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Calcium channels play a crucial role in neuronal signaling by helping the synaptic vesicles to fuse through the synaptic active zone and release their neurotransmitters. My question is, at a given moment, what determines the influx of calcium ions in these channels? Where are the ions coming from and what is their source? Is the influx determined by the source by changing the ion flow, or the ion flow is constant and the influx of ions into the synapse is only regulated by opening/closing of the channels?

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So, let's use a neuron to neuron connection as an example.

Calcium is released into the neuron when voltage gated ion channels, specific to calcium, open. These channels can only open when there is a significant depolarisation in the axon terminal of the neuron.

Where do the calcium ions come from? The majority of them are outside of the cell by the plasma membrane and adjacent to the synaptic vesicles.

I'm not too sure what you mean when you ask what the calcium ion's "source" is. It is common for calcium ions to sit outside the cell, not just in neurons, to help maintain potential difference for example.

And yes, the influx of calcium in a neuron is determined by the opening of voltage gated calcium channels.

Sources: https://web.williams.edu/imput/synapse/pages/IIA1.htm , https://en.wikipedia.org/wiki/Calcium, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249630/

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