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Nerve impulses of course go really fast, but the neurotransmitters have to travel through millions of nerves, so how can it go that fast?

Is it a sort of relay with lots of neurotransmitters taking over at each nerve cell, or do the neurotransmitters go all the way from A to B?

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    $\begingroup$ What sparked your question? Were you reading something? Watching a movie? If so, what? A little background might help me answer it in a way that fills the gap you want to fill. The reason I ask is that your question seems to hint at a subtle misunderstanding about what a neurotransmitter is. Have you read this? $\endgroup$ – Kevin Ford The Submariner Mar 4 '13 at 18:11
  • $\begingroup$ It's beginners level Psychology. I guess they're more like chemicals that aid passing of electrical signals? $\endgroup$ – DarkLightA Mar 4 '13 at 18:20
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You've got a few things mixed up here:

A signal may travel down a pathway, passing through several neurons (e.g. around 4-8). However, this signal is not always in the same form: From one end of a neuron to the other, it is merely an electric potential which travels down the neuron's cell membrane (from the dendrite to the axon hillock, where the cell decides whether to carry the signal on; if yes, a new potential is sparked to travel down the axon).

When the potential reaches the end of a neuron, i.e. an axon terminal, it causes exocytosis of neurotransmitter. The signal has become chemical now. This diffuses through the synaptic cleft to the next neuron, where it acts on ion channels in the membrane to cause a new electric potential.

So the actual distance a neurotransmitter travels is only the distance between the two membranes at the synapse, i.e. the synaptic cleft. This is only about 20-40 nm wide, so it's fairly rapid :)

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  • $\begingroup$ Huh, thanks! You mention it only being 4-8 neurons. I can't imagine there being so few between, say, the fingers and the spine? How long are these things? $\endgroup$ – DarkLightA Mar 4 '13 at 18:34
  • $\begingroup$ @DarkLightA some are pretty long. In a human body, they can be more than a meter in length. $\endgroup$ – terdon Mar 4 '13 at 19:46
  • $\begingroup$ Wow! I read there are several billion of them? Is this because every nerve ending has its own connection to the spinal chord? $\endgroup$ – DarkLightA Mar 4 '13 at 20:14
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    $\begingroup$ @DarkLightA Yes, that's probably around the number of neurons you have in total in your body. The actual pathway that a single signal will follow doesn't usually comprise that many cells though. Example: pain (e.g. papercut) -> cell 1: sensory neuron from periphery (e.g. finger) towards spinal chord -> cell 2: afferent neuron from spinal chord towards the thalamus in the brain -> cell 3: thalamic neuron into various brain regions (exact number of cells here varies as far as I know) -> result: you feel pain. Could be I forgot the odd neuron here or there in this pathway but that's the gist. $\endgroup$ – Armatus Mar 5 '13 at 13:12
  • $\begingroup$ The bit inside the brain could be quite a lot I think, hundreds or thousands even. Plus, a single stimulus doesn't activate just a single sensory neuron, and there are also cross-interactions of adjacent pathways. $\endgroup$ – Armatus Mar 5 '13 at 13:14

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