Question

in our discretized circuit model for the neuron (shown above), we may represent the extracellular fluid (the region outside the axon) as a long conducting wire with no resistance even though the resistivity of this extracellular fluid is about the same as that of the axoplasm inside the axon. why can we do this?

Answer 1

We can represent the extracellular fluid as a long conducting wire with no resistance in the neuron circuit model because the extracellular fluid and axoplasm have similar resistivity. The resistivity of the extracellular fluid is low compared to the neuronal membrane, so we can neglect its effect on the overall resistance.

Step-by-step explanation:

The reason why we can represent the extracellular fluid as a long conducting wire with no resistance in our discretized circuit model for the neuron is because the extracellular fluid and the axoplasm have approximately the same resistivity. This means that the flow of current through both of these conductors will be similar, and therefore we can simplify the model by considering the extracellular fluid as a wire. Additionally, the resistivity of the extracellular fluid is relatively low compared to the resistivity of the neuronal membrane, so we can neglect its effect on the overall resistance in the circuit.

Answer 2

We can represent the extracellular fluid (ECF) in a discretized circuit model for a neuron as a long conducting wire with no resistance as the ECF has a much larger cross-sectional area through which current can flow.

Compared to the narrow axon, the ECF has a much larger cross-sectional area through which current can flow and effectively allows for many parallel pathways for current to travel, significantly reducing the overall resistance. Imagine a highway compared to a single-lane road. The highway, with its multiple lanes, offers much less resistance to traffic flow than the single-lane road, even if the road material itself is the same.

The distances over which current needs to travel in the ECF are much shorter than those within the axon. This is because the ECF surrounds the axon closely, minimizing the lateral spread of current.