I just looked in the introductory textbook we use for general biology, and it provides a good explanation of chemical neurotransmission between neurons. If you’ve studied basic biology, a diagram like this probably looks familiar:
I was impressed that the textbook mentioned for chemical neuromodulation. The presynaptic cell releases chemicals across the synaptic cleft, but the receptors don’t open ion channels. Instead, they interact with metabotropic receptors that cause biochemical cascades inside the neurons. These cause any variety of slow, long-lasting effects.
But there are at least three more ways that neurons can communicate.
Third, there are gasotransmitters: small, short lived gases that are made on the spot and zip through cell membranes like they weren’t even there. There are at least four different gases that do this (reviewed in Wang 2014). Nitric oxide is the best known in nervous systems.
Fourth there are electrical synapses. Neurons can be connected by gap junctions, which create pores in the membranes that allow ions to flow freely from one neuron to another. Consequently, an action potential spreads from one cell to the next as though they were one big cell. These were first described in crayfish, by the way (Furshpan & Potter 1957).
Fifth, there are ephatic signals, where the electrical potentials generated by spikes in one neuron create electrical potentials in adjacent neurons, even with no synapses or gap junctions between them. It’s just a spread of electrical activity. This was described at least as early as 1940 (Katz & Schmitt 1940), but I am ashamed to admit I had never heard of this until a few years ago (Su et al. 2012), even though the original effect was shown in the 1940s using motor neurons in crab legs! And learning about those was a big chunk of my doctorate.
I often find myself griping about how many people assume that brains work like computers. And I think part of that is because signalling by chemical neurotransmission seems very computational. I wish more people knew that there are at least four other ways that neurons can interact and influence each other. Maybe they wouldn’t be so quick to think that downloading our brain activity into computers is going to be easy.
References
Furshpan EJ, Potter DD. 1957. Mechanism of nerve-impulse transmission at a crayfish synapse. Nature 180(4581): 342-343. http://www.nature.com/nature/journal/v180/n4581/pdf/180342a0.pdf
Katz B, Schmitt OH. 1940. Electric interaction between two adjacent nerve fibres. The Journal of Physiology 97(4): 471-488. https://doi.org/10.1113/jphysiol.1940.sp003823
Su C-Y, Menuz K, Reisert J, Carlson JR. 2012. Non-synaptic inhibition between grouped neurons in an olfactory circuit. Nature 492: 66-71. https://doi.org/10.1038/nature11712
Wang R. 2014. Gasotransmitters: growing pains and joys. Trends in Biochemical Sciences 39(5): 227-232. https://doi.org/10.1016/j.tibs.2014.03.003
Picture from here.
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