must be replaced by other vesicles that are in different degrees of readiness. This process takes time, so if one action potential follows very quickly after another, there may be no vesicles ready to fuse during the second action potential. If there are no vesicles fused, no transmitter is released, so there will be no EPSP, and the post-synaptic neuron will not fire. 2)Neurotransmitter depletion. If the available pool of neurotransmitter is used up by the firing of previous action potentials, there is no neurotransmitter available to be packaged into vesicles (more must be synthesized, which takes time). Vesicles will continue to fuse, but no transmitter will be released so there will be no EPSP, and the post-synaptic neuron will not fire. 3)Voltage gated calcium channel inactivation. After a period of being open due to previous action potentials, voltage gated calcium channels inactivate, and it takes time for them to recover and be able to open again.
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