How does a signal jump the gap between two nerve cells?

At most junctions between neurons, a nerve signal crosses the gap as a chemical message. Electrical activity at the end of the first cell is turned into the release of a chemical, a neurotransmitter, which binds to receptors on the next cell. The meeting point between the two cells is called a synapse.

At a synapse the membrane of the signal-passing cell, the presynaptic neuron, sits in very close contact with the membrane of the target cell, the postsynaptic one. In a chemical synapse, electrical activity in the sender is converted into the release of a neurotransmitter, which binds to receptors on the receiver. The message can say go or stop: glutamate acts as an excitatory neurotransmitter, in contrast to GABA, which acts as an inhibitory one. And the message is cleaned up fast. A released neurotransmitter is swiftly eliminated, absorbed back by the terminal that released it, taken up by nearby glial cells, or broken down by enzymes.

Nobody knew this a hundred years ago. Scientists argued about whether nerve cells were separate at all, and whether signals crossed between them electrically or chemically. In 1921 Otto Loewi dreamed of an experiment that would prove the transmission was chemical, not electrical, and went on to show that a chemical released by a nerve slows a frog's heart. Medicines such as Prozac work at this tiny gap: Prozac prevents the absorption of the neurotransmitter serotonin.