synaptic cleft

The synaptic cleft is the tiny gap between two neurons at the point where one passes a message to the next. Even though we often picture brain cells as wired together, most of them never actually touch. Instead, the sending cell stops just short of the receiving cell, leaving a sliver of space — only about twenty millionths of a millimeter wide — filled with watery fluid. That sliver is the synaptic cleft, and it is where the conversation between neurons happens.

An electrical signal cannot simply leap across this gap, so the sending cell uses a chemical messenger instead. When its signal arrives, the sending side releases tiny packets of a chemical called a neurotransmitter into the cleft. These molecules drift across the narrow space and land on matching receptors on the receiving cell, like a key sliding into a lock, telling that cell to fire its own signal or to quiet down. Because the gap is so small, the message crosses in well under a millisecond; and because it is a separate, watery space, the brain can mop up leftover neurotransmitter quickly and keep each signal crisp and brief rather than letting it smear on.