A synapse is the specialized junction where one neuron communicates with another cell—usually another neuron, but sometimes a muscle or gland cell. Most synapses in the mammalian brain are chemical: the presynaptic terminal releases neurotransmitter into a narrow synaptic cleft; the transmitter binds to receptors on the postsynaptic membrane and alters its excitability.

Electrical synapses also exist. At these gap junctions ions flow directly from one cell to the next, allowing very rapid and often bidirectional transmission. Chemical synapses, by contrast, are typically unidirectional and can be modulated in strength—the basis of much learning and memory.

A single neuron may form thousands of synapses. The collective pattern of excitation and inhibition arriving at a cell determines whether it will fire. Synaptic plasticity—long-term potentiation and depression—allows experience to reshape these connections.

• Junction for intercellular communication
• Chemical or electrical
• Site of neurotransmitter release and reception
• Capable of lasting changes in strength

Almost every aspect of neural computation depends on the properties of synapses.