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There are two stereotyped statements that I have seen during my coursework regarding electric properties of neurons:

  1. Large diameter axons propagate action potentials more quickly than small diameter axons
  2. Large motor neurons are the last motor neurons to be recruited

Separately, I have always understood these statements through the following lens:

Explanation for 1. (for arguments sake, lets talk about unmyelinated neurons...so, for example, a C-fiber with a small diameter and a C-fiber with a slightly larger diameter). The reason the larger diameter neuron will propagate its action potential more quickly is because, in a given cross section of the axon, there are more voltage gated sodium channels present along the circumference. Consequently, the speed with which the incoming sodium signal is generated is quicker. I am aware of "reduced longitudinal resistance" for larger axons, but I am purposely ignoring this...please continue reading.

Explanation for 2. Idealizing the soma of a neuron as a sphere, we know that capacitance increases with increasing radius (related to the distribution of charge along a 'longer' circumference). Consequently, for a large soma, the amount of charge required in order to observe a change in potential difference across the membrane is MORE than it would be for a smaller soma.

When treated individually, both of these statements are intuitively "okay". However, when I think of these two statements together, confusion arises:

A. Shouldn't the increased total number of ion channels observed in the larger diameter axon also be observed in the larger soma?

B. Shouldn't the capacitance increase observed in the larger soma also be observed in the larger unmyelinated axon?

If both of these statements are equivalently applicable in both cases, why do the two statements "1" and "2" seem fundamentally different?

The only rationale I can think is the following: The % of surface area occupied by ion channels on the soma membrane is significantly less than the % of surface area occupied by ion channels on the axon membrane. Consequently, the axon can offset its increased capacitance by the gain in additional ion channels. However, the soma cannot offset its increased capacitance because the % of ion channels occupying its surface is below this "capacitance offset" value.

Edit:

I have provided two pictures to help clarify my question:

Soma Size and Passive Propagation of Membrane Potential

Axon Size and Passive Propagation of Membrane Potential Through Myelinated Segments

As you can see from the above two images, the values in the two tables are the exact same. Summarizing, the larger soma and larger myelinated segment of axon both exhibit:

  1. Less Longitudinal Resistance

  2. Greater Capacitance

  3. Greater Surface Area Available for Ion Channels

I have previously asked two separate questions:

Here_1: Are large cell bodies of neurons harder to depolarize than small cell bodies of neurons?

and

Here_2: Why do larger diameter myelinated axons have greater conduction velocities than small diameter myelinated axons?

The conclusions from the two questions were that larger somas take longer to depolarize and larger axons transmit electric signal more quickly.

Even though both 'entities' exhibit shared electrical properties, it appears as though the outcome of having these electrical properties is fundamentally different. In one case, the membrane is taking longer to depolarize...but in another case, the membrane is depolarizing quickly!

What am I missing that results in my perception that electrical properties are somehow being differently applied to the soma of a neuron and the axon of a neuron?

(Note that this question involves the myelinated section of the neuron so there is no need to talk about nodes of ranvier or any sort of "active" electric process)

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  • $\begingroup$ Can you cite where your assumptions are coming from? I think you are taking things to be rules that are not rules. Also: where is the stimulus coming from? why are you talking about receptors on axons? why are you applying motor neuron ideas to C-fibers? $\endgroup$ – Bryan Krause Apr 22 at 0:49
  • $\begingroup$ The "receptor" was a typo. I have corrected some other typos accordingly. My assumptions are coming from my own intuitive understanding of physics and other general sources. C fibers were chosen because they are unmyelinated...I didn't want to involve myelin in this discussion...and why should the electrical phenomenon affecting motor neurons not be extendable to the electrical phenomenon affecting any other sort of neuron? $\endgroup$ – S.Cramer Apr 22 at 0:53
  • $\begingroup$ Because the "size principle" is an experimental finding with motor neurons. It is not a rule for all neurons. Please separate out your question a bit too, there is far too much here to unpack, and ask a single answerable question. $\endgroup$ – Bryan Krause Apr 22 at 0:55
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    $\begingroup$ I never said that. $\endgroup$ – Bryan Krause Apr 22 at 2:35
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    $\begingroup$ What I said is that size is not the only determining factor, and you should not assume that a larger cell is activated at higher threshold if you aren't talking about a motor neuron: that principle counts for motor neurons only, others you have to approach on a case by case basis. $\endgroup$ – Bryan Krause Apr 22 at 4:43

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