system
Given the following post, respond as if you're commenting on Reddit:
human
Why do axons with a larger circumferences transmit action potentials faster?

Explanations that I've had from profs use resistivity differences in electrical wires to compare, but I don't see how a action potential (localized at the membrane) can compare to an electrical wire. Action potential currents and electrical currents don't seem to follow the same physics to me.

Do any of the neuroscientists or biophysicists have a better explanation for this?

(3rd year undergrad in biochem, currently in physiology and upper year neuroscience.... don't be afraid to use technical terms)
gpt
Background/Setup:

The progression of an action potential is driven by the diffusion of charged ions axially down the length of the nerve. After ions from a section of nerve reach the next section down, they travel across the membrane. This results in a flow of charge across the membrane which has small conductance (1/resistance) in its initial resting state. Following ohms lab, the flow of charge results in a large voltage change, kicking off the AP.

Answer to question:

Cross-section for the progressing ions to flow axially through the nerve is dependent on the square of the radius, therefore the time needed for one section of nerve to build up enough charged ions stimulate the next section scales with the inverse square of radius (This is the resistance of the nerve to the axial conduction of ions that your professors have been mentioning)

The conductance of the membrane (mentioned in the first paragraph) scales with the radius, this is the conductance of the membrane to the flow of ions through it.  Therefore the amount of ions needed in the next section of nerve to result in an AP scales with the radius due ohms law.

The speed with which an AP can travel the nerve axially is dependent both on how quickly the ions can build up in the next section of nerve and also how great the conductance of the section of nerve is. This results in a relation where speed= r^2 *  1/r = r, so speed of the conduction scales with r.

Sorry if that was confusing, its hard to put concepts like this in words.

Source: Various physiology courses and a bioelectric phenomenon course.