How the signal shuts off at the synapse

Two neurons never touch: a tiny gap is left between them, the synaptic cleft. To talk to each other, the first one releases substances called neurotransmitters into that gap, and they cross it and are read on the other side. Reuptake is what comes next: it means removing that neurotransmitter from the cleft, and either the neuron itself or the cells around it can do it.

The same neuron that released it can pick it back up, using specialized doorways on its surface called transporters. It is like picking up off the floor the piece of paper you just dropped instead of leaving it there. In the case of norepinephrine, this is the main way it leaves the gap.

It is not the only cleanup crew. Glial cells, the support cells of the nervous system, live around the neurons and also take part. With glutamate, for example, that is how it works.

Not everything can be picked back up, though. Only small-molecule neurotransmitters get taken back in. Neuropeptides, which are larger messengers made of chains of amino acids, are cleared by other routes, such as being broken into pieces.