Neurotransmitters are stored in the axon terminal in small membrane-bound sacs called synaptic vesicles. These vesicles are located in the presynaptic neuron, specifically in the axon terminal, where they are prepared to release their contents into the synaptic cleft in response to an action potential.
The process of storing neurotransmitters in these vesicles involves several steps:
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Synthesis: Neurotransmitters are synthesized in the neuron, either in the cell body (for neuropeptides) or in the axon terminal (for smaller neurotransmitters).
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Loading into Vesicles: Once synthesized, neurotransmitters are transported into the synaptic vesicles by specific transporter proteins located in the vesicular membrane.
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Storage: The filled vesicles are then stored in the axon terminal until an action potential arrives. The presence of calcium ions (Ca²⁺) plays a critical role in triggering the release of these vesicles when an action potential reaches the axon terminal.
When the action potential arrives, it causes depolarization of the presynaptic membrane and the opening of voltage-gated calcium channels, leading to an influx of calcium ions. This increase in intracellular calcium concentration prompts the synaptic vesicles to merge with the presynaptic membrane and release their neurotransmitter contents into the synaptic cleft, facilitating communication between neurons.
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