Graded Potential

For a membrane at the resting potential, a graded potential represents a change in the voltage either above or below -70 mV. All graded potentials in the postsynaptic membrane (postsynaptic potential – PSP) will result in either depolarization (excitatory) or hyperpolarization (inhibitory).

Depolarizing graded potential or excitatory postsynaptic potential (EPSP) results from Na⁺ or Ca²⁺ entering the cell. When a neurotransmitter attaches to its receptor, it opens ion channels, allowing positively charged ions to flow into the neuron. This influx of positive charge reduces the voltage difference across the neuron’s membrane, bringing it closer to the threshold required for an action potential to occur. Suppose the depolarization is strong enough and reaches the threshold; in that case, an action potential develops, transmitting information to other neurons.

During the resting state, Na⁺ and Ca²⁺ have higher concentrations outside the cell than inside. Due to their high positive charge, movement of Na⁺ or Ca²⁺ cause the cell interior to become less negative than outside.

On the other hand, hyperpolarizing graded potential or inhibitory postsynaptic potential (IPSP) occur when the binding of a neurotransmitter causes ion channels to open, allowing negatively charged ions such as Cl^– to enter the cell or positively charged ions to leave.

Graded potentials are crucial for integrating and processing information in the brain. They conduct neurons to respond selectively to specific inputs, thus integrating signaling within neural networks. The response to specific stimuli is modulated based on the strength and frequency of graded potentials. It contributes to our ability to perceive and interact with the environment.

Action and graded potentials are two fundamental electrical events occurring in neurons. However, there are some differences:

Graded potentials are different-strength signals over small distances for a brief time. They can be either depolarizing (excitatory) or hyperpolarizing (inhibitory) localized in the regions where they form.

In contrast, an action potential is a rapid, all-or-nothing electrical event that occurs in the axon initiated when the membrane potential reaches the threshold level. Action potentials are very-brief events having a fixed magnitude.