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In order for the axon to initiate an action potential, we know that the axon initial segment must be brought to threshold. So my question is as follows:

Say we have the minimum charge input, "X", necessary to depolarize the axon initial segment of Neuron 1.

Now, we have Neuron 2, which has a larger soma. Will this same input X be sufficient to depolarize the axon initial segment of Neuron 2?

I am trying to explore how physical concepts like capacitance manifest in biological systems. Neuron 1's soma (approximated as a sphere) presumably has a lower capacitance than Neuron 2's soma (approximated as a sphere), due to the difference in cross sectional area of the somas. Therefore, I would assume that Neuron 2's axon initial segment requires greater input to depolarize than Neuron 1's axon initial segment.

Is this simplistic idealization of somas actually observed in experiments?

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  • $\begingroup$ Your intuition is correct, in order to attain the depolarization necessary to trigger an action potential you need more charge in the bigger neuron. This is simply because you have a larger capacitor. $\endgroup$
    – BPinto
    Apr 22, 2019 at 3:22
  • $\begingroup$ Great! Knowing this, could you please explain then, WHY larger diameter axons convey action potentials more quickly than smaller diameter axons? (assume they are both equally myelinated) Shouldn't the larger diameter axons "require more charge to depolarize" and therefore actually take longer to convey an action potential? $\endgroup$
    – S.C.
    Apr 22, 2019 at 3:29
  • $\begingroup$ Larger axons also have more sodium channels, so they move more charge. The reason why larger axons have faster conduction is because the have less internal resistance. $\endgroup$
    – BPinto
    Apr 23, 2019 at 1:41
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    $\begingroup$ I will say that the main source of your confusion is between propagation velocity and charge needed for depolarization. They are related, but they don`t mean the same. $\endgroup$
    – BPinto
    Apr 23, 2019 at 2:40
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    $\begingroup$ I really recommend that you run neuron simulations to see how the different parameters you`re interested affect conduction velocity. This program will allow you to play with myelination and axon diameter neuron.yale.edu/neuron $\endgroup$
    – BPinto
    Apr 23, 2019 at 11:53

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