The primary difference between a local potential and an action potential lies in where they occur on a neuron and how they are initiated.
Local potentials and action potentials are both electrical signals within neurons, but they serve distinct roles and have different characteristics. Based on fundamental neurobiology, including the provided reference:
Where They Happen: Location Matters
According to the reference, a key distinction is their location: "First, local potentials occur on dendrites and soma of a neuron whereas action potentials originate at the axon hillock (or the part of the axon closest to the soma)."
- Local Potentials: These changes in membrane potential happen primarily on the dendrites and the cell body (soma) of a neuron. These are the parts of the neuron that typically receive signals from other neurons.
- Action Potentials: These large, rapid electrical events begin at the axon hillock, which is the region where the soma transitions into the axon. From the axon hillock, the action potential propagates down the axon.
How They Start: Initiation Triggers
The initiation of these potentials also differs significantly, as stated in the reference: "Local potentials occur as a result of a stimulus whereas action potentials occur as a result of local potentials."
- Local Potentials: These are triggered by a specific stimulus, such as the binding of a neurotransmitter to a receptor on the dendrite or soma. This opens or closes ion channels, causing a small, localized change in the membrane voltage.
- Action Potentials: Action potentials are triggered by local potentials. If the summation of local potentials reaching the axon hillock is strong enough to depolarize the membrane to a critical level called the threshold potential, an action potential is initiated.
Other Important Contrasts
Beyond location and initiation, local and action potentials differ in several other ways:
- Amplitude:
- Local Potentials: Have variable amplitudes; their strength depends on the strength of the stimulus. Stronger stimulus means larger local potential.
- Action Potentials: Are "all-or-none." Once the threshold is reached, the action potential always has the same maximum amplitude for that specific neuron. Increasing the stimulus strength above the threshold does not make the action potential larger.
- Summation:
- Local Potentials: Are summable. Multiple local potentials can add together (summation) if they occur close in time or space. This summation is crucial for reaching the threshold for an action potential.
- Action Potentials: Are not summable in the same way. They are discrete events.
- Conduction:
- Local Potentials: Are conducted decrementally. They spread passively along the membrane, decreasing in strength with distance from the point of origin.
- Action Potentials: Are conducted regeneratively and non-decrementally. They propagate actively down the axon without losing strength, effectively regenerating themselves at each point along the way.
- Polarity:
- Local Potentials: Can be depolarizing (making the membrane potential less negative) or hyperpolarizing (making it more negative).
- Action Potentials: Are primarily depolarizing events, though they include a repolarization phase.
The Relationship: From Local Signal to Long-Distance Communication
Think of local potentials as the initial receiving and processing signals within the neuron's input zone (dendrites and soma). They are like small ripples on a pond, spreading out and fading. The neuron constantly integrates these local potentials. If the sum of these ripples reaching the axon hillock (the "trigger zone") is strong enough to cross a certain threshold, it unleashes a powerful, self-propagating wave – the action potential. This action potential then travels down the axon, acting as a robust, long-distance signal to communicate with other neurons or target cells.
Summary Table
Here is a comparison of the key differences:
| Feature | Local Potential | Action Potential |
|---|---|---|
| Location | Dendrites and Soma | Axon Hillock and Axon |
| Initiation | Stimulus (e.g., neurotransmitter binding) | Summation of Local Potentials reaching threshold |
| Amplitude | Graded (variable) | All-or-None (fixed magnitude) |
| Summation | Yes (Temporal and Spatial) | No (discrete event) |
| Conduction | Decremental (fades with distance) | Non-decremental (propagates actively) |
| Duration | Varies (milliseconds) | Relatively fixed (typically 1-2 milliseconds) |
| Polarity | Depolarizing or Hyperpolarizing | Primarily Depolarizing, followed by Repolarization |
Understanding the differences between local and action potentials is fundamental to understanding how neurons process information and communicate throughout the nervous system.
