Why is the Membrane Potential More Negative When Potassium is Higher Than Sodium in a Neuron?

The membrane potential of a neuron is a crucial factor that determines its electrical state and ability to send signals. This potential is influenced by the concentrations of various ions across the cell membrane, with potassium (K ) and sodium (Na ) ions playing pivotal roles. While a typical resting membrane potential is around -70mV, it can become more negative, a condition known as hyperpolarization, under certain conditions. Let's explore why a higher concentration of potassium relative to sodium leads to a more negative membrane potential.

Ion Concentration Gradients

In neurons, the inside of the cell has a higher concentration of potassium ions (K ) compared to the outside. This results in a concentration gradient that drives K ions to move out of the cell through K channels. Conversely, sodium ions (Na ) are more concentrated outside the cell, making their movement into the cell less than that of K out.

Nernst Equation

The Nernst equation is a key tool in understanding ion movements across cell membranes. It calculates the equilibrium potential for each ion based on its concentration gradient. For potassium (K ), the equilibrium potential EK is typically around -90mV. For sodium (Na ), ENa is around 60mV. The resting membrane potential (Em) is a weighted average of these potentials, influenced by the relative permeability of the membrane to each ion.

Resting Membrane Potential

At rest, the neuron's membrane is much more permeable to K than to Na due to the presence of more open potassium channels. This means that the resting membrane potential is closer to EK than to ENa.

Increased K Conductance

When the concentration of K in the extracellular fluid increases relative to Na , it can lead to increased K efflux (outflow). This increased movement of K ions outside the cell makes the inside of the cell more negative. Specifically:

The outward flow of positively charged K ions makes the inside of the cell more negative. If the membrane becomes more permeable to K due to more open potassium channels or a higher extracellular K concentration, the membrane potential will approach EK, which is more negative.

Hyperpolarization

When K levels are significantly elevated, the neuron may become hyperpolarized, meaning the membrane potential becomes even more negative than the typical resting potential. This can affect the neuron's excitability, making it less likely to fire an action potential.

Summary

In summary, when potassium (K ) concentrations are higher than sodium (Na ) concentrations, the greater permeability of the membrane to K and the resulting outward flow of K ions lead to a more negative membrane potential. This can result in hyperpolarization of the neuron, affecting its ability to generate electrical signals.