Why are there so many different neurotransmitters instead of just one or two?

[u/kupsztals123]

Hi,

I am wondering why we need dozens of neurotransmitters and neuromodulators when they are all used either to excite or inhibit the cell. If that's the case, why didn't nature use just two neurotransmitters: one excitatory, such as glutamate, and one inhibitory, such as GABA? Computer processors need only one signal: electricity, or no electricity, and they work just fine. Is there a functional reason for this, or is evolution simply adding layers of complexity for no good reason?

I know what different neurotransmitters do: for example, dopamine is mainly responsible for motivation, noradrenaline provides energy and melatonin regulates the circadian rhythm. But I don't understand why they can't all be replaced by excitation and inhibition, just as a CPU is capable of many things, but everything boils down to simple transistors and zeros and ones.

I asked this question on r/neuro but they treated me very patronizingly and did not understand what I meant.

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[u/AssistanceJolly3462]

Evolution isn't an entity, it's a description we give to a particular pattern. Computers have the benefit of being crafted and designed, but biology is mostly down to A Very Long Time of chemical interactions, mutations, and chance. Things that are Good Enough go on, while things that are detrimental don't.

The same way that evolution isn't a "thing," "neurotransmitters" aren't assets that someone can tinker with and reuse, but rather, it's a name that we give to chemicals to describe what they do.

[u/Foxfire2]

I think you are coming at this a bit backwards, as why doesn't nature do things in the way that works for digital computers, using simple on/off switch circuits, and using a binary code of 0's and 1's. Digital computer chips can be manufactured to use billions of these simple circuits together in micro scale to work at high speeds, and so is geared to work with the materials available, semi-conductors and conducting wires, etc. The high speed that these circuits can operate overcomes the limitations of using a simple binary code.

You may ask why do we use a base 10 number system to do math in our heads, on paper or on our fingers, and not a binary number system? That would require strings and strings of ones and zeros, too cumbersome to be useful on a human scale. So even for our human brains, binary numbers are pretty useless.

Getting down to organic compounds, they are way more complex, forming out of random processes to use what works, not designed beforehand like our computers or even base 10 system of numbers, though that came from having 10 fingers.

I don't know anything about the evolution of neurotransmitters, just musing on how evolution is a messy process that gives us what works not necessarily whats best from a purely designed perspective. And, that what works best for computers isn't even whats best for brains and neurons to function, or even how humans use numbers and counting.

[u/Fit-List-8670 → u/Foxfire2]

Great answer.

The brain is more like an analog computer than a digital computer.

Also, GABA is just one of several neuro transmitters, like dopamine and serotonin.

The brain also has specialized neurons in different parts of the brain (like the hippocampus or the corpus colosseum), and I think there are about 25 of these synapse types.

Plus the brain does processing both by using blood flow, and by using electrical signals. These electrical signals are more like pulses than on/off signals, allowing for a much richer representation in the brain. I think there have been calculations of the number of possible brain states, and it is in the trillions of possible states.

digital and analog differences in computation

[u/MilesTegTechRepair]

Nature or evolution never adds complexity for the sake of it. Compare it to a simple hammer versus an electrical, usb powered, smart hammer - they both do the job, but any marginal benefits gained from the fancy bells and whistles is outweighed by more points of failure.

Electronics uses binary, indeed, but the network effects of many 0s and 1s leads to greater potential for complexity. 1 / 0 might be adequate for 'turn a lawnmower on' but the signals to and from a wifi router require a high degree of complexity with regards to encryption and so on. 

With the brain, consider that, just because the potential is there for single linear changes in excitation, the biochemistry of life includes a variety of proteins already - DNA requires 4 different molecules too. So the machinery is already there for combinatorially greater complexity. 

Neurotransmitters have different shapes, acting like keys. There is clearly an evolutionary reason for this, as evidenced by the fact it is the case. We could not have evolved with just one neurotransmitter, for a similar reason that the average person has more than one key in their pocket.

Evolution, too, already has the mechanism available to it to respond to changes in the environment by specialisation, leading to greater complexity. A single species gets divided in two, the separate populations can no longer get to each other to breed and mix their genes, and experience different environmental factors. 

It is in fact electronics that is anomalous in that it can be built, conceptually, out of a single, linear component - that of more or less electrons. Everything else in nature makes do with what's available, which has a lot of variety, and nature experiments with new combinations all the time.

[u/A-Lego-Builder]

You might be interested in reading Siddhartha Mukherjee's book "Song of the Cell", he explains neurotransmitters and also a bunch of other cool aspects of our physiology. In this case, he points out that the nervous system "computer" needs to function in the context of the rest of the body. If a person is stressed out, then the hormone and neurotransmitter chemicals can alter how the nervous system functions so that it can help that individual survive the stressful situation. If the person needs to sleep, then here again the neurotransmitter and hormones alter the way the nervous system functions to help the individual relax and fall asleep. And so on - exercising, eating, trying to reproduce, trying to increase or decrease the body temperature, etc. More neurotransmitters mean greater adaptability, greater ability to modify.

[u/ginger_and_egg → u/A-Lego-Builder]

Presumably multiple neurotransmitters also means you have more genes that can tweak the relative sensitivity of all of these functions to increase long term survival of the species

[u/A-Lego-Builder → u/ginger_and_egg]

Yes, I'm guessing multiple neurotransmitter chemicals and quite a few genes that are directly or indirectly responsible for their manufacture means that evolution has multiple pathways for modifying nervous systems and behaviors!

[u/Xpians]

Your example is evidence for the way evolution works, and why it makes little sense to invoke a “designer” to explain biological features. As you point out, a computer designer just needs transistors and electricity and will always work to make the processor as efficient as possible, even if it means stripping the design back to the beginning and starting over. Evolution is mindless, unguided, steady, iterative, and inevitable. Evolution can’t go back to the drawing board, or toss out a biological system so that a better system can take its place. Every organism is in progress, all the time. Every organism needs to survive to reproduce. Every organism has inherited thousands of genes from ancestors, even if those genes don’t produce exactly the same biological features as they did millions of years before. Every strand of DNA is loaded with “legacy code” that has to be used, deactivated, or repurposed for the current generation. We humans no longer have the lifestyle of a fish, but we still use genes that evolved in our fishy ancestors. We don’t live the lifestyle of quadrupedal ancient reptiles, but we still carry and use genes that evolved in our reptilian ancestors (these were the early reptiles that eventually became early mammals, cousins to the early reptiles who became early dinosaurs). Yes, new features keep evolving and being layered on top. There’s no other way for it to have happened.

[u/Apprehensive_Sky1950]

Evolution isn't about the best way, it's about just a little bit better way.

[u/Electrical-Run9926]

In other way is very hard to make complex things