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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 [[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.

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.

==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.

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.

==Functions of the GIT==

==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; 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; 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 and growth and repair.

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 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 and 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 neighbouring target cells; neurotransmitters from nerves and other cells can be used and the autonomic nervous system is superimposed over the local control.

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.

==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.  

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.  

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 2 plexuses are connected by interneurones to co-ordinate control. The autonomic nervous system is 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'''. They include; ACh (muscarinic M1 and M2), Serotonin and Substance P.

'''Excitatory neurotransmitters''' are '''parasympathomimetic'''. They include: ACh (muscarinic M1 and M2), Serotonin and Substance P.

'''Inhibitory neurotransmitters''' are '''sympathomimetic'''. They include; Vasoactive intestinal polypeptide (VIP), Nitric oxide (NO), ATP and Enkephalins.

'''Inhibitory neurotransmitters''' are '''sympathomimetic'''. They include: Vasoactive intestinal polypeptide (VIP), Nitric oxide (NO), ATP and Enkephalins.

==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 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.

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.

[[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|250px|Cephalic phase of secretion - Copyright RVC 2008]]

This pahse 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 from 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.

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.

[[Image:gastric control of secretion diagram.jpg|thumb|right|250px|Gastric control of secretion - Copyright RVC 2008]]

[[Image:gastric control of secretion diagram.jpg|thumb|right|250px|Gastric 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 sectretions and motility.

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.

[[Image:Pancreatic control of secretion diagram.jpg|thumb|right|250px|Pancretic control of secretion - Copyright RVC 2008]]

[[Image:Pancreatic control of secretion diagram.jpg|thumb|right|250px|Pancretic control of secretion - Copyright RVC 2008]]

===Small Intestinal===

===Small Intestinal===

Involves secretin, [[Gut Endocrine Function - Anatomy & Physiology|GIP]] and [[Gut Endocrine Function - Anatomy & Physiology|Gastrin]]. Contolled by vagal tone and the parasympathomimetic reflex.

Involves secretin, [[Gut Endocrine Function - Anatomy & Physiology|GIP]] and [[Gut Endocrine Function - Anatomy & Physiology|gastrin]]. Controlled by vagal tone and the 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]].

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]].

===Gut Peptides===

===Gut Peptides===