I'm trying to reproduce a common observation that a larger neuron tends to get activated at a lower current threshold, via extracellular stimulation, than a smaller neuron. Unfortunately I am working with magical neurons that the opposite seems to be the case. Doubting my results and magical neuron, I'm stuck trying to figure out why I am observing the difference from expected results. Working through the logic, it seems that for a given extracellular current, the the applied extracellular voltage would be the same regardless of the diameter of the neurons (assuming the membrane potential on the surface of a small ring patch is equal to the potential at the center of that ring). In this regard, it seems that it should be the change in resistance by changing diameter that would cause different current densities and thus different levels of excitability for a given extracellular potential.Along these same lines, if a neuron is more likely to be excited at a given level, I would think the current density would need to be higher at that level than the current density of a less excitable neuron. When examining i_membrane for a completely passive neuron this doesn't seem to be the case, the larger diameter neuron has a smaller current density (except for at stimulus onset due to capacitive currents).Thoughts on why the current density might be lower for a larger neuron? How exactly is i_membrane calculated, or rather the component that comes from the the extracellular potential? It would seem to be dependent on the difference in extracellular potential between a node of interest and neighboring nodes, and the resistance in between. As the diameter increases this would cause a decrease in axial resistance, and an increase in "injected current" from the applied voltage.
I didn't find the right solution from the Internet.
https://www.neuron.yale.edu/phpBB/viewt ... f=8&t=1086
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