One would assume that a faster response time in the nervous system would be beneficial. However, nerve cells have to convert electrical impulses to chemical signals and cross a synapse. Why didn't nerve cells evolve in such a way as to transmit signals purely electronically, and what was the evolutionary need for the current, more complicated structure of the nerve cell?

I've seen answers on how synapses ALLOW for more complicated structures, but never how the synapse came to be. Is it even possible to have a brain structure without converting between electrical and chemical signals? If not, why not? If so, why synapses?

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    $\begingroup$ You are mingling two questions together. I would suggest editing this one to only ask about the difference between electrical and chemical synapses, and then open a new question asking about the evolutionary story of synapses. Be careful not to ask a question that's too similar to this one. $\endgroup$ – James Dec 2 '15 at 6:28
  • $\begingroup$ I concur with @James - there's a concoction of evolution (question body) and neurophysiology (title). The answer below is quite apt but receives the dreaded "yes, but that's not what I'm asking". Please follow James' instructions and make the current questions the physiological counterpart, because that received an answer from Ben2209 $\endgroup$ – AliceD Dec 7 '15 at 8:40

There are two types of synapses:

  1. Chemical synapse
  2. Electrical synapse

The first one is the one you are asking about. The second one corresponds to the faster synapse you are imagining. It consists of two neurones connected by a gap junction. Gap junction form a cytoplasmic bridge between the neurones and thereby allow electrical signal to directly go from one neuron to the next through ion diffusion without converting to chemical signal.

The second one implies release of chemicals (neurotransmitter) in the synaptic cleft, the extracellular space between the two neurons. It is indeed slower than the electrical synapse. However, unlike the electrical synapse, it enables a variety of postsynaptic effects. Some neurotransmitter elicit excitatory effects, some elicit inhibitory effects (electrical synapses where the presynaptic side is an axon only allow excitatory effects). Some postsynaptic receptors to neurotransmitter are ionotropic whereas some are metabotropic. The former are neurotransmitter-gated ion channels, which means that they let specific ion cross the postsynaptic plasma membrane when they bind neurotransmitter. The latter trigger intracellular chemical reactions, such as production of cAMP, which is a companent of a signalling pathway and can for example influence the expression level of specific genes.

This more diverse set of postsynaptic response enabled by chemical synapses explains why they dominate our nervous system.

Reference: Purves et al., Neuroscience, 5th ed., 2012 ISBN 978-0-87893-695-3

  • $\begingroup$ Thank you for the answer. I understand now why chemical synapses are more useful, but I still wonder how they came to be evolutionarily. How could there have been a jump from the creation of electrical synapses in organisms to chemical synapses (or vice-versa)? $\endgroup$ – rainwaffles Dec 1 '15 at 7:00
  • $\begingroup$ Please consider adding a couple of references to you answer. $\endgroup$ – fileunderwater Dec 2 '15 at 10:47
  • $\begingroup$ why would an electrical synapse only allow for excitatory events? Gap junctions couple cells in a tissue and mediate, e.g. oscillatory behavior, i.e. wave-like spread of activity. I think you lay out an incorrect view of electrical synapses. $\endgroup$ – AliceD Dec 7 '15 at 8:36
  • $\begingroup$ +1 for your answer btw, but some additional referencing and adjustment of the function of gap junctions may improve your answer. $\endgroup$ – AliceD Dec 7 '15 at 8:53
  • $\begingroup$ @Christiaan: yes you are right the sign of postsynaptic current equals the sign of presynaptic current in case of electrical synapses. When saying they are excitatory I was referring to synapses involving an axon. An action potential is always hyperpolarizing, thus the postsynaptic effect is excitatory. $\endgroup$ – Ben2209 Dec 7 '15 at 21:25

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