Membrane Potential: What is it? Depolarization, Hyperpolarization, Ion Types, Channels and Equilibrium Potential

It is the term used to describe the difference in voltage (or electrical potential) between the inside and outside a cell.

The membrane potential is due to disparities in the concentration and permeability of essential ions across a membrane.

Due to the varying concentrations of ions across a membrane, the membrane has an electrical charge.

Changes in the membrane potential trigger action potentials and give cells the ability to send messages throughout the body.

More specifically, action potentials are electrical signals; these signals carry efferent messages to the central nervous system for processing and afferent messages outside the brain to elicit a specific reaction or movement.

Numerous active transports embedded within the cell membrane contribute to the creation of membrane potentials and the cellular structure of the lipid bilayer.

The chemistry involved in membrane potentials reaches into many scientific disciplines. Chemically it involves molarity, concentration, electrochemistry, and the Nernst equation.


From a physiological point of view, there is an electrical potential difference between the interior of the cell and the surrounding extracellular fluid in cells of all types.

While this phenomenon is present in all cells, it is essential in nerve and muscle cells, as changes in their membrane potentials are used to encode and transmit information.

Cell biology is fundamentally connected with electrochemistry and physiology.

Depolarization and hyperpolarization

A resting neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential.

The resting potential is determined by the ion concentration gradients across the membrane and by the membrane’s permeability to each type of ion.

When a nerve or muscle cell is “resting,” its membrane potential is called the resting membrane potential. In a typical neuron, this is about -70 millivolts.

The minus sign indicates that the cell’s interior is negative concerning the surrounding extracellular fluid.

Because there is a potential difference across the cell membrane, the membrane is said to be polarized.

Changes in membrane potential are associated with depolarization and hyperpolarization:

  • 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 harmful than it is at the resting potential, the membrane is said to be hyperpolarized.

Types of ions found in neurons

In neurons and their surrounding fluid, the most abundant ions are:

  • Positively charged (cations): sodium and potassium.
  • Negatively charged (anions): chloride and organic anions.

In most neurons, potassium and organic anions (such as those found in proteins and amino acids) are present in higher concentrations inside the cell than outside. Unlike chlorine and sodium, which are generally present in higher concentrations outside the cell.

This means stable concentration gradients across the membrane for all the most abundant ion types.

Thus, K + is more concentrated inside the cell than outside the cell. Organic anions are more concentrated inside than outside the cell, Cl- is more focused outside than inside, and Na + is more concentrated outside. The cell.

Mechanisms for ions to cross the membrane

Because they are charged, the ions cannot pass directly through the hydrophobic lipid regions of the membrane.

Instead, they have to use specialized channel proteins that provide a hydrophilic tunnel through the membrane. Some channels, known as leak channels, are open in resting neurons.

Others close in resting neurons and only open in response to a signal.

Ion channels

Some ion channels are highly selective for one type of ions, but others pass various ions.

The ion channels that primarily allow sodium to pass are called potassium channels, and the ion channels that primarily allow potassium to pass are called sodium channels.

The channels provide a path for the ions through the membrane, allowing them to move down any electrochemical gradient.

The equilibrium potential

The difference in electrical potential across the cell membrane that exactly balances the concentration gradient of an ion is known as the equilibrium potential.

Because the system is in equilibrium, the membrane potential will remain at the equilibrium potential.

For a cell with only one permeable ionic species (only one type of ion that can cross the membrane), the resting membrane potential will be equal to the equilibrium potential for that ion.

The steeper the concentration gradient, the greater the electrical balancing potential.

In a neuron, the resting membrane potential is closer to potassium’s equilibrium potential than sodium’s equilibrium potential.

The membrane at rest is much more permeable to potassium than sodium.

If more potassium channels were to open, making it even easier for potassium to cross the cell membrane, the membrane would hyperpolarize, moving even closer to the equilibrium potential of potassium.

If, on the other hand, additional sodium channels are opened, making it easier for sodium to cross the membrane, the cell membrane would depolarize towards the equilibrium potential of sodium.

Changing the number of open ion channels provides a way to control the cell’s membrane potential and an excellent way to produce electrical signals.

The concentration gradient

The concentration gradient, of course, also applies to uncharged molecules. But with ions, the electric potential difference must always be considered.

Therefore, the total energy change for the movement of an ion through the membrane is the sum of the energy change due to the concentration gradient and the energy change due to the electric potential difference.

These two factors can act in the same direction or opposite directions.