Graded potentials are temporary, localized changes in the membrane potential of neurons. Unlike action potentials, which are all-or-nothing, graded potentials vary in strength and decrease over distance. These signals occur in the dendrites and soma of neurons and play a critical role in whether or not an action potential will be triggered.
There are two main types of graded potentials: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).
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1. Excitatory Postsynaptic Potentials (EPSPs)
An EPSP is a type of graded potential that depolarizes the neuron’s membrane, bringing it closer to threshold (typically around -55 mV) and making it more likely to fire an action potential.
Key Features:
- Cause: Opening of ligand-gated Na⁺ or Ca²⁺ channels.
- Effect: Influx of positive ions into the cell → membrane becomes less negative.
- Location: Dendrites or soma.
Example:
A neurotransmitter like glutamate binds to receptors, opening sodium channels and allowing Na⁺ to flow into the neuron.
Learn more about EPSPs and their impact on neuron excitability at Khan Academy – Synapse overview.
2. Inhibitory Postsynaptic Potentials (IPSPs)
An IPSP is a graded potential that hyperpolarizes the membrane, making the neuron less likely to fire an action potential.
Key Features:
- Cause: Opening of ligand-gated K⁺ or Cl⁻ channels.
- Effect: K⁺ flows out or Cl⁻ flows in → membrane becomes more negative.
- Location: Dendrites or soma.
Example:
Neurotransmitters like GABA bind to receptors, causing Cl⁻ channels to open and hyperpolarize the membrane.
Explore how GABA functions in the brain from Visible Body’s neurotransmitter guide.
Comparison Table
| Feature | EPSP | IPSP |
|---|---|---|
| Ion Movement | Na⁺ or Ca²⁺ in | K⁺ out or Cl⁻ in |
| Membrane Effect | Depolarization | Hyperpolarization |
| Outcome | Increases chance of action potential | Decreases chance of action potential |
| Neurotransmitters | Glutamate, acetylcholine | GABA, glycine |
| Channel Types | Ligand-gated Na⁺/Ca²⁺ | Ligand-gated K⁺/Cl⁻ |
3. Characteristics of All Graded Potentials
Regardless of type, graded potentials share the following traits:
- Localized: Confined to the area near the point of stimulation.
- Variable strength: Proportional to the strength of the stimulus.
- Decremental: Decrease in magnitude over distance.
- Summation: Can combine to strengthen or cancel each other (spatial or temporal summation).
You can explore how summation works in TeachMeAnatomy’s neural integration guide.
Conclusion
The two types of graded potentials—EPSPs and IPSPs—play opposite roles in neuron signaling. EPSPs make action potentials more likely by depolarizing the membrane, while IPSPs make them less likely by hyperpolarizing it. Together, these signals integrate at the axon hillock to determine whether the neuron fires. Understanding these differences is critical for mastering neurophysiology and cellular communication.
For an interactive animation of neuron signals, visit InnerBody’s Nervous System Explorer