seconds.
Role: Propels contents toward the pylorus.
Tonic contractions:
Occur in the proximal stomach.
Duration: Minutes.
Role: Maintains pressure to aid in storing food.
Phases of Gastric Motility
Fasting Motility:
Phase I: Motor quiescence (resting state).
Phase II: Irregular contractility.
Phase III: Strong propulsive contractions occurring every 90 to 120 minutes, lasting 5 to 10 minutes.
Accommodation:
Adjusts stomach to the amount of food ingested.
Postprandial Motility:
Follows food intake to aid in digestion and gastric emptying.
Migrating Motor Complex (MMC)
Majority of MMCs are initiated in the stomach.
Small intestine can also serve as a pacemaker for phase III MMC contractions.
Location of the initiation of MMCs (i.e., stomach vs duodenum and jejunum) affects the periodicity and duration of contractions, as well as the relative proportion of phase I, Il, or Ill contractions that occur.
MMCs originating in the stomach are more often phase Ill and are more likely to propagate distally than those originating in the small bowel.
Ghrelin: Key Features and Functions
Structure and Forms
Ghrelin: A 28-amino-acid peptide with a unique octanoyl group on serine-3, essential for its biologic activity.
Forms:
Active ghrelin: Undergoes acylation at serine-3 and is responsible for most of its hormonal activity.
Desacyl ghrelin: Comprises up to 90% of circulating ghrelin but has little biologic activity.
Synthesis and Secretion
Secreted by X/A-like cells in the mucosal layer of the gastric body, especially in the oxyntic glands.
Functions
Stimulates gastric motility.
Sequence homology with motilin, another gastrointestinal hormone.
Increases appetite and feeding behavior.
Associated with increased adiposity (fat accumulation).