Final answer:
The neuron membrane potential is closest to its Nernst equilibrium for Na+ at the resting membrane potential, option d). At this state, despite the interventions of the Na+/K+ transporter, the membrane potential is closest to the equilibrium condition for Na+.
Step-by-step explanation:
When discussing the membrane potential of a neuron and its relation to the Nernst equilibrium for Na+, it is crucial to understand that the Nernst equilibrium potential is the membrane potential at which the net flow of that ion (in this case, Na+) across the membrane is zero. This is important when answering when the neuron membrane potential is closest to its Nernst equilibrium for Na+.
During the peak of the action potential, Na+ channels have closed, and the K+ channels are open, causing K+ to leave the cell and leading to repolarization. The membrane potential is far from the Nernst potential for Na+. During hyperpolarization and the absolute refractory period, the cell is even more negative than the resting membrane potential due to increased K+ ion permeability. Therefore, the membrane potential is still not close to Na+ Nernst equilibrium.
The correct answer is d) At resting membrane potential. The resting potential, typically around -70 mV, is primarily determined by the K+ ion concentration gradient due to the cell's permeability to K+ ions. However, the Na+/K+ transporter actively maintains the concentrations of Na+ and K+ ions across the membrane, thus preventing the potential from exactly equilibrating to the Nernst potential for either ion. Nevertheless, of all the options, the resting membrane potential is closest to the Nernst equilibrium for Na+.