Difference Between Depolarization and Hyperpolarization

Main Difference – Depolarization vs Hyperpolarization

The transmission of signals in the nervous system occurs in the form of electrical impulses. These electrical impulses are generated on the membrane of the nerve cells. Different types of ion channels are involved in the transmission of electrical impulses through nerve cells. Typically, the sodium ion concentration outside the nerve cell membrane is high while the concentration of the potassium ions inside the nerve cell membrane is high. The potential at this stage is known as the resting membrane potential. Depolarization and hyperpolarization are two variations of the resting membrane potential. The main difference between depolarization and hyperpolarization is that depolarization refers to a decrease in the resting membrane potential whereas hyperpolarization refers to an increase in the resting membrane potential. 

Key Areas Covered

1. What is Depolarization
      – Definition, Occurrence, Role
2. What is Hyperpolarization
      – Definition, Occurrence, Role
3. What are the Similarities Between Depolarization and Hyperpolarization
      – Outline of Common Features
4. What is the Difference Between Depolarization and Hyperpolarization
      – Comparison of Key Differences

Key Terms: Action Potential, Depolarization, Hyperpolarization, Resting Membrane Potential, Sodium Ions, Threshold

What is Depolarization

Depolarization refers to the loss of polarization which is caused by the change in the permeability of sodium ions. This leads to the migration of sodium ions to the interior of a nerve cell or a muscle cell. The potential when a neuron is at rest is known as resting potential. The resting membrane potential is -70 mV. However, when a signal transmits through a neuron, an action potential is created by a depolarizing current. The depolarizing current is generated by the opening of sodium ion channels. Sodium ions migrate inside the cell from the outside. When the membrane potential reaches -55 mV, the action potential is generated. The -55 mV is called the threshold. The membrane potential at the action potential is +30 mV. The change in the membrane potential during depolarization is shown in figure 1.

Figure 1: Depolarization

Since action potential is a fixed value, the depolarizing potential is also a fixed value. The membrane potentials that are less than the depolarizing potentials are called graded potentials. Graded potentials decay during the transmission, while the action potentials do not lose their strength during the transmission.

What is Hyperpolarization

Hyperpolarization refers to an increase in the amount of the electrical charge, making the resting membrane potential more negative. Hyperpolarization is the opposite of the depolarization. Since it increases the negative charge outside the membrane, the initiation of an action potential is prevented by hyperpolarization. Hyperpolarization occurs due to the opening of the potassium ions. Potassium ions migrate outside the cell while chloride ions migrate inside the cell. The movement of ions during resting potential, depolarization, and hyperpolarization are shown in figure 2.

Figure 2: Movement of ions during resting potential, depolarization, and hyperpolarization

Nerve cells enter a hyperpolarization state following an action potential. Refractory period is the time between two action potentials. Hyperpolarization is one of the events which occurs in the refractory period.

Similarities Between Depolarization and Hyperpolarization

• Both depolarization and hyperpolarization are two alterations of the resting membrane potential.
• Both depolarization and hyperpolarization are caused by the opening of ion channels.

Difference Between Depolarization and Hyperpolarization

Definition

Depolarization: Depolarization refers to the loss of polarization, which is caused by the change in the permeability of sodium ions to the interior of a nerve cell or a muscle cell.

Hyperpolarization: Hyperpolarization refers to an increase in the amount of the electrical charge, making the resting membrane potential more negative.

Charge Difference

Depolarization: Depolarization makes the exterior of the cell membrane negatively-charged and the interior of the membrane positively-charged.

Hyperpolarization: Hyperpolarization makes the interior of the cell membrane more negatively-charged and the exterior of the membrane more positively-charged when compared to the resting membrane potential. 

Membrane Potential

Depolarization: Depolarization decreases the membrane potential.

Hyperpolarization: Hyperpolarization increases the membrane potential.

Ion Channels

Depolarization: Depolarization is caused by the opening of sodium ion channels.

Hyperpolarization: Hyperpolarization is caused by the closing of the sodium channels and the opening of the sodium channels.

Action Potential

Depolarization: Depolarization causes a firing of an action potential.

Hyperpolarization: Hyperpolarization prevents the firing of an action potential.

Conclusion

Depolarization and hyperpolarization are two types of membrane potentials that occur in the cell membrane of nerve cells. Depolarization is a decrease of the membrane potential, which generates an action potential. Hyperpolarization is an increase of the membrane potential, which prevents the generation of an action potential. The main difference between depolarization and hyperpolarization is the change in the membrane potential in each type of membrane potentials.

Reference:

1.“ Action Potentials.” Action Potentials, Available here.
2.“Hyperpolarization: Last Phase of the Action Potential.” Interactive Biology, with Leslie Samuel, 9 Jan. 2016, Available here.

Image Courtesy:

1. “1221 Action Potential” By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Ion channel activity before during and after polarization” By Robert Bear and David Rintoul – (CC BY 4.0) via Commons Wikimedia