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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.

canadianer
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bonCodigo
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    "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? – Bryan Krause Apr 27 '17 at 00:28
  • Also what do you mean by "an regularity or irregularity help in propagation of AP"? As far as fast vs "normal" sodium channels...I don't know what you mean there either. Different voltage gated channels can have slightly different speeds, but in general all voltage gated sodium channels are pretty fast. At the level you are learning at there is no reason to distinguish between faster and slower sodium channels, unless you talk about hypotheticals like "what would happen if sodium channels were faster/slower?" – Bryan Krause Apr 27 '17 at 00:31
  • @BryanKrause that's precisely my confusion as well. This was a test paper question not homework. Let me type the question exactly as it is, perhaps you may get a better grasp of the context than I do. Also not a duplicate at all. Because I am really not concern about the myelinated or unmyelinated factor of the axon. It is obvious which is stated in my question. I added the image from wiki (which just happened to be referring to saltatory conduction), I am eager to know the general, main concept of AP conduction on any neuron. – bonCodigo Apr 27 '17 at 04:30
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    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." – Bryan Krause Apr 27 '17 at 05:53
  • My only guess as to why (1) is supposed to be correct is that an electrical synapse would make an axon electrotonically longer (this is a bit hand wavy in my mind). But honestly, your Prof did a terrible job writing that question. – Devon Ryan Apr 27 '17 at 07:02
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    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. – Oliver Houston Apr 27 '17 at 07:24
  • @OliverHouston and the rest here, could you please clarify this a bit?depolarisation suppression through.... I understand the basic concept of how myelinated sheath supports the AP propagation. However I do not get the suppression bit. So for my own learning and knowledge, if you could elaborate or point to a comprehensible resource (to 1st year Bio level) : that would be great. – bonCodigo Apr 27 '17 at 07:31
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    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. – Oliver Houston Apr 27 '17 at 07:46
  • @OliverHouston a lot of articles is referring to Depolarisation induced suppression. After this discussion and thanks to you all, as a take away I want to understand depolarisation suppression. Perhaps the jargon is strangling me a little. – bonCodigo Apr 27 '17 at 07:50
  • @DevonRyan I really wish it was my Prof who wrote it. It is from a national test paper... actually international - sample revision paper. I shall take the rest of the 120 questions with a pinch of salt... – bonCodigo Apr 27 '17 at 07:52
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    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. – Devon Ryan Apr 27 '17 at 11:13
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    @bonCodigo Eek, good luck with that then! – Devon Ryan Apr 27 '17 at 11:14

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