The human brain contains about 10 billion nerve cells, or neurons. On average, each neuron is connected to other neurons through about 10,000 synapses.
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell.
Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, and synapses are the means by which they do so.
At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process.