What’s the point of neurotransmitters, and why not just have a long neuron?

Tim Chiswell has an interesting answer, but it doesn’t technically address this question.

Neurotransmitters likely evolved because of long neurons, not in spite of them. A long neuron is a useful tool, but it’s just a cellular extension that doesn’t do much by itself. It is the neurotransmitters that actually communicate information from that long neuron to its downstream targets (i.e. the dendrites of other neurons).

There are several reasons why you can’t just have a “direct” connection between two cells, including poor conduction, lack of signal regulation, and (as Tim talked about) just plain inefficiency.

Neurotransmitters are short distance communication molecules—they don’t work over distances much longer than a few Nano-meters, because neurons and glia clean them up very quickly.

Hormones, on the other hand, can circulate through the bloodstream and effect many targets throughout a wide area. But they are entirely uncontrolled. You couldn’t signal to a specific cell with a hormone.

Therefore, neurotransmitters evolved to facilitate long-distance communication through very short-distance, precision signalling.

Neurotransmitter release happens at synapses, which are tiny tiny gaps between axons and dendrites.

Any neurotransmitter that leaks out beyond the synaptic cleft gets recycled back into cells.

Long story short, neurotransmitters exist because of the extreme length of axons.

Reference: Ben Callif