Physiology of Digestion Flashcards
Name the 3 phases of digestion and how much acid is secreted in each?
- Cephalic phase: 35%
- Gastric phase: 60%
- Intestinal phase: 5%
Describe the Cephalic phase?
Stimulated by sight/smell/taste of food.
Causes vagal post-ganglionic muscarinic nerves to release ACh which stimulates parietal cells to produce HCl.
Gastrin is released from G cells.
ECL cells secrete histamine.
Chief cells secrete pepsinogen.
Describe the Gastric phase?
Stimulated by food distending the stomach which activates vago-vagal reflexes causing more ACh secretion.
G cells release gastrin.
ECL cells release histamine.
When food enters stomach, the pH is raised, more acid brings pH back down.
When pH reaches <2, gastrin and histamine secretion is inhibited by somatostatin. It also directly inhibits parietal cells to reduce acid secretion.
Describe the Intestinal phase?
Occurs in the duodenum in response to arriving chyme.
Chyme triggers enterogastric reflex and gastric secretory activity is suppressed due to;
- Secretin released by S cells of dunodenal mucosa in response to acid. Reaches stomach via bloodstream and inhibits gastrin release and inhibits parietal cells affinity for gastrin.
- CCK and GIP are released in the presence of lipids and carbohydrates and inhibit gastrin.
What sphincters are under voluntary control?
Upper oesophageal and external anal.
What is the main function of the Enteric NS?
Carry out autonomous functions and can act independently of sympathetic and parasympathetic NS (although can be influenced by them).
What are the 2 ganglial plexus’ of the Enteric nervous system and where are they found?
Meissners Plexus - Submucosa
Myenteric Plexus - In between circular and longitudinal muscle layer.
What extrinsic factors can influence the Enteric NS?
- Vagal control - excitatory to non-sphincter muscle.
- Sympathetic control - inhibitory to non-sphincter muscle and excitatory to sphincter muscle.
- Neurohormonal Influences - 5 Hydroxyl Tryptamine, Motilin and Opioid receptors.
What is the role of the interstitial cells of Cajal?
Mediate enteric neurotransmission and serves as a pacemaker for contraction of smooth muscle.
What is the Migrating Motor Complex?
Waves of electrical activity creating a cyclical contraction sequence around every 90mins in the Interparadial period.
Describe the 4 phases of the MMC?
I. Prolonged period of quiescence.
II. Increased frequency of contractility.
III. A few mins of peak electrical and mechanical activity.
IV. Declining activity merging to next phase I.
What hormone regulates the MMC?
Motilin - a polypeptide hormone produced by M cells in the small intestine.
Secreted at 90-min intervals.
Stimulates contraction of fundus and enhances gastric emptying.
Describe what replaces the MMC in the gastric phase after eating?
Replaced by contractions of variable amplitude and frequency for mixing and digestion.
Frequency and direction of contraction is controlled by the gastric pacemaker zone in the body of stomach.
Pacemaker generates rhythmic depolarisations of 3 cycles per min which trigger smooth muscle contractions (with neurohumoral input).
Liquids leave before solids, solids empty every 3-4hours.
What replaces the MMC in the intestinal phase?
Segmentation (haustral) Contractions: Non-synchronised contractions of circular SM to mix chyme.
Peristaltic Waves: Arise from dueodenal bulb which generates its own basal electrical rhythm of about 12 per min. It is the major form of propulsion.
Pedular Contractions: Rythmic contractions of the longitudinal SM to mix chyme.
Villous Movements: Contraction of muscularis mucosae causing micro-mixing at surface of mucosa.
