Control of Feeding - Anatomy & Physiology
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BACK TO ALIMENTARY - ANATOMY & PHYSIOLOGY
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 acton neighbouring target cells
- Neurotransmitters from nerves andother 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
| Conrtol 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)
- Gastrin secreted from endocrine cells in pyloric mucosa
- Histamine secreted from paracrine action
- Increase stomach motility by release of hydrochloric acid and pepsinogen
Gastric
- Chemical and mechanical receptors in stomach
- Histamine and gastrin released
- Negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit gastrin secretion (when pH falls below 3)
Intestinal
File:/intestinal control of secretion diagram.jpg
Intestinal control of secretion - Copyright RVC 2008
- Chyme in the duodenum inhibits acid secretion and motility
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