Communication between two neurons begins when an electrical impulse called an action potential travels along the axon of a presinaptic neuron toward the axon terminal. The action potential cannot cross the synaptic space. When it reaches the axon terminal, it causes membranous sacks called vesicles to move toward the membrane of the axon terminal. The membrane of the vesicle fuses with the membrane of the axon terminal, enabling the vesicle to release its contents into the synaptic space. The molecules released from the vesicles are chemicals called neurotransmitters. They drift across the synaptic space and bind to special proteins called receptors on the postsaptic neuron. The binding of a neurotransmitter to its receptor can trigger an action potential in the postsaptic neuron. That electrical signal then moves toward the cell body of the postsaptic neuron. Now that the neurotransmitter has relayed its message, it releases from the receptor into the synaptic space. Some of the neurotransmitter is degraded by enzymes in the synaptic space and some of the neurotransmitter is carried back into the presinaptic neuron through transporter proteins. The neurotransmitter that is taken back up into the presinaptic neuron is then repackaged into vesicles that can be released the next time an action potential reaches the axon terminal. The entire process repeats each time an action potential reaches the axon terminal of the presinaptic neuron.