The texts, online materials describe the velocity of conduction of action potential depends on axon's length and cross sectional area e.g. short length and large diameter decreases the resistance. Myelin sheath increases the speed of conductivity as well.

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Can someone explain why chemical synapses allow quicker conduction than regular arrangement of sodium fast channels along the axon? Can regular arrangement of Na channels help in propagation of AP (I guess it's referring to the nodes of Ranvier)? What's the difference between fast sodium channel and a 'normal' sodium channel?

The question is: Of the following, which allows action potentials to conduct quicker? 1. Chemical synapses 2. Regular arrangement of sodium fast channels along the axon 3. Small axon diameter 4. Depolarisation suppression through the majority of the axon's length

I picked 2, but the answer is 1. I want to know the reason behind me being wrong and that being correct. 1 doesn't make sense at all to me. Come to think of it, I see that 4 could be the most relevant.

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    $\begingroup$ "Can someone explain why chemical synapses allow quicker conduction than regular arrangement of sodium fast channels along the axon?" - This part of your question doesn't make any sense to me. Chemical synapses are for communication between cells. Do you mean nodes of Ranvier and saltatory conduction? $\endgroup$ – Bryan Krause Apr 27 '17 at 0:28
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    $\begingroup$ 1 is definitely wrong, it is a nonsense answer in this context - your confusion is understandable. None of those answers make much sense to me actually...2 is the closest, but that's not what allows an action potential to conduct "quicker" it's what allows it to conduct "at all." $\endgroup$ – Bryan Krause Apr 27 '17 at 5:53
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    $\begingroup$ Agreed, the question asks about action potentials, strictly speaking, chemical synapses have nothing to do with "conduction" of action potentials, only the transfer of information (initiation/suppression of AP's in the postsynaptic cell). 4. i.e. myelination looks the most likely to me to. $\endgroup$ – Oliver Houston Apr 27 '17 at 7:24
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    $\begingroup$ Basically, the myelin sheath prevents AP's in coated areas, as there's no sodium channels. Therefore action potentials only occur at the nodes of ranvier... Ions diffuse along the axon from one node to the next, depolarising the next node of ranvier enough to trigger an AP, more sodium floods in and diffuses along and so on. It's not well covered, the wikipedia description is OK, but the idea of hopping? I think it's a bit outdated and oversimplified by professors. $\endgroup$ – Oliver Houston Apr 27 '17 at 7:46
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    $\begingroup$ One note about "depolarisation suppression" is that it's not even correct. You don't need depolarisation, you just need current moving (e.g., a chloride shunt) to change the electrotonic structure. $\endgroup$ – Devon Ryan Apr 27 '17 at 11:13

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