WK 1

• myenteric stimulatory motor neurons = acetylcholine
• myenteric inhibitory motor neurons = nitric oxide
• myenteric ascending/descending interneurons = acetylcholine, 5-hydroxytryptamine
• myenteric sensory neurons = substance P
• submucosal non-cholinergic secretomotor neurons = vasoactive intestinal polypeptide
• submucosal cholinergic secretomotor neurons = acetylcholine
• submucosal sensory neurons = substance P
• IBS = 10-20% population = pain, constipation, diarrhoea = alteration in signalling pathway in serotonin = 5HT3 receptor antagonist Alosetron for IBS-induced diarrhoea (Aus not approved), 5HT4 receptor agonist Tegaserod (banned)
• Presence of food bolus itself triggers peristaltic reflex via vagus
• Peristaltic reflex involves signals for contraction of circular muscle proximal to bolus and relaxation of circular muscle distal to bolus
• Stomach capacity 0.8-1.5L. 2 sections; body (upper 2/3) and antrum (lower 1/3)
• Food processed in the stomach is forced through the pylorus into the duodenum
• Stomach 1) storage 2) mixes food with gastric secretions to form a semifluid mixture called chyme 3) empties chyme at suitable rate for digestion
• When food is present, peristaltic constrictor waves begin in the stomach body and move toward the antrum
• COnstrictor waves dig deep into the good contents of the antrum. This exposes more food to digestive juices.
• chyme is transferred to the duodenum with the aid of the migrating myoeletric complex (MMC). CHaracterised by 10-20 minutes of intense antral contractions every 1-2 hours during fasting to clear particles from the stomach
• at the pylorus, some chyme is passed into the duodenum but most is pushed back upstream by strong contractions to increase homogeneity
• Particles passing through the pylorus as <3mm diameter
• Trituration = mixing of food with gastric juices through stomach contractions
• control by the duodenum prevents excess food moving into the intestines (this would limit nutrient absorption ability)
• Inhibition of stomach emptying by the duodenum is mediated by nervous reflexes and hormones
• Factors that inhibit stomach emptying: duodenum distension, high chyme acidity, hyper/hypotonic chyme, protein breakdown products, irritation
• small intestine stretching causes peristaltic contractions that cause forward movement and mixing. movement is slow to allow greater absorption of nutrients (1cm/min). Takes 3-5hrs to pass through SI. Motility upregulated via gastrin, CCK, insulin, motilin and serotonin. Inhibited by secretin and glucagon.
• Irritation causes peristaltic rush (eg infectious diarrhoea) --> autonomic NS + intrinsic control of myenteric plexus
• faeces enters the rectum, stretches the walls and stimulates the myenteric plexus to initiate peristaltic contractions in the descending colon downward. Myenteric plexus relaxes internal sphincter. If the external sphincter is voluntarily relaxed, defecation occurs. Augmented by parasympathetic activity.
• external anal sphincter = skeletal motor nerve
• internal anal sphincter = parasympathetic nerve fibre
• as ex. anal sphincter relaxes, pelvic floor descends, increases anorectal angle from 90 --> 140 degrees. relaxation of puborectalis
• squatting increases anorectal angle
• paradoxical puborectalis syndrome = external anal sphincter can't relax = history of chronic constipation, fear surrounding defecation
• Melaena = dark black, tarry faeces = upper GI bleed. Colour + odour = haemoglobin altered by digestive enzymes and intestinal bacteria.
• Peristaltic reflex = signals for contraction of circular muscle proximal to bolus, relaxation of longitudinal muscle distal to bolus