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Any animal with an eye has photoreceptors, which are essentially light-sensitive neurons. Green algae have channelrhodopsin, which are ion channels that open and close in response to light. Clearly, the nervous system and its molecular components have evolved to use light effectively.

I'm wondering if light can act as a neurotransmitter in the classical sense; that is, the presynaptic cell releases a photon, which travels across the synaptic cleft and interacts with a receptor, depolarizing the postsynaptic cell. Light can be generated in vivo through chemiluminescence, and as with channelrhodopsin, different proteins can activate in response to different wavelengths or intensities. I suspect this would be much faster than traditional chemical signaling.

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  • $\begingroup$ I'd be hesitant about using the term 'neurotransmitter' as it specifically refers to a chemical synthesised by the presynaptic neuron. But I see no reason why a neuron couldn't transfer electromagnetic radiation that is then picked up by another neuron (possibly via rhodopsin). Imo you could probably have a functioning signalling network with bioluminescent/light-sensitive cells but whether that actually occurs in nature, I'm not sure. $\endgroup$ – Jam Dec 19 '18 at 13:05
  • $\begingroup$ Hmmm i feel like a paper was just published about this. Let me go digging around to see if i can find it $\endgroup$ – theforestecologist Dec 20 '18 at 14:26
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I don't think there is a theoretical reason why this wouldn't be possible. However, I think there might be some practical problems with a system that relied upon light signaling.

1) It would be susceptible to interference. Many bio-luminescent organisms are transparent, so other sources of light could depolarize a post-synaptic cell. Light also penetrates tissue in organisms that are not transparent. Blue light can penetrate 2 mm of human tissue and higher wavelengths like red can more than double that. I could see light acting to replace signal molecules in quorum sensing though.

2) Neurotransmitters aren't really much of a limiting factor in nervous system speed. A monosynaptic reflex travelling a short distance might take 30 milliseconds, but neurotransmitter transmission is only 1 of those milliseconds. In more complicated neural pathways, the speed of neurotransmitters might only account for 1/100th of the time. So, if we're just imagining time-saving nervous system improvement, fiber optic neurons are the way to go.

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  • $\begingroup$ An emitted photon could go any direction, also go "inwards'. Indeed an optics would be needed. $\endgroup$ – Alchimista Dec 18 '18 at 16:16
  • $\begingroup$ Adding to the list of why it might be practically infeasible: it might be less energetically favourable for cells to create light than for them to keep using chemical transmitters. Also, it would require the ability of the organism to evolve a specialised way mechanism of bioluminescence, which might not confer any advantage to the organism. $\endgroup$ – Jam Dec 19 '18 at 13:10
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There are four main criteria for identifying neurotransmitters, these are very strict for the definition of something as a neurotransmitter or not.

  1. The chemical (in your case light) must be synthesized in the neuron or otherwise be present in it.
  2. When the neuron is active, the chemical must be released and produce a response in some target.
  3. The same response must be obtained when the chemical is experimentally placed on the target.
  4. A mechanism must exist for removing the chemical from its site of activation after its work is done.

I'm currently not aware of light fulfilling these criteria so it cannot be cataloged as a neurotransmitter. This is the definition I found in wikipedia, but you'll also find it in any neuroscience textbook like Kandel or Purves

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    $\begingroup$ The markup you used is reserved for program code. I deleted it and replaced it with bullets. Can you add references? $\endgroup$ – AliceD Dec 18 '18 at 17:54

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