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Communication between two neurons begins when an electrical impulse, called an action potential,

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travels along the axon of a presynaptic neuron toward the axon terminal.

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The action potential cannot cross the synaptic space.

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When it reaches the axon terminal, it causes membranous sacs, called vesicles,

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to move toward the membrane of the axon terminal.

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The membrane of the vesicle fuses with the membrane of the axon terminal,

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enabling the vesicle to release its contents into the synaptic space.

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The molecules released from the vesicles are chemicals called neurotransmitters.

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They drift across the synaptic space and bind to special proteins,

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called receptors, on the postsynaptic neuron.

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The binding of a neurotransmitter to its receptor

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can trigger an action potential in the postsynaptic neuron.

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That electrical signal then moves toward the cell body of the postsynaptic neuron.

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Now that the neurotransmitter has relayed its message,

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it releases from the receptor into the synaptic space.

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Some of the neurotransmitter is degraded by enzymes in the synaptic space,

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and some of the neurotransmitter is carried back into the presynaptic neuron

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through transporter proteins.

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The neurotransmitter that is taken back up into the presynaptic neuron

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is then repackaged into vesicles that can be released the next time

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an action potential reaches the axon terminal.

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The entire process repeats each time an action potential

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reaches the axon terminal of the presynaptic neuron.