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In neuroscience we learn that when the membrane potential of a neuron reaches a threshold (typically around -55mV) it "spikes": That is, it actively propagates a signal. I have two related questions with this respect:

  1. The spike initiation zone is typically (e.g. for mammals) at the axon hillock - from there the action potential is actively (opening of ion channels..) propagated through the axon. But what happens then at and after the (chemical) synapse? Is the propagation after the synapse to the postsynaptic cell passive?

  2. Newer imaging techniques (e.g. calcium imaging) can capture sub-threshold changes in the membrane potential. How are these sub-threshold potentials relevant for information processing? Are they propagated to postsynaptic cells, albeit again only in a passive manner?

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    $\begingroup$ For an interesting reversal, check out C. elegans neuroscience. In C. elegans most communication occurs by sub-threshold potentials, and detection of action potentials is rare 2008, 2018 and controversial. Not my field, but it might be worth checking out C. elegans neuro-transmitter release absent action potentials. $\endgroup$
    – Luke
    Aug 17, 2020 at 0:12

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This type of passive conduction (sub-threshold) is called electrotonic conduction. When an action potential reaches the axon terminal (pre-synaptic knob) it induces Post-Synaptic Potential (PSP), through chemical or electrical synapse. Now if there is EPSP (i.e. excitatory) generation, then in the post-synaptic neuron there will be electrotonic potential, that will move towards 'axon hillock'. enter image description here

Upto the axon hillock, the conduction is mostly electrotonic and hence we need this type of conduction to actually generate Action Potential.

enter image description here

In theoretical neuroscience this electrotonic conduction along dendrites is computed by using Cable Theory. It will eventually die out with distance as$-$

$V(x)={V_o}\, e^{-\frac{x}{\sqrt{r_m/r_i}}}$ ; standard notations used.

ref article: https://www.sciencedirect.com/science/article/pii/B9780123971791000178

Hence we can conclude that for information transmission, sub-threshold potentials are extremely important.

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  • $\begingroup$ If these are subthreshold, does that theoretically also means that they can travel both way (as closed ion channels do not prohibit that)? $\endgroup$
    – TestGuest
    Aug 16, 2020 at 21:16
  • $\begingroup$ Theoretically yes. If there is potential gradient it will flow. $\endgroup$
    – ANA negative
    Aug 17, 2020 at 1:59
  • $\begingroup$ All voltage perturbations can propagate in both directions. Usually, they are called orthodromic and antidromic if propagation is from dendrites to axon or viceversa respectively. $\endgroup$
    – heracho
    Aug 20, 2020 at 22:43
  • $\begingroup$ @heracho what then actually hinders the current flow in the dendrites, stemming from an action potential, do reverse direction? In axons we know that channels close to hinder exactly that - but how about dendrites? $\endgroup$
    – TestGuest
    Nov 22, 2020 at 23:15
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    $\begingroup$ @TestGuest As I understand, the voltage perturbation propagates passively through the dendrites. This is because they don't have the Sodium channels (or another) necessary to actively propagate the signal. To be more precise in your comment, it's Sodium channel inactivation (not just closing) the mechanism that hinders the action potential to propagate antidromically in the axon. $\endgroup$
    – heracho
    Nov 24, 2020 at 16:02
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Electrical synapses (Gap junctions) can generate current to other cells without the emission of a spike. This sub-threshold interaction is proven to have functional implications in neural activity (eg. Retina).

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  • $\begingroup$ Thank you, that is good to know, though I was also interested in this property wrt chemical synapses. $\endgroup$
    – TestGuest
    Aug 16, 2020 at 21:17

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