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Synapses are the connections between neurons, where one neuron can affect the activity of another. The signals from one neuron travel down its axon (or nerve fiber) to axon terminals (or terminal buttons) There axon terminals almost touch the dendrite of another neuron to form a synapse. The "typical" synaptic connection sends signals from the axon terminal button of one neuron to the dendrite of another neuron.
Figure 1 shows the main parts of a "typical" synapse.
Most synapses use chemical neurotransmitters (often called simply transmitters) to send signals at synapses. The nervous system has more than 50 different identified chemicals that act as transmitters. (Some of them are called modulators, because they only change the dendrites' sensitivity to other transmitters, but do not trigger signals themselves. I will not distinguish between these two kinds of transmitters.) It is estimated that perhaps hundreds more have yet to be identified.