The three parts of a synapse are the presynaptic ending, the synaptic cleft and the postsynaptic ending. An impulse traveling from one nerve cell to another must pass through all three parts to successfully cross the synapse and to travel to the next neuron in the sequence.
Nerve impulses travel from the body of the nerve cell to its end through the axon terminal, which is a long straightaway. At the far end of this terminal lies the presynaptic ending. This structure contains pockets of chemical neurotransmitters that are released in response to the arrival of the impulse from the nerve.
Once released, neurotransmitters cross the synaptic cleft, which is just a short gap between the nerve cells. Unused neurotransmitters are taken back up by the presynaptic ending after the transmission is complete. The neurotransmitter proteins that reach their target are taken up by the third part of the synapse, the postsynaptic ending.
The postsynaptic ending has specialized receptor structures on its surface. These receptors accept neurotransmitters in the way that a lock accepts a key. The arrival of a transmission protein triggers an impulse in the postsynaptic ending that travels up a short terminal to the main body of the cell.