The membrane potential
- 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 and . 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 than to , so the resting potential is close to the equilibrium potential of (the potential that would be generated by if it were the only ion in the system).
The resting membrane potential
- If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.
- If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.
Where does the resting membrane potential come from?
Types of ions found in neurons
- Positively charged (cations): Sodium () and potassium ()
- Negatively charged (anions): Chloride () and organic anions
- K+ is more concentrated inside than outside the cell.
- Organic anions are more concentrated inside than outside the cell.
- Cl- is more concentrated outside than inside the cell.
- Na+ is more concentrated outside than inside the cell.
How ions cross the membrane
What happens if only can cross the membrane?
The equilibrium potential
Does membrane potential equal equilibrium potential?
Both and contribute to resting potential in neurons
- will try to drag the membrane potential toward its (positive) equilibrium potential.
- will try to drag the membrane potential toward its (negative) equilibrium potential.
Opening and closing ion channels alters the membrane potential
- If more potassium channels were to open up—making it even easier for to cross the cell membrane—the membrane would hyperpolarize, getting even closer to the potassium equilibrium potential.
- If, on the other hand, additional sodium channels were to open up—making it easier for to cross the membrane—the cell membrane would depolarize toward the sodium equilibrium potential.
The -pump maintains and gradients
- Three sodium ions bind to the sodium-potassium pump, which is open to the interior of the cell.
- The pump hydrolyzes ATP, phosphorylating itself (attaching a phosphate group to itself) and releasing ATP. This phosphorylation event causes a shape change in the pump, in which it closes off on the inside of the cell and opens up to the exterior of the cell. The three sodium ions are released, and two potassium ions bind to the interior of the pump.
- The binding of the potassium ions triggers another shape change in the pump, which loses its phosphate group and returns to its inward-facing shape. The potassium ions are released into the interior of the cell, and the pump cycle can begin again.