Final answer:
The most negative voltage, as represented by the resting membrane potential typically occurs inside the axon when compared to the outside, with controlled potassium levels in the extracellular fluid, due to the active transport of ions by the sodium-potassium pump.
Step-by-step explanation:
The student's question concerns membrane potential in neurons and the effect of ion concentration on electrical charge, particularly focusing on potassium (K+) ions in extracellular fluid (ECF) and their influence on voltage differences. To address which scenario has the most negative voltage, one must consider the distribution of ions across the neuronal membrane and the role of the sodium-potassium pump.
At resting membrane potential, neurons have a voltage of approximately -70 millivolts (mV) inside the cell compared to the outside. This is due to the action of the sodium-potassium pump which, through ATP consumption, expels three Na+ ions for every two K+ ions it brings in. This activity contributes to a net negative charge inside the cell while the outside remains relatively positive.
Considering the student's options, the most negative voltage, meaning the most negative relative to a reference point, would occur where there is the highest concentration of positive ions outside the cell and the most active sodium-potassium pumping maintaining a negative interior. This would typically be the case with control K+ in the ECF -- scenario 3: between the inside of the axon and the outside of the axon with control K+ ECF -- since raising the K+ concentration outside the cell, as in scenarios with High K+ ECF, would reduce the voltage gradient.