The tiny gap through which communication between two neurons takes place.
Every thought, movement, and sensation occurs due to communication between different neurons, which provide information throughout the nervous system. Within a single neuron, information proceeds through electrical signals, but when information must be transmitted from one neuron to a succeeding neuron, the transmission is chemical.
For two neurons to communicate, chemical messengers, or neurotransmitters, are released into the synaptic cleft (a tiny gap about one thousandth of a millimeter between neurons), at which point they migrate to the next neuron and attach themselves to locations called receptor sites. The result is an initiation of electrical current that moves through that neuron toward the next one. After the neurotransmitter exerts its effect, it is either destroyed by other chemicals in the synaptic cleft or is reabsorbed into the original neuron. This action prevents the neurons from becoming overstimulated.
When neurons communicate, the effect can be either stimulation or inhibition of the next neuron. For example, when a person pays attention to one conversation and ignore others, the neurons in the brain are actively seeking out that information (stimulation) and actively ignoring the rest (inhibition). Neurons come in different shapes and sizes, affecting many other neurons, and can have different numbers of synapses. Some neurons, called Purkinje cells, may have as many as 100,000 synapses.