The key difference lies in their nature: membrane potential is the baseline voltage difference across a cell membrane, while an action potential is a rapid, temporary change in this voltage that travels along the cell.
Understanding Membrane Potential
Potential means voltage. The membrane potential refers to the electrical voltage difference that exists across the membrane of a cell. According to the reference, membrane voltage is measured from the inside to the outside of the membrane. In a resting cell, this potential is typically negative on the inside relative to the outside, often referred to as the resting membrane potential. This difference is maintained by the uneven distribution of ions across the membrane, mediated by ion pumps and channels.
- Key Characteristics:
- Represents the electrical state of the membrane at any given time.
- Exists in virtually all cells.
- Can change in response to stimuli.
Understanding Action Potential
An action potential, in contrast, is a specific type of rapid, transient change in the membrane potential that occurs in excitable cells like neurons and muscle cells. The reference describes it as a traveling wave of depolarization (zero voltage). It is explicitly stated that it's only temporary.
Action potentials are triggered when the membrane potential reaches a certain threshold voltage. As the reference mentions, generally speaking, receptors will trigger (fire off) when a certain voltage is achieved. This triggering leads to a swift sequence of events involving the opening and closing of voltage-gated ion channels, causing a rapid depolarization (becoming less negative or even positive inside, temporarily reaching zero voltage as noted) followed by repolarization back to the resting potential. This electrical signal then propagates along the cell membrane.
- Key Characteristics:
- A brief, all-or-nothing electrical impulse.
- Occurs in excitable cells.
- Involves rapid depolarization followed by repolarization.
- Travels or propagates along the membrane.
- Triggered by reaching a threshold voltage.
Comparing Action Potential and Membrane Potential
Think of membrane potential as the continuous state of voltage across the membrane, like the water level in a tank. An action potential is like a sudden, brief splash or wave that occurs in that tank when a specific condition is met (the threshold voltage), and this wave travels across the surface.
Here's a simple table highlighting the main differences:
| Feature | Membrane Potential | Action Potential |
|---|---|---|
| Definition | Voltage difference across the cell membrane | Rapid, temporary, traveling change in membrane potential |
| Nature | Continuous state; baseline voltage | Discrete electrical event or impulse |
| Occurrence | Exists in most cells (resting or changing) | Occurs in excitable cells only |
| Trigger | Can change due to various stimuli (e.g., neurotransmitters) | Triggered specifically by reaching a threshold voltage |
| Duration | Can be stable (resting potential) or sustained | Very brief (milliseconds) |
| Description | Baseline voltage (inside to outside) | Traveling wave of depolarization (zero voltage) |
| Temporality | Can be stable or change slowly | Only temporary |
In essence, the action potential is a dynamic event based on changes in the membrane potential. The membrane potential provides the context, and the action potential is the specific signal generated by a rapid, triggered alteration of that potential.
