Difference between revisions of "Control of Feeding - Anatomy & Physiology"

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==Introduction==
 
==Introduction==
  
 
Different hormones, neurotransmitters and reflexes are involved in the complicated process of feeding in animals. Secretions and motility of the gastrointestinal tract are stimulated and carefully regulated by numerous factors, including environmental stimuli and the presence of food in different parts of the gastrointestinal tract from the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]] right through to the intestines.  
 
Different hormones, neurotransmitters and reflexes are involved in the complicated process of feeding in animals. Secretions and motility of the gastrointestinal tract are stimulated and carefully regulated by numerous factors, including environmental stimuli and the presence of food in different parts of the gastrointestinal tract from the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]] right through to the intestines.  
  
−
When a harmful substance is ingested the body acts to eliminate it in different ways to prevent the animal becoming ill, for example, through [[Vomiting#The Vomit Reflex|vomiting]] and [[Diarrhoea|diarrhoea]]. If one or more of the pathways in controlling feeding is damaged or inhibited, then problems such as obesity occurs.
+
When a harmful substance is ingested the body acts to eliminate it in different ways to prevent the animal becoming ill, for example, through [[Control of Feeding - Anatomy & Physiology#The Vomit Reflex|vomiting]] and [[Diarrhoea|diarrhoea]].
 +
 
 +
If one or more of the pathways in controlling feeding is damaged or inhibited, then problems such as obesity occurs.
  
 
==Feeding Methods==
 
==Feeding Methods==
  
−
There are a number of feeding methods, including absorption over the body surface, filter feeding, mucous trapping, fluid feeding, piercing and sucking, cutting and biting, seizing prey, the use of jaws, [[:Category:Teeth - Anatomy & Physiology|teeth]], [[Integument of Exotic Species - Anatomy & Physiology#The Beak|beaks]] and the use of toxins.
+
*Absorption over body surface
 +
 
 +
*Filter feeding
 +
 
 +
*Mucous trapping
 +
 
 +
*Fluid feeding
 +
**Piercing and sucking
 +
**Cutting and biting
 +
 
 +
*Seizing prey
 +
**Jaws, [[Oral Cavity - Teeth & Gingiva - Anatomy & Physiology|teeth]], [[The Integument of Exotic Species - Anatomy & Physiology#The Beak|beak]]
 +
**Toxins
 +
 
 +
*Herbivores and grazing
 +
**Invertebrates
 +
**Vertebrates- bony plates or [[Oral Cavity - Teeth & Gingiva - Anatomy & Physiology|teeth]]
  
 
==Functions of the GIT==
 
==Functions of the GIT==
  
−
The main function of the GIT is to efficiently breakdown feed to the essential nutrients that can then be absorbed. The GIT then acts to propel the waste fed material out of the body. The following constitute this main function:
+
The main function of the GIT is to efficently breakdown feed to the essential nutrients that can then be absorbed. The GIT then acts to propell the waste fed material out of the body. The following functions constitute this main function;
−
:The 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
+
*Secretion of enzymes and co-factors for digestion including water, ions and mucous
−
:Migrating myoelectric complex to prevent debris accumulation
+
 
−
:Peristalsis
+
*Motility for forward propulsion, mechanical breakdown, mixing of ingesta and sphincter tone
−
:Haustration
+
**Migrating myoelectric complex to prevent debris accumulation
−
:Segmental motility
+
**Peristalsis
−
:Blood flow to muscles, submucosa and epithelial surfaces to sustain secretion, motility and the uptake of products of digestion and growth and repair.
+
**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==
 
==Control of the GIT==
  
−
Endocrine hormones are released into the circulation by cells within the GIT or an accessory organ; paracrine mediators are released by cells within the tract and diffuse locally to act on neighboring target cells; neurotransmitters from nerves and other cells can be used and the autonomic nervous system is superimposed over the local control.
+
*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==
 
==Control of Motility==
  
−
The control of motility is intrinsic. It uses muscle pacemaker cells (Cajal cells) to set a basic electric rhythm (BER) of 3-20 per minute. It passes through gap junctions and an action potential is created, producing a slow wave of contraction. The likelihood of an action potential is increased through the stretch of food in the lumen and chemical food stimulation.
+
*Intrinsic
 +
**Muscle pacemaker cells (Cajal cells)
 +
**Set a basic electric rhythm (BER) of 3-20 per minute
 +
**Passes through gap junctions
 +
**Action potential created producing a slow wave of contraction
 +
**Likelihood 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 plexus controls muscle movement
 +
**Submucosa plexus controls secretion and blood flow
 +
**2 plexuses connected by interneurones to co-ordinate control
 +
**Autonomic nervous system superimposed
  
−
Endocrine, paracrine or neural signals enter the enteric nervous system via sensory neurones. The neurons interact with plexuses in the GIT wall. The '''myenteric plexus''' controls muscle movement and the '''submucosa plexus''' controls secretion and blood flow. The two plexuses are connected by interneurones to co-ordinate control. The autonomic nervous system is superimposed.
+
*Excitatory neurotransmitters are parasympathomimetic
 +
**ACh (muscarinic M1 and M2)
 +
**Serotonin
 +
**Substance P
  
−
'''Excitatory neurotransmitters''' are '''parasympathomimetic'''. They include: ACh (muscarinic M1 and M2), Serotonin and Substance P.
+
*Inhibitory neurotransmitters are sympathomimetic
 +
**Vasoactive intestinal polypeptide (VIP)
 +
**Nitric oxide (NO)
 +
**ATP
 +
**Enkephalins
  
−
'''Inhibitory neurotransmitters''' are '''sympathomimetic'''. They include: Vasoactive intestinal polypeptide (VIP), Nitric oxide (NO), ATP and Enkephalins.
+
*Extrinsic - Autonomic 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==
 
==Control of GIT Secretions==
  
−
The presence of food in the GIT is detected by open chemoreceptors. Signals to the endocrine cells or via the autonomic nervous system (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 unconditioned responses act via the ANS.
+
*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 unconditioned responses act via the ANS
  
  
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===Cephalic===
 
===Cephalic===
−
[[Image:Cephalic phase of secretion diagram.jpg|thumb|right|250px|Cephalic phase of secretion - Copyright RVC 2008]]
+
[[Image:Cephalic phase of secretion diagram.jpg|thumb|right|150px|Cephalic phase of secretion - Copyright RVC 2008]]
 +
*Unconditioned reflex
  
−
This phase is an unconditioned reflex. It is stimulated by the sight, smell and taste of food. It involves the vagus nerve ([[Cranial Nerves - Anatomy & Physiology|CN X]]) and parasympathetic fibres that synapse in the submucosal plexus. [[Gut Endocrine Function - Anatomy & Physiology|Gastrin]] is secreted from the endocrine cells in the pyloric mucosa. Postganglionic fibres activate chief, parietal, mucous and G cells. Histamine is secreted due to paracrine action. There is an increase in [[Monogastric Stomach - Anatomy & Physiology|stomach]] motility by release of hydrochloric acid and pepsinogen. It is a short phase and overlaps with the gastric phase of secretion. It is inhibited by stress and increased by aggression.
+
*Sight, smell, taste of food
−
[[Image:gastric control of secretion diagram.jpg|thumb|right|250px|Gastric control of secretion - Copyright RVC 2008]]
 
  
 +
*Vagus nerve ([[Cranial Nerves - Anatomy & Physiology|CN X]])
 +
**Parasympathetic fibres
 +
**Synapse in submucosal plexus
 +
 +
*[[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
 +
 +
*Increase [[Forestomach - Anatomy & Physiology|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]]
 
===Gastric===
 
===Gastric===
  
−
Chemical and mechanical receptors in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] respond to stretch and chemical stimulation by an increase in the pH of gastric contents. It is a response to undigested materials, especially proteins. Histamine and [[Gut Endocrine Function - Anatomy & Physiology|gastrin]] are released. It is a negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit gastrin secretion (when pH falls below 3). The submucosal and myenteric plexuses are activated in vagus reflex arcs causing the postganglionic release of ACh at parietal cells. The neural response and presence of peptides in chyme stimulates gastrin release. Parietal and chief cells stimulate via gastrin acting in the bloodstream. It is a long response.
+
*Chemical and mechanical receptors in [[Forestomach - Anatomy & Physiology|stomach]] respond to stretch and chemical stimulation
  
−
[[Image:Intestinal control of secretion diagram.jpg|thumb|right|250px|Intestinal control of secretion - Copyright RVC 2008]]
+
*Increase in pH of gastric contents
  
 +
*Response to undigested materials, especially proteins
 +
 +
*Histamine and [[Endocrine System - Gut - Anatomy & Physiology|gastrin]] released
 +
 +
*Negative feedback loop by sympathomimetic somatostatin released by paracrine method to inhibit [[Endocrine System - Gut - Anatomy & Physiology|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]]
 
===Intestinal===
 
===Intestinal===
  
−
Chyme in the [[Duodenum - Anatomy & Physiology|duodenum]] inhibits acid secretion and motility by decreasing the [[Monogastric Stomach - Anatomy & Physiology|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 stimulates buffer release from the [[Pancreas - Anatomy & Physiology|pancreas]]. Chyme causes the release of secretin, [[Gut Endocrine Function - Anatomy & Physiology|GIP]] and [[Gut Endocrine Function - Anatomy & Physiology|CCK]] decreasing gastric secretions and motility.
+
*Chyme in the [[Duodenum - Anatomy & Physiology|duodenum]] inhibits acid secretion and motility by decreasing the [[Forestomach - Anatomy & Physiology|stomach]] distension and increasing the stretch of the [[Duodenum - Anatomy & Physiology|duodenum]] leading to the enterogastric reflex
−
[[Image:Pancreatic control of secretion diagram.jpg|thumb|right|250px|Pancretic control of secretion - Copyright RVC 2008]]
 
  
 +
*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]] decreasing gastric sectreions and motility
 +
[[Image:Pancreatic control of secretion diagram.jpg|thumb|right|150px|Pancretic control of secretion - Copyright RVC 2008]]
 
===Pancreatic===
 
===Pancreatic===
  
−
Involves [[Gut Endocrine Function - Anatomy & Physiology|CCK]], secretin and [[Gut Endocrine Function - Anatomy & Physiology|Gastrin]]. Parasympathetic stimulation during cephalic and gastric phases. Negative feedback occurs from paracrine sympathomimetics (somatostatin and enkephalins).
+
*[[Endocrine System - Gut - Anatomy & Physiology|CCK]], secretin and [[Endocrine System - Gut - Anatomy & Physiology|Gastrin]]
 +
 
 +
*Parasympathetic stimulation during cephalic and gastric phases
 +
 
 +
*Negative feedback from paracrine sympathomimetics (somatostatin and enkephalins)
  
 
===Biliary===
 
===Biliary===
  
−
[[Gut Endocrine Function - Anatomy & Physiology|CCK]] empties the [[Gall Bladder - Anatomy & Physiology|gall bladder]] and secretin stimulates hydrogencarbonate ions from the bile duct.
+
*[[Endocrine System - Gut - Anatomy & Physiology|CCK]] empties [[Gall Bladder - Anatomy & Physiology|gall bladder]]
 +
 
 +
*Secretin stimulates hydrogencarbonate ions from bile duct
  
 
===Small Intestinal===
 
===Small Intestinal===
  
−
Involves secretin, [[Gut Endocrine Function - Anatomy & Physiology|GIP]] and [[Gut Endocrine Function - Anatomy & Physiology|gastrin]]. Controlled by vagal tone and the parasympathomimetic reflex.
+
*Succus entericus
 +
 
 +
*Secretin, [[Endocrine System - Gut - Anatomy & Physiology|GIP]] and [[Endocrine System - Gut - Anatomy & Physiology|Gastrin]]
 +
 
 +
*Vagal tone and parasympathomimetic reflex
  
 
==Neuroendocrine Regulation of Feeding==
 
==Neuroendocrine Regulation of Feeding==
  
−
The [[Hypothalamus - Anatomy & Physiology|hypothalamus]] is the critical region of feeding control. The major hypothalamic nuclei involved are; [[Hypothalamus - Anatomy & Physiology#Nuclei|arcuate]], [[Hypothalamus - Anatomy & Physiology#Nuclei|ventromedial hypothalamus]], [[Hypothalamus - Anatomy & Physiology#Nuclei|dorsomedial hypothalamus]], [[Hypothalamus - Anatomy & Physiology#Nuclei|lateral hypothalamus]] and [[Hypothalamus - Anatomy & Physiology#Nuclei|paraventricular hypothalamus]].
+
*[[Hypothalamus - Anatomy & Physiology|Hypothalamus]] is the critical region of feeding control
 +
 
 +
*Major hypothalamic nuclei involved:
 +
**[[Hypothalamus - Anatomy & Physiology#Nuclei|Arcuate]]
 +
**[[Hypothalamus - Anatomy & Physiology#Nuclei|Ventromedial hypothalamus]]
 +
**[[Hypothalamus - Anatomy & Physiology#Nuclei|Dorsomedial hypothalamus]]
 +
**[[Hypothalamus - Anatomy & Physiology#Nuclei|Lateral hypothalamus]]
 +
**[[Hypothalamus - Anatomy & Physiology#Nuclei|Paraventricular hypothalamus]]
  
 
===Gut Peptides===
 
===Gut Peptides===
 +
 
{| style="width:75%; height:300px" border="1"
 
{| style="width:75%; height:300px" border="1"
  
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|-
 
|-
 
| '''Motilin'''
 
| '''Motilin'''
−
| [[Small Intestine Overview - Anatomy & Physiology|Small intestine]]
+
| [[Small Intestine - Anatomy & Physiology|Small intestine]]
 
| Indirectly inhibits food intake
 
| Indirectly inhibits food intake
 
|-
 
|-
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|-
 
|-
 
|}
 
|}
 +
 +
==[[Vomiting#The Vomit Reflex|The Vomit Reflex]]==
  
 
==Species Differences==
 
==Species Differences==
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The anatomy of the equine stomach, the close proximity of the exiting duodenum and the entering oesophagus along with the well developed cardiac sphincter, make it difficult for the horse to vomit.  However on rare occasions it does occur.
 
The anatomy of the equine stomach, the close proximity of the exiting duodenum and the entering oesophagus along with the well developed cardiac sphincter, make it difficult for the horse to vomit.  However on rare occasions it does occur.
  
−
'''Click here for more information on [[Vomiting#The Vomit Reflex|The Vomit Reflex]]'''.
 
−
 
−
{{OpenPages}}
 
 
[[Category:Feeding Control]]
 
[[Category:Feeding Control]]
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[[Category:A&P Done]]
 

Revision as of 15:56, 2 September 2010

Introduction

Different hormones, neurotransmitters and reflexes are involved in the complicated process of feeding in animals. Secretions and motility of the gastrointestinal tract are stimulated and carefully regulated by numerous factors, including environmental stimuli and the presence of food in different parts of the gastrointestinal tract from the oral cavity right through to the intestines.

When a harmful substance is ingested the body acts to eliminate it in different ways to prevent the animal becoming ill, for example, through vomiting and diarrhoea.

If one or more of the pathways in controlling feeding is damaged or inhibited, then problems such as obesity occurs.

Feeding Methods

  • Absorption over body surface
  • Filter feeding
  • Mucous trapping
  • Fluid feeding
    • Piercing and sucking
    • Cutting and biting
  • Herbivores and grazing
    • Invertebrates
    • Vertebrates- bony plates or teeth

Functions of the GIT

The main function of the GIT is to efficently breakdown feed to the essential nutrients that can then be absorbed. The GIT then acts to propell the waste fed material out of the body. The following functions constitute this main function;

  • 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)
    • Set a basic electric rhythm (BER) of 3-20 per minute
    • Passes through gap junctions
    • Action potential created producing a slow wave of contraction
    • Likelihood 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 plexus controls muscle movement
    • Submucosa plexus 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)
    • Nitric oxide (NO)
    • ATP
    • Enkephalins
  • Extrinsic - Autonomic 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 unconditioned 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
  • 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

  • Parasympathetic stimulation during cephalic and gastric phases
  • Negative feedback from paracrine sympathomimetics (somatostatin and enkephalins)

Biliary

  • Secretin stimulates hydrogencarbonate ions from bile duct

Small Intestinal

  • Succus entericus
  • Vagal tone and parasympathomimetic reflex

Neuroendocrine Regulation of Feeding

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

Species Differences

Equine

The anatomy of the equine stomach, the close proximity of the exiting duodenum and the entering oesophagus along with the well developed cardiac sphincter, make it difficult for the horse to vomit. However on rare occasions it does occur.

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