The net diffusion of potassium ions (K⁺) out of a neuron through K⁺ leak channels is primarily driven by two factors: the concentration gradient and the electrical gradient (or membrane potential) established across the neuronal membrane.
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Concentration Gradient: Normally, there is a higher concentration of K⁺ ions inside the neuron compared to the outside. This concentration gradient encourages K⁺ ions to move out of the cell.
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Electrical Gradient: As K⁺ ions diffuse out of the neuron, they leave behind negatively charged ions (like proteins and other anions), which creates a negative charge inside the cell relative to the outside. This negative charge tends to attract K⁺ ions back into the cell, resulting in an opposing electrical gradient.
When the extracellular concentration of K⁺ increases, the following occurs:
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Reduced Concentration Gradient: Increasing the extracellular concentration of K⁺ diminishes the concentration gradient between the inside and outside of the neuron. Since the driving force for K⁺ to diffuse outward is the difference in concentration, a smaller gradient means that there is less "push" for K⁺ ions to leave the cell.
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Equilibrium Potential Shift: The equilibrium potential for K⁺ (calculated using the Nernst equation) is influenced by the concentration of K⁺ inside and outside the cell. When extracellular K⁺ increases, the equilibrium potential becomes less negative, meaning that the driving force for K⁺ to exit the neuron decreases. If the membrane potential approaches the new equilibrium potential (which is less negative due to increased extracellular K⁺), the net movement of K⁺ out of the neuron will be reduced.
In summary, increasing extracellular K⁺ reduces the net diffusion of K⁺ out of the neuron because it lowers the concentration gradient driving K⁺ outward and alters the equilibrium potential, decreasing the driving force for K⁺ ions to leave the cell.