Motility II. Stomach, Small Intestine & Colon PDF

Motility II. Stomach, Small intestine & Colon GASTRIC MOTILITY The CNS regulates everything we receive, including the activity of the stomach: PS innervation: everything starts in the hypothalamus, which will send neurons into the dorsal vagal nucleus from which the X CN, the vagus nerve, descends....

Motility II. Stomach, Small intestine & Colon GASTRIC MOTILITY The CNS regulates everything we receive, including the activity of the stomach: PS innervation: everything starts in the hypothalamus, which will send neurons into the dorsal vagal nucleus from which the X CN, the vagus nerve, descends. It will give branches to provide parasympathetic innervation to the correlated) Revision for Visceral Efferent stomach. sympatheti Sympathetic innervation: sympathetic neurons arise from from T1-L2 levels. The ones corresponding to the stomach exit the ganglia sympathetic trunk through the major splanchnic nerve, to greater synapse in the Celiac ganglion (preaortic). Info SPLANCHNIC NERVES · · > - : 2 · 11 to 12 divide into : ↳ -15 which ASCEND to experior * 15-412 Thoracic SpLanchnic - - - masse herer (19-11) last (412) · Ga Inst ↳ L-L2 ALSO CALLED GREATER SPLANCHNIC NERVE (descend) (15-19) LUMBAR SPLANCHNIC (descend) and synapse LUMBAR GANGLIA/ part of Sympathetic trunk) in part of sympathetic trunk Stimuli for PS pathway: touch and taste (tongue), smell, sight of food,... Stimuli for sympathetic pathway: stress, fight-flight response (inhibits peristaltism and acid secretion) Stomach main functions: 1. Storage (in the fundus) 2. Mixing of food with gastric secretions. 3 stages: - Propulsion (food is propelled through the stomach and into the duodenum) - Grinding (mixing, breaking down) - Retropulsion (chyme is forced back against the stomach walls, increasing grinding. Essential to the digestion and breakdown of food in the stomach) 3. Emptying (through propulsion) Mixing and Propulsion & 2 The oral portion (fundus) is in charge of receiving the bolus, storage and some digestion. The caudal portion (body and antrum) carries out digestion and propulsion. - Mixing waves = slow waves. From body to antrum - Propulsion waves = spikes. From antrum to pylorus Vagal reflex The incoming bolus causes distension of the stomach walls, which is detected by mechanoreceptors. These connect with the myenteric plexus, which’ll convey the info to the CNS. The PS reflex comes back through the X cranial nerve and slow waves begin, which go back and forth (segmentation movement → mixing). Next, at the antrum, the propulsion waves (spike) push the chyme from antrum to pylorus, which is RETRAPULSION usually still closed, so it allows for more mixing and chyme formation in the stomach. Once relaxation in the pylorus is induced (X CN), the chyme can continue its way. This depends on different hormones. - Retropulsion and mixing As the peristaltic contraction reaches the pylorus, the sphincter contracts and chyme can no longer be emptied into the duodenum. This is retropulsion, and allows for more efficient mixing with the gastric secretions, as chyme is forced back up to the antrum. 3 Gastric emptying It occurs as chyme passes through the pyloric sphincter, which remains partially open during all times until the peristaltic wave reaches it. Chyme enters the duodenum. Stimulus for emptying: - Gastric factors such as: ○ Gastric food volume. The stretching of stomach walls initiates local myenteric reflexes (mechanism) ○ ↑ gastrin secretion, released by G cells in the antrum - Effects are: ○ Pyloric sphincter relaxation ○ Pyloric pump activation ○ Gastrin secretion also promotes the secretion of gastric acidic juices (HCl) by stomach gastric glands des not always happen Inhibition of emptying: - Duodenal factors (if the duodenum receives too much chyme, it will inhibit gastric emptying. It has to regulate pyloric function to protect itself) ○ Duodenal distension ○ Toxic components which cause duodenal mucosa irritation ○ Chyme acidity ○ Osmolarity ○ Protein/fat rich chyme - Mechanism: entero-gastic reflexes (ex: vagal reaction, you become dizzy and start vomiting as the vagus inhibits gastric emptying) ○ Directly from the duodenum to the stomach via the myenteric plexus ○ Sympathetic nervous system (extrinsic nerves) ○ Inhibitory reflex of the vagus ○ Hormonal mechanisms, carried by the blood to receptors in the stomach: - CCK released by the jejunum in response to fat rich chyme - Secretin released by the duodenal mucosa in response to acid - Gastric inhibitory peptide (GIP) in response to fat and carbohydrate 6 rich chyme secreted high density present - Effects: genear ○ Increase tone (contraction) of the pyloric sphincter ○ Inhibit pyloric pump (therefore inhibit emptying) * 6 * CHOLELYSTOKININ in in duodenum and Upper jejunur ; but in whole small intestine in Summary of the neurohormonal regulation of the entero-gastric axis: (receptors will NOT be asked) Protection mechanisms of the gastric mucosa - - Principal/Mucus cells are very important, as they’re in charge of the production of bicarbonate mucus, which neutralizes the acid in gastric secretions and protects the stomach epithelial lining. otyntic Parietal cells will be producing chloridric acid There are also some important endocrine cells cells CHIEf CELLS (symogenic als) : Lipase secrete persinogen and precursors of Cannin and Bicarbonate mixes with the protons of the acid content of the stomach and becomes water, neutralizing and protecting the lining. MOTILITY OF THE SMALL INTESTINE GI tract reflexes Segmentation movements: Short contractions of different parts of the small intestine allow mixing of the chyme with bile, pancreatic and intestinal fluids. Also increases the absorption surface as it spreads the chyme over the epithelium. Antrograde and retrograde movements are carried out in order to mix. Propulsion. Only anterograde, forward-driven movements. - Stimulated by the PS nerves, induced by stretching and irritants. - Inhibited by the sympathetic NS (induced by fight-flight response) Migrating motor complex. During fasting, in between meals, there is a small wave of contraction which cleans the remaining food, dead cells, bacteria,... Actually happening all the time, but mostly during fasting. Carried out through slow waves induced by a very important hormone, Motilin, (secreted by endocrine cells in intestinal crypts) Ileocecal valve; always closed. Opened by the presence of Gastrin. When gastrin comes, (gastroileal reflex), it activates the ileocecal reflex (=valve opening). MOTILITY OF THE LARGE INTESTINE Austrae contractions: - Stimulus = stretching / distension ENIDE / - Segmentation movements are produced by tinae coli when there is food. In other words, when there’s stretching of the walls. - Mainly in the ascendent and transverse colon - Increases the absorption surface, for: water, electrolytes and vitamins B1/B5/B6 (produced by the abundant large intestine microbiome) Austrae are the segments, tinae coli are the muscular bands which delimit them ↑ MASS COLL PERISTALSIS Peristalsis de Masa: - Also stimulated by stretching + irritants and gastro-colic reflex - Small propulsion movements, continuous and directed towards the rectum. - Mainly in the transverse and descending colon - Huge lengths of contractions (20 cm), which happen even during meals. Both ↳ during and after meals - Increase the absorption surface for water and electrolytes LENGTH OF COLON THAT IS CONTRACTED Defecation reflex - - Stimulus is the distension of the sigmoid colon + rectum. Distension of the rectum is sensed and conveyed by sensory neurons through PS nerves from S2-S4 levels (pelvic splanchnic nerves). The info arrives to the dorsal horn of the spinal cord The reflex begins and travels through efferent fibres of these same S2-S4 PS nerves arising from the ventrolateral horn, which will induce propulsion (contractions) However, there is a concious relaxation of the anal sphincter, which is the only conscious part of the whole process. It’s performed by the external muscles of the sphincter at the anus, controlled by regular, voluntary motor neurons (ventral horn) > - somatic INTERNAL ANAL SPHINCTER instead AUTONOMIC NERVOUS SYSTEM innervation(S2-S4) PUDENDAL NERVE is controlled by