There is a much greater K concentration inside the cell than outside. Why, then, is the resting membrane potential negative?

The resting membrane potential is negative despite the higher concentration of potassium ions (K⁺) inside the cell due to the selective permeability of the neuronal membrane. Here are the key reasons:

1. Potassium Permeability: The neuronal membrane is more permeable to K⁺ than to other ions at rest. As K⁺ ions diffuse out of the cell down their concentration gradient, they leave behind negatively charged proteins and other anions inside the cell. This movement creates a net negative charge inside.
2. Equilibrium Potential: The equilibrium potential for K⁺ (Eₖ) is around -80 mV, meaning that if the membrane were solely permeable to K⁺, the inside of the cell would be at this negative potential. The actual resting membrane potential is influenced by K⁺ but is typically around -65 mV, indicating a balance between the concentration gradient and the electrical gradient.
3. Other Ion Influences: While K⁺ is the primary contributor to the resting membrane potential, sodium ions (Na⁺) and chloride ions (Cl⁻) also play roles. The lower permeability to Na⁺ at rest prevents it from significantly affecting the potential, allowing K⁺ to dominate.