What is a synapse and how do neurotransmitters cross it?

A synapse is the tiny gap between the axon terminal of one neuron and the dendrite of the next neuron. When an action potential reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters from synaptic vesicles into the synaptic cleft. These neurotransmitters cross the gap and bind to receptor sites on the receiving neuron, either exciting or inhibiting it. After transmission, neurotransmitters are cleared from the synapse through reuptake back into the sending neuron, enzymatic breakdown, or diffusion.