ANS Control of the Gastrointestinal Tract vs Vasculature






Compare and contrast the autonomic control of the intestine and the systemic vascular system. Your answer should be weighted equally (50%) between each system.

The enteric nervous system is often refered to the 'small brain' in popular culture, due to the presence of around 100-500 million intrinsically active neurones in the gastrointestinal area. However the ENS is also well connected to the autonomic nervous system, with parasympathetic and sympathetic input. The descending vagus is the main conveyor of the parasympathetic signal (with output at the sacral level of the spinal cord) with stimulatory in both sensory and motor functions. With stretch, osmolarity and temperature conveyed to the CNS along the vagus, allowing for motor output of gastrointestinal secretion an motility from the vagus; termed the vago-vagal reflex.
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Sympathetic preganlionic innervation arises from the T8-L2 region synapsing at the coeliac, the superior and inferior mesenteric ganglia. The main function of the SNS being the redirection of blood from the GI tract, along with contraction of sphincters.
In the early 1900s Bayliss and Starling  initially characterized the enteric nervous system, noting that peristalsis of bolus occurred in the right direction even following a vagectomy.
The ENS is a complex 3D structure containing multiple ganglia and arises in development from embryonic neural crest cells. There are two main plexus, the myenteric plexus which is found between the longitudinal and circular muscle layer and submucosal plexus found is the submucosa.
The Myenteric plexus is found within the whole of the GI tract from mouth to anus. It has a densely packed ganglia and is arranged regularly in a a polygonal network. It mainly contains motor neurones supplying the muscularis, with a main role in  muscle relaxation associated peristalsis being a main function.
Image result for Gastrointestinal neurotransmittersWhile the submucosal plexus has both motor and sensory functions, mainly being involved in the the absorption of nutrients and secretions of enzymes, mucous etc. along with some involvement in gut motility. It is arranged more diffusely across the submucosa, with a less densely packed network of ganglions. The SNS (mostly postganglionic) and PNS (preganglionic) innervates both the myenteric and submuosal plexus as well as the GI tract directly. The ANS as a whole has a neuromdoculatary role. The ANS also synapses at neruoendocrine cells such as the enterochromaffin cells in order to modulate hormone release. The main neurotransmitters of the ANS include Acetylcholine from parasympathetic nerve causing smooth muscle cells contraction, sphincter relaxation and secretion from salivary, pancreatic and gastric glands. Vasoactive Intestinal Peptide (VIP) also released from parasympathetic and enteric nerve ending cause s sphincter relaxation and increased secretion from GI glands. While Noradrenaline (NA) from the sympathetic nervous system causes relaxation of the smooth muscle, contraction of sphincter muscle and decrease of salivary secretion. Neuropeptide Y of the SNS also effect similar results of sphincter contraction and smooth muscle relaxation. While Enkephalins (opioid receptor agonists) of the enteric nervous system causes sphincter constriction and a decrease secretion.
Overall the ENS is essential for Motility, Secretions, Water and electrolyte transport, Regeneration of epithelial cells, Blood flow and endocrine functions,  Mediation of immune system and May play a role in diseases such as ulcerative colitis and Crohn's Disease, though the exact cause and morphology of these disease are not yet fully understood.

The autonomic innervation of vasculature on the other hand has many similarities though does differ vastly. The sympathetic nervous innervation of the vasculature arises from the thoracic-lumbar section of the spinal cord similarly to that of the intestine While the parasympathetic innervation is also conveyed via the vagus though in contrast from a supraspinal source. In particular, the ANS control of blood vessel is essential for maintenance of the systemic arterial  blood pressure, with systemic arterial vasoconstriction being able to raise blood pressure several folds above normal. In contrast global vasodilation, which is common in sepsis, is life-threatening drop in BP. Though the vasculature is essential for the circulation and transport of blood around the body, unlike the GI tract and the ENS there is no intrinsic nervous activity of the vasculature and all input is from the autonomic branch of the peripheral nervous system (ANS). Vascular tone is maintained by the SNS acting on smooth muscle,in order to homeostatically maintain blood pressure, unlike in the intestine where sympathetic innervation results in smooth muscle relaxation. The main neurotransmitter used by the ANS in blood vessels similar to the within the GI tract, with noradrenaline acting mainly at alpha-1 receptors on arteries, to cause vasoconstriction. While Parasympathetic neurotransmitter Acetycholine acts on muscarinic receptor on target organ, in this instance vascular smooth muscle and effect vasodilation chiefly through the release of nitric oxide. The ANS neurotransmitters are released from varicosities which interface closely with the smooth muscle layer of blood vessels. However both SNS and PNS act on nicotonic cholinergic receptors on ganglia.



Image result for AND control of blood vessels








Image result for parasympathetic receptor types


Thus infusion of a nACh antagonist such as hexamethonium results in blockage of nACh activation in SNS and PNS ganglia. Blood vessels thus are vasolidated due to the availability of mACh of vascular smooth muscle not being affected. Thus the ANS control of vasulature is also contributes a major part of temperature homeostasis. Vasodilation is needed for heat loss through convection in the hot environments. With vasoconstriction of blood vessels, and shunting taking place in order to redirect blood to the core during cooler temperatures. Similarly in exercise, there is also a great vasodilatory response in vessels supplying the major muscle as well as in coronary vessels in order to accommodate increased muscle contraction and an increased heart rate. In contrast during exercise there is vasoconstriction of intestinal blood vessels in order to redirect to to the coronary and skeletal muscle circulation, and there is also maintenance of the GI tone, via sympathetic innervation.

The wide reaching effect of the autonomic nervous system is essential for life as seen though its actions on the systemic vasculature and within the GI tract.




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