Question

The ratio between extracellular and intracellular K+ concentration

A) will cause depolarisation of resting membrane potential if extracellular K+ concentration decreases
B) has little effect on neuronal resting membrane potential
C) is the main factor controlling neuronal resting membrane potential
D) is able to vary freely in brain tissue
E) will cause hyperpolarisation of resting membrane potential if intracellular K+ concentration increases

Please explain the answer! Thankyou :)

Answer 1

Answer 2

Answer:

C) is the main factor controlling neuronal resting membrane potential

Explanation:

The resting membrane potential (RMP) of neurons is primarily determined by the ratio of extracellular to intracellular K⁺ concentration, as described by the Nernst equation for potassium ($E_K$):

$$E_K=\frac{RT}{zF}\ln \left(\frac{[{\text{K}}^{+}{]}_{\text{out}}}{[{\text{K}}^{+}{]}_{\text{in}}}\right)$$

• At body temperature (37°C), this simplifies to:

  $$E_K\approx 61.5\text{mV}\cdot {\log }_{10}\left(\frac{[{\text{K}}^{+}{]}_{\text{out}}}{[{\text{K}}^{+}{]}_{\text{in}}}\right)$$

• Typical values:

• $[{\text{K}}^{+}{]}_{\text{out}}\approx 5\text{mM}$,

• $[{\text{K}}^{+}{]}_{\text{in}}\approx 150\text{mM}$.

• Substituting:

  $$E_K\approx 61.5\cdot {\log }_{10}\left(\frac{5}{150}\right)=-90\text{mV}(\text{close to RMP of }-70\text{mV})\mathrm{.}$$

Why not other options?

• A: Incorrect. Decreasing extracellular K⁺ hyperpolarizes RMP (reduces $E_K$).

• B: Incorrect. K⁺ gradient is dominant for RMP.

• D: Incorrect. Brain tightly regulates K⁺ to prevent excitotoxicity.

• E: Incorrect. Increasing intracellular K⁺ depolarizes RMP (reduces gradient).