5. Gastric Motility

Gastric Motility

• Gastric motility is the movement of food through the stomach, applying to both simple and ruminant stomachs.
• The stomach performs four main tasks:
  • Receives ingested food
  • Stores food
  • Mixes and grinds food
  • Propels food into the duodenum
• Gastric digestion involves the formation of acid chyme, a mixture of dissolved food, HCl, and pepsin.

Proximal Stomach Motility

• Motility pattern is tonic, not phasic.
• Tonic contractions are dominated by the circular smooth muscle layer.
• The circular smooth muscles relax during meals to allow storage.
• As the stomach empties, the inhibitory neurons turn off, the circular SM proximal starts contracting, forcing the content distally.

Distal Stomach Motility

• Basic motility is peristalsis, driven by a pacemaker in the mid-region of the stomach.
• Functions include propulsion of contents through the antrum and emptying via the pylorus, mixing and grinding of gastric contents, and retropulsion of solid matter.

Gastric Emptying

• Empties faster with liquids compared to solids.
• Low-fat diets empty faster compared to high-fat diets.
• Gastric emptying is controlled by both neural and hormonal mechanisms.

Neural Control

• Mechanoreceptors monitor gastric volume and chemoreceptors detect amino acids, pH, and osmolarity.
• These signals activate enteric nervous afferents, then interneurons activating ENS effectors and visceral afferents to the CNS/vagus parasympathetic, affecting ENS effectors.
• This leads to increased gastric motility and emptying.

Hormonal Control

• Gastrin is stimulated by filling (amino acids, dietary, acid hydrolysis, pepsin).
• Gastrin increases gastric motility and emptying via action on smooth muscle.
• Other hormones such as CCK and GLP-1 regulate gastric motility.

Feedback Control

• Feedback mechanisms, primarily through the duodenum, inhibit gastric emptying.
• Duodenal receptors respond to osmolality, pH, and stretch.
• Reduced gastric motility/emptying is achieved by activation of sympathetic efferents and deactivation of parasympathetic efferents, or hormonal factors.
• Hormones like Cholecystokinin (CCK) slow gastric emptying, while Gastric Inhibitory Peptide (GIP) might affect it.

Migrating Motor Complex (MMC)

• MMCs are interdigestive periods (between meals), where the stomach and small bowel are cleared of indigestible content.
• MMCs consist of phases, featuring slow waves followed by peristaltic contractions.
• Interdigestive motor patterns are mediated by a CNS timing mechanism, involving MMCs that help empty the gut.
• Motilin provides neurocrine function in Phase III, driving peristaltic contractions, and maintaining the pylorus open to clear the stomach.

Hunger Contractions

• Occur 12-24 hours post-eating without food intake.
• Increased intensity of MMCs (Migrating Motor Complexes).
• Intense rhythmic peristaltic contractions fuse into tetanic spasms, causing hunger pangs.