In the cell, Na+ ions are not at electrochemical equilibrium when the [Na+] is equal on both sides of the membrane. Why?
A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential.
The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.
In a resting neuron, there are concentration gradients across the membrane for Na+ and K+. Ions move down their gradients via channels, leading to a separation of charge that creates the resting potential.
The membrane is much more permeable to K+ than to Na+, so the resting potential is close to the equilibrium potential of K+ (the potential that would be generated by K+ if it were the only ion in the system)
The resting membrane potential is determined by the uneven distribution of ions (charged particles) between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions.
Types of ions found in neurons
In neurons and their surrounding fluid, the most abundant ions are:
In most neurons, K+ and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast, Na+ and Cl− are usually present at higher concentrations outside the cell. This means there are stable concentration gradients across the membrane for all of the most abundant ion types.
The equilibrium potential
The electrical potential difference across the cell membrane that exactly balances the concentration gradient for an ion is known as the equilibrium potential. Because the system is in equilibrium, the membrane potential will tend to stay at the equilibrium potential. For a cell where there is only one permeant ionic species (only one type of ion that can cross the membrane), the resting membrane potential will equal the equilibrium potential for that ion.
The steeper the concentration gradient is, the larger the electrical potential that balances it has to be. You can get an intuitive feeling for this by imagining the ion concentrations on either side of the membrane as hills of different sizes and thinking of the equilibrium potential as the force you'd need to exert to keep a boulder from rolling down the slopes between them.
If you know the K+ concentration on both sides of the cell membrane, then you can predict the size of the potassium equilibrium potential.
Both K+ and Na+ contribute to resting potential in neurons
As it turns out, most resting neurons are permeable to Na+ and Cl− as well as K+. Permeability to Na+, in particular, is the main reason why the resting membrane potential is different from the potassium equilibrium potential.
Let’s go back to our model of a cell permeable to just one type of ion and imagine that Na+ (K+) is the only ion that can cross the membrane. Na+ is usually present at a much higher concentration outside of a cell than inside, so it will move down its concentration gradient into the cell, making the interior of the cell positive relative to the outside.
Because of this, the sodium equilibrium potential—the electrical potential difference across the cell membrane that exactly balances the Na+concentration gradient—will be positive. So, in a system where Na+ is the only permeant ion, the membrane potential will be positive.
In a resting neuron, both Na+ and K+ are permeant, or able to cross the membrane.
Na+ will try to drag the membrane potential toward its (positive) equilibrium potential.
K+ will try to drag the membrane potential toward its (negative) equilibrium potential.
You can think of this as being like a tug-of-war. The real membrane potential will be in between the Na+ equilibrium potential and the K+ equilibrium potential. However, it will be closer to the equilibrium potential of the ion type with higher permeability (the one that can more readily cross the membrane).
In the cell, Na+ ions are not at electrochemical equilibrium when the [Na+] is equal on...