Difference between revisions of "Control of Feeding - Anatomy & Physiology"
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*Vagus nerve (CN X) | *Vagus nerve (CN X) | ||
+ | **Parasympathetic fibres | ||
+ | **Synapse in submucosal plexus | ||
*[[Endocrine System - Gut - Anatomy & Physiology|Gastrin]] secreted from endocrine cells in pyloric mucosa | *[[Endocrine System - Gut - Anatomy & Physiology|Gastrin]] secreted from endocrine cells in pyloric mucosa | ||
+ | |||
+ | *Postganglionic fibres activate chief, parietal, mucous and G cells | ||
*Histamine secreted from paracrine action | *Histamine secreted from paracrine action | ||
*Increase stomach motility by release of hydrochloric acid and pepsinogen | *Increase stomach motility by release of hydrochloric acid and pepsinogen | ||
+ | |||
+ | *Short phase | ||
+ | |||
+ | *Overlaps with gastric phase of secretion | ||
+ | |||
+ | *Inhibited by stress, increased by aggression | ||
[[Image:gastric control of secretion diagram.jpg|thumb|right|150px|Gastric control of secretion - Copyright RVC 2008]] | [[Image:gastric control of secretion diagram.jpg|thumb|right|150px|Gastric control of secretion - Copyright RVC 2008]] | ||
===Gastric=== | ===Gastric=== | ||
− | *Chemical and mechanical receptors in stomach | + | *Chemical and mechanical receptors in stomach respond to stretch and chemical stimulation |
+ | |||
+ | *Increase in pH of gastric contents | ||
+ | |||
+ | *Response to undigested materials, especially proteins | ||
*Histamine and gastrin released | *Histamine and gastrin released | ||
*Negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit gastrin secretion (when pH falls below 3) | *Negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit gastrin secretion (when pH falls below 3) | ||
+ | |||
+ | *Submucosal and myenteric plexuses activated in vagus reflex arcs | ||
+ | |||
+ | *Postganglionic release of ACh at parietal cells | ||
+ | |||
+ | *Neural response and presence of peptides in chyme stimulate gastrin release | ||
+ | |||
+ | *Parietal and chief cells stimulate via gastrin acting in the bloodstream | ||
+ | |||
+ | *Long response | ||
[[Image:Intestinal control of secretion diagram.jpg|thumb|right|150px|Intestinal control of secretion - Copyright RVC 2008]] | [[Image:Intestinal control of secretion diagram.jpg|thumb|right|150px|Intestinal control of secretion - Copyright RVC 2008]] | ||
===Intestinal=== | ===Intestinal=== | ||
− | *Chyme in the duodenum inhibits acid secretion and motility | + | *Chyme in the [[Duodenum - Anatomy & Physiology|duodenum]] inhibits acid secretion and motility by decreasing the stomach distension and increasing the stretch of the [[Duodenum - Anatomy & Physiology|duodenum]] leading to the enterogastric reflex |
+ | |||
+ | *A drop in pH below 4.5 causes release of secretin which inhibits parietal and chief cells and stimulate buffer release from the [[Pancreas - Anatomy & Physiology|pancreas]] | ||
− | *Chyme causes release of secretin, [[Endocrine System - Gut - Anatomy & Physiology|GIP]] and [[Endocrine System - Gut - Anatomy & Physiology|CCK]] | + | *Chyme causes release of secretin, [[Endocrine System - Gut - Anatomy & Physiology|GIP]] and [[Endocrine System - Gut - Anatomy & Physiology|CCK]] decreasing gastric sectreions and motility |
[[Image:Pancreatic control of secretion diagram.jpg|thumb|right|150px|Pancretic control of secretion - Copyright RVC 2008]] | [[Image:Pancreatic control of secretion diagram.jpg|thumb|right|150px|Pancretic control of secretion - Copyright RVC 2008]] | ||
===Pancreatic=== | ===Pancreatic=== |
Revision as of 08:19, 17 July 2008
Feeding Methods
- Absorption over body surface
- Filter feeding
- Mucous trapping
- Fluid feeding
- Piercing and sucking
- Cutting and biting
- Seizing prey
- Jaws, teeth, beak
- Toxins
- Herbivores and grazing
- Invertebrates
- Vertebrates- bony plates or teeth
Functions of the GIT
- Secretion of enzymes and co-factors for digestion including water, ions and mucous
- Motility for forward propulsion, mechanical breakdown, mixing of ingesta and sphincter tone
- Migrating myoelectric complex to prevent debris accumulation
- Peristalsis
- Haustration
- Segmental motility
- Blood flow to muscles, submucosa and epithelial surfaces to sustain secretion, motility and the uptake of products of digestion
- Growth and repair
Control of the GIT
- Endocrine hormones released into the circulation by cells within the GIT or an accessory organ
- Paracrine mediators released by cells within the tract and diffuse locally to act on neighbouring target cells
- Neurotransmitters from nerves and other cells
- Autonomic nervous system superimposed over the local control
Control of Motility
- Intrinsic
- Muscle pacemaker cells (Cajal cells)
- Basic electric rhythm (BER) created of 3-20 per minute
- Passes through gap junctions
- Action potential created producing a slow wave of contraction
- Liklihood of an action potential is increased through the stretch of food in the lumen and chemical food stimulation
- Endocrine, paracrine or neural enter the enteric nervous system via sensory neurones
- Neurons interact with plexuses in the GIT wall
- Myenteric controls muscle movement
- Submucosa controls secretion and blood flow
- 2 plexuses connected by interneurones to co-ordinate control
- Autonomic nervous system superimposed
- Excitatory neurotransmitters are parasympathomimetic
- ACh (muscarinic M1 and M2)
- Serotonin
- Substance P
- Inhibitory neurotransmitters are sympathomimetic
- Vasoactive intestinal polypeptide (VIP)
- Nitroc oxide (NO)
- ATP
- Enkephalins
- Extrinsic nervous system (ANS)
- Sympathetic via norepinephrine (A1 and B2)
- Thoraco-lumbar innervation
- Parasympathetic via ACh (M1 and M2)
- Cranio-sacral innervation
- Sympathetic via norepinephrine (A1 and B2)
Control of GIT Secretions
- Presence of food in the GIT is detected by open chemoreceptors
- Signals to the endocrine cells or via the ANS releases gastrointestinal peptide hormones
- These hormones act to promote secretion, provide negative feedback or affect motility
- Closed mechanoreceptors also act to alter secretions
- Conditioned (associative) and unconditiones responses act via the ANS
Control Method | Neural | Endocrine |
---|---|---|
Saliva | Yes | No |
Stomach | Yes | Yes |
Small Intestine | No | Yes |
Phases of Gastric Secretion
Cephalic
- Unconditioned reflex
- Sight, smell, taste of food
- Vagus nerve (CN X)
- Parasympathetic fibres
- Synapse in submucosal plexus
- Gastrin secreted from endocrine cells in pyloric mucosa
- Postganglionic fibres activate chief, parietal, mucous and G cells
- Histamine secreted from paracrine action
- Increase stomach motility by release of hydrochloric acid and pepsinogen
- Short phase
- Overlaps with gastric phase of secretion
- Inhibited by stress, increased by aggression
Gastric
- Chemical and mechanical receptors in stomach respond to stretch and chemical stimulation
- Increase in pH of gastric contents
- Response to undigested materials, especially proteins
- Histamine and gastrin released
- Negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit gastrin secretion (when pH falls below 3)
- Submucosal and myenteric plexuses activated in vagus reflex arcs
- Postganglionic release of ACh at parietal cells
- Neural response and presence of peptides in chyme stimulate gastrin release
- Parietal and chief cells stimulate via gastrin acting in the bloodstream
- Long response
Intestinal
- Chyme in the duodenum inhibits acid secretion and motility by decreasing the stomach distension and increasing the stretch of the duodenum leading to the enterogastric reflex
- A drop in pH below 4.5 causes release of secretin which inhibits parietal and chief cells and stimulate buffer release from the pancreas
Pancreatic
- Parasympathetic stimulation during cephalic and gastric phases
- Negative feedback from paracrine sympathomimetics (somatostatin and enkephalins)
Biliary
- CCK empties gall bladder
- Secretin stimulates hydrogencarbonate ions from bile duct
Small Intestinal
- Succus entericus
- Vagal tone and parasympathomimetic reflex
Neuroendocrine Regulation of Feeding
- Hypothalamus is the critical region of feeding control
- Major hypothalamic nuclei involved:
Gut Peptides
Peptide | Site of Release | Effect on Feeding |
---|---|---|
CCK | Duodenum | Reduces food intake |
Ghrelin | Gastric fundus | Increases food intake |
GLP-2 | Intestine | Reduces food intake |
Motilin | Small intestine | Indirectly inhibits food intake |
Oxyntomodulin | Large intestine | Inhibits food intake |
Pancreatic Polypeptide | Pancreas | Inhibits food intake |
PYY3-36 | Intestine | Inhibits food intake |
Somatostatin | Pancreas | Inhibits food intake |
Other Peptides
Peptide | Site of Release | Effect on Feeding |
---|---|---|
Leptin | Adipocyte | Reduces food intake |
NPY | Arcuate nucleus | Increases food intake |
Agrp | Arcuate nucleus | Increases food intake |
MCH | Lateral hypothalamus | Increases food intake |
Orexins | Lateral hypothalamus | Increases food intake |
CART | Arcuate nucleus | Inhibits food intake |
α MSH | Arcuate nucleus | Inhibits food intake |
Bdnf | Ventromedial nucleus | Inhibits food intake |
Serotonin | Brainstem | Inhibits food intake |
Cannabinoids | CNS | Increases food intake |