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The equilibrium potential for sodium (ENa) is around +60 mV. This is the membrane potential at which there is no net movement of sodium ions across the membrane, as the concentration gradient is balanced by the electrical gradient.
The resting membrane potential value for sodium is closer to the equilibrium of potassium because the sodium-potassium pump actively maintains a higher concentration of potassium inside the cell and a higher concentration of sodium outside the cell. This leads to a higher permeability of potassium ions at rest, resulting in the resting membrane potential being closer to the equilibrium potential of potassium.
Membrane potential is the difference in electric charge between the inside and outside of a cell membrane. Equilibrium potential is the membrane potential at which the electrical and concentration gradients of a specific ion are balanced, resulting in no net movement of that ion across the membrane.
The neuronal membrane also has ion channels for other ions besides potassium, such as sodium or chloride, that can influence the resting membrane potential. These other ions contribute to the overall equilibrium potential of the neuron, which affects its resting membrane potential. Additionally, the activity of Na+/K+ pumps helps establish and maintain the resting membrane potential, contributing to the slight difference from the potassium equilibrium potential.
The equilibrium potential of sodium (Na) is primarily determined by the concentration of Na ions inside and outside the cell, as described by the Nernst equation. Changing the concentration of potassium (K) inside the cell does not directly affect the equilibrium potential of Na. However, alterations in K concentration can influence the overall membrane potential and the activity of sodium channels, which may indirectly affect the dynamics of Na influx during action potentials. Thus, while the Na equilibrium potential remains unchanged, the cell's excitability and response to stimuli could be affected.
A change in extracellular sodium concentration would not alter the resting membrane potential of a neuron because the resting potential is primarily determined by the relative concentrations of sodium and potassium ions inside and outside the cell, as mediated by the sodium-potassium pump and leak channels. Changes in extracellular sodium concentration would not directly affect this equilibrium.