Difference Between Graded Potential and Action Potential

Main Difference – Graded Potential vs Action Potential

The plasma membrane of the nerve cells is usually at the resting membrane potential. The interior of the plasma membrane is negatively charged while the exterior is positively charged. The signals of the nervous system are transmitted through the nerve cells in the form of potential differences. The loss of resting membrane potentials is known as the depolarization. Graded potential and action potential are the two types of potential differences that can be generated during depolarization. The main difference between graded potential and action potential is that graded potentials are the variable-strength signals that can be transmitted over short distances whereas action potentials are large depolarizations that can be transmitted over long distances. Graded potential may lose the strength as they are transmitted through the neuron but, action potentials do not lose their strength during the transmission.

Key Areas Covered

1. What is a Graded Potential
      – Definition, Features, Role
2. What is an Action Potential
      – Definition, Features, Role
3. What are the Similarities Between Graded Potential and Action Potential
      – Outline of Common Features
4. What is the Difference Between Graded Potential and Action Potential
      – Comparison of Key Differences

Key Terms: Action Potential, Depolarization, Graded Potential, Nerve Cell, Resting Membrane Potential 

What is a Graded Potential

Graded potential refers to a membrane potential that can vary in amplitude. The amplitude is proportional to the size of the input stimuli. Graded potentials can be either depolarizing or hyperpolarizing. Several graded potentials can be integrated either temporally or spatially. The transmission of graded potential can occur uniformly in all directions. The generation of graded potentials occurs by the opening of ligand-gated ion channels. The strength of the signal decays with the distance. Examples of graded potentials are shown in figure 1.

Figure 1: Graded Potentials

The three primary forms of the graded potentials are receptor potential, postsynaptic potentials, and end plate potentials. Receptor potentials are generated in the specialized sensory receptor cells. Postsynaptic potentials are generated in the nerve cells. Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are the two types of postsynaptic potentials. EPSPs occur during depolarization whereas IPSPs occur during hyperpolarization. The end plate potentials are generated in the muscle cells.

What is an Action Potential

Action potential refers to a change in the electrical potential, which is associated with the transmission of impulses along the membrane of a nerve cell or muscle cell. The three main stages of an action potential are the depolarization, repolarization, and refractory period. A sudden change in the membrane potential is referred to as a depolarization. Here, the internal charge changes from negative to positive. The opening of the ion-gated channels causes the membrane depolarization. As the sodium channels are opened, the migration of the positively-charged sodium ions into the nerve cell causes more positive charge inside the cell. The three stages of the action potential are shown in figure 2.

Figure 2: Stages of Action Potential

 The restoration of the negative charge inside the nerve cell is known as the repolarization. This is caused by the opening of the potassium channels. The influx of potassium ions into the outside of the nerve cell causes the reduction of the positive charge inside the cell. Refectory period refers to the time period between two action potentials. During the refectory period, sodium-potassium channels are opened to restore the resting potential. In the resting potential, the concentration of the sodium ions is high outside of the nerve cell while the concentration of the potassium ions is high inside the nerve cell.

Similarities Between Graded Potential and Action Potential

• Both graded potential and action potential are two types of membrane depolarizations of the nerve cells.
• Both graded potential and action potential are generated as a result of transmission of signals.

Difference Between Graded Potential and Action Potential

Definition

Graded Potential: Graded potential refers to a membrane potential, which can vary in amplitude.

Action Potential: Action potential refers to a change in the electrical potential, which is associated with the transmission of impulses along the membrane of a nerve cell or muscle cell.

Depolarization/Hyperpolarization

Graded Potential: Graded potential can occur either due to depolarization or hyperpolarization.

Action Potential: Action potential can only occur due to depolarization.

Strength of Depolarization

Graded Potential: Graded potential may have variable signal strengths which are less than an action potential.

Action Potential: Action potential is a large depolarization, which reaches the threshold (+40 mV).

Ion Channels

Graded Potential: Graded potential is generated by ligand-gated ion channels.

Action Potential: Action potential is generated by voltage-gated ion channels.

Distance

Graded Potential: Graded potential may be transmitted over short distances.

Action Potential: Action potential may be transmitted over long distances.

Strength

Graded Potential: Graded potential may lose its strength during transmission.

Action Potential: Action potential does not lose its strength during transmission.

Addition

Graded Potential: Two graded potentials can be added together.

Action Potential: Two action potentials cannot be added together.

Conclusion

Graded potential and action potential are two types of membrane potentials that can be generated in the nerve cells during the transmission of signals. A graded potential consists of a low amplitude than the action potential. Therefore, it decays during the transmission. But, action potentials do not decay during the transmission. The main difference between graded potential and action potential is the characteristics of each type of membrane potentials.   

Reference:

1. “2014 Neural Communication.” Graded potentials, Available here.
2.“Brent Cornell.” Action Potential | BioNinja, Available here.

Image Courtesy:

1. “1223 Graded Potentials-02” By OpenStax (CC BY 4.0) via Commons Wikimedia
2. “Action potential” By Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom – Own work (CC BY-SA 3.0) via Commons Wikimedia