The vesicles containing neurotransmitters fuse in response to calcium signals at the synapse.

Chemical synapses are specialized junctions through which the cells of the nervous system signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow the neurons of the central nervous system to form interconnected neural circuits. They are thus crucial to the biological computations that underlie perception and thought. They provide the means through which the nervous system connects to and controls the other systems of the body. A chemical synapse between a motor neuron and a muscle cell is called a neuromuscular junction; this type of synapse is well-understood. (wikipedia)