Synapse

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The synapse is a the specific site where neuronal signals are transmitted from cell to cell. It is a structure where the electical signals of one neuron are converted to chemical signals in order to transmit information from one nerve cell to the next nerve cell(s). Postsynaptic cells are separated from presynaptic cells by a synaptic cleft. The electrical potential in the presynaptic cell changes which causes the cell to release a neurotransmitter signalling molecule by exocytosis. The neurotransmitter moves across the synpatic cleft and triggers an electrical change in the postsynaptic cell. [1]

What happens at the Cholinergic Synapse?

Transmisson of an action potential across a synapse commonly involves Acetylcholine (Ach), a common neurocrine, neurones which secrete Ach are cholinergic.[2]

  • An action potential arrives at the pre-synaptic membrane causing voltage-gated Ca2+ ion channels to open. Ca2+ moves in through the pre-synaptic memebrane, down the chemical gradient. 
  • This influx of Ca2+ results in vesicles containing Ach to fuse with the pre-synaptic membrane, Ach is released into the synaptic cleft by exocytosis
  • Ach diffuses across the synaptic cleft and binds to specific neuroreceptor sites on the post-synaptic membrane. 
  • Binding of Ach opens Na+ ion channels on the post-synaptic membrane. Na+ ions diffuse across the post synaptic membrane into the post-synaptic neurone (down the chemical gradient).  This causes depolarisation of the membrane which in-turn initiates an action potential
  • The enzyme acetylcholinesterase rapidly breaks down Ach in the synaptic cleft (into choline and acetyl CoA) and the choline is transported back into the axon terminal where more Ach is synthesised [3]

References

  1. Bruce, A, Johnson, A, Lewis, J, Raff, M, Roberts, K and Walter, P (2008). Molecular Biology of the Cell. Fifth Edition. New York: Garland Science, Taylor & Francis Group, LLC. p682.
  2. Silverthorn. D.U., 2007. Human Physiology an Integrated Approach. 4th Edition. San Francisco: Pearson Education Inc.
  3. Silverthorn. D.U., 2007. Human Physiology an Integrated Approach. 4th Edition. San Francisco: Pearson Education Inc.
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