Resting membrane potential of nerve fibre is close to isoelectric potential of:
First, I need to recall the core concept here. The resting membrane potential (RMP) is mainly due to the concentration gradients of ions across the cell membrane and the selective permeability of the membrane to those ions. The Nernst equation calculates the equilibrium potential for a specific ion, which is the potential difference that would exactly balance the concentration gradient for that ion.
Potassium (K+) has a much higher concentration inside the cell compared to outside. The cell membrane is more permeable to K+ at rest, so K+ tends to leak out, making the inside more negative. The equilibrium potential for K+ is around -90 mV. However, the actual resting membrane potential is about -70 mV. Why is there a difference? Because there's a slight permeability to Na+ as well, which leaks in, making the actual RMP less negative than the K+ equilibrium potential. But overall, the RMP is still closer to the K+ equilibrium potential than to Na+.
So the correct answer should be K+ because the membrane potential is closer to the K+ equilibrium potential. The other ions like Na+ have a much higher equilibrium potential (around +60 mV), which is way off. Cl- and Ca2+ are less relevant here. Cl- might be a distractor since some cells have Cl- equilibrium potential, but in nerve cells, it's not the main driver. Ca2+ is involved in other processes like neurotransmitter release but not in the resting potential.
The clinical pearl here is that the resting potential is primarily due to K+ efflux, and the Goldman equation is used to calculate the actual membrane potential considering multiple ions. Students often confuse this with the Nernst equation for a single ion.
**Core Concept**
The resting membrane potential of a nerve fiber is primarily determined by the **potassium (K⁺) ion concentration gradient** and the membrane's relative permeability to K⁺. The Nernst equation calculates the equilibrium potential for a specific ion, while the Goldman equation accounts for multiple ions. At rest, the membrane is most permeable to K⁺, making its equilibrium potential closest to the resting membrane potential (~-70 mV vs. K⁺'s -90 mV).
**Why the Correct Answer is Right**
Potassium ions (K⁺) are most influential in setting the resting membrane potential because the nerve cell membrane is highly permeable to K⁺ at rest. K⁺ diffuses out of the cell down its concentration gradient (intracellular concentration ~140 mM vs. extracellular ~5 mM), leaving behind negatively charged proteins. This creates a negative intracellular environment (~-70 mV). While the Nernst equation predicts a K⁺ equilibrium potential of ~-90 mV, the actual resting potential is less negative due to minor Na⁺ influx through leak channels. Thus, the RMP is closest to the **K⁺ equilibrium potential**.
**Why Each Wrong Option is Incorrect**
**Option A:** Sodium (Na⁺) has an equilibrium potential of ~+60 mV. The membrane is far