There are two stereotyped statements that I have seen during my coursework regarding electric properties of neurons:
- Large diameter axons propagate action potentials more quickly than small diameter axons
- 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.
I have provided two pictures to help clarify my question:
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:
Less Longitudinal Resistance
Greater Surface Area Available for Ion Channels
I have previously asked two separate questions:
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)