Control of Feeding - Anatomy & Physiology

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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

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

• Chyme in the duodenum inhibits acid secretion and motility

• Chyme causes release of secretin, GIP and CCK

Pancreatic

• CCK, secretin and Gastrin

• 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

• Secretin, GIP and Gastrin

• Vagal tone and parasympathomimetic reflex