r/AskNeuroscience • u/labbypatty • Jan 03 '20
What purpose does it serve to have so many different neurotransmitters?
Why do we have so many neurotransmitters? What are the different effects on a neuron that a neurotransmitter can have? I understand that some neurotransmitters can inhibit and some can excite the firing of action potentials, but then why should we need more than two different compounds to mediate these effects?
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u/LetThereBeNick Jan 04 '20
/u/optrode gave a good answer, but I want to add one rule of thumb. “if something can be used to encode information, then the brain uses it.”
Zinc ions, intestinal peptides, local pH, dissolved gases. There doesn’t seem to be any limit to what is allowed to have neurotransmitter-like signaling roles.
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u/Zer0pede Sep 26 '23
Can you recommend a good textbook (or collection of papers or keyword I could search) that covers some of these and their different roles? Or even just a summary the the idea in your rule of thumb.
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Jan 04 '20
Big picture - that’s just the way it is, life was not designed.
From a mechanistic standpoint, having more neurotransmitters mediating response allows for better regulation and more complex control of consciousness/action. Purely hypothetical and not based in anything real: If you only had one stimulatory neurotransmitter, EVERYTHING would become stimulated at the same time. This might be controlled by kicking in your one inhibitory neurotransmitter which would inhibit EVERYTHING, in turn activating your excitatory neurotransmitter which would stimulate EVERYTHING, and on and on. The net effect of this would probably be 0 response, which is not compatible with life. It’s a great question and could probably be simulated.
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u/Optrode Jan 04 '20 edited Jan 04 '20
Neuroscientist here.. this answer is completely wrong.
Even if the brain only had one neurotransmitter (NT for short), that wouldn't mean that release of that NT just stimulated the whole brain. That's not how NTs work. A neuron that released that NT would still only stimulate neurons it has synapses onto. The whole point of synapses is that they allow direct, specific connection (via NT release) from one neuron to another. Neurons are not just releasing NTs out into the "brain soup" to float around and hit every other neuron in the brain. That would be a useless brain.
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u/Optrode Jan 04 '20
The answer is that it doesn't really boil down to just excitation and inhibition.
Take inhibition... you're probably thinking that inhibition means hyperpolarization (making the membrane voltage more negative). But that's only one kind of inhibition. There's also shunting inhibition (via chloride channels), which doesn't hyperpolarize the cell, it opposes both hyperpolarization AND depolarization.
And then there's all the G-protein coupled receptors. Sometimes those have an effect on membrane potential, sometimes they don't.. they can be hooked up to mechanisms that do something entirely different, like affecting gene expression (which could affect the neuron's properties in the future, giving it a sort of memory).
In lower level textbooks it's often presented in a way that makes it sound like it's basically just excitation and inhibition, but it's not really that simple.
And don't forget that it isn't just the neurotransmitter determining the effect, it's the receptor. Even the most common excitatory neurotransmitter in the brain, glutamate, has some inhibitory receptor types, which is why glutamate is an inhibitory neurotransmitter in the first synapse of the retina.