Synapses are microscopic gaps that separate the terminal buttons of one neuron from receptors (usually, located on the dendrites) of another neuron. When neurons communicate, they release chemicals that must travel across this gap to stimulate the post-synaptic receptors. The human brain contains trillions of neurons, although it contains about 100 billion neurons. Thus, on average, each neuron communicates through tens of synapses, although in reality the range is very wide – some neurons terminate in few synapses, while others may be involved in communicating through thousands of synapses. Neurons also communicate with other cells. For example, neurons that control the motor system communicate with muscle cells and synapse with them. Although the typical structure of a synapse involves a gap that separates axonic terminal buttons of the pre-synaptic neurons with dendritic receptors of the post-synaptic cells, other types of synapses also exist.