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 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

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

Cephalic phase of secretion - Copyright RVC 2008

• 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 control of secretion - Copyright RVC 2008

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 control of secretion - Copyright RVC 2008

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

• Chyme causes release of secretin, GIP and CCK decreasing gastric sectreions and motility

Pancretic control of secretion - Copyright RVC 2008

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

Neuroendocrine Regulation of Feeding

• Hypothalamus is the critical region of feeding control

• Major hypothalamic nuclei involved:
  • Arcuate
  • Ventromedial hypothalamus
  • Dorsomedial hypothalamus
  • Lateral hypothalamus
  • Paraventricular hypothalamus

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

The Vomit Reflex

• Emesis is the process of vomiting

• Retching involves the abdominal and chest walls contracting

• Vomiting includes retching and the action of the diaphragm

• Diaphragm moves caudal to open the cardia

• Gastrointestinal tract have protective stimuli to recognise harmful products ingested. The mechanoreceptors and chemoreceptors respond using viscerent afferent pathways.

• Medulla co-ordinates process

• Chemoreceptive trigger zone in the 4th ventricle responds to blood and CSF

• Inputs also from inner ear and higher centres

• Emetic agents
  • Histamine
  • ACh
  • Dopamine
  • Catecholamines
  • 5-hydroxytryptamine

• Anti-emetic agents

(INCLUDE HERE)

Species Differences

Equine

• The horse cannot vomit