Acetylcholine is a prevalent neurotransmitter that is used bynerve cells to exhibit control over the heart, lungs and muscles.

This is foundin the central nervous system as well as the peripheral nervous system. It is released as a response of theparasympathetic nervous system, which is a calming, rest response. Electricalimpulses are received from the heart through the vagus nerve and sympatheticnervous system fibre’s. As the parasympatheticnervous system dictates this will cause a reduction in heart rate. The heartcontains muscarinic G-protein metabotropic receptors. The ligand, acetylcholinewill bind leading to a cascade of events.

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A second messenger produced willprolong the duration of which the potassium ion channels are open for and theclosing of calcium channels. As the potassium channels are open for longer periodsof time, the cell will be become hyperpolarized for longer due to the membranepotential dropping. When the cell is hyperpolarized a new action potential isnot able to be initiated therefore a reduction of heart rate will be a result.The closing of calcium channels will also aid in the reduction of the actionpotential and reduced force of contraction.This demonstrates amyogenic heart, which is found in all vertebrates and some invertebrates.

Daphnia however is unique in that acetylcholine does not have an excitatoryeffect like it does on most arthropods. Rather it has a depressing effect onheart rate. Another distinction yet similarity to humans is that daphniado not have a neurogenic pace maker unlike most arthropods but a myogenic pacemaker(Bekker and Krijgsman, 1951)A slowdown in heart rate will cause the rate of metabolism tobe reduced, it could be hypothesised that an increase in production ofacetylcholine is a way to survive in colder temperatures where the level ofenergy available is limited.



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