Particular ions are distributed unequally across the cell membrane.
Concentrations of Na+ and Cl− are greater on the outside of the cell, while concentrations of K+ and organic anions, such as charged amino acids and proteins, are greater on the inside of the cell .
The organic ions are incapable of passing across the cell membrane.
This arrangement leads to an electrochemical gradient called the resting potential.
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State: The neuron is at rest (around –70 mV).
Ionic conditions:
Inside: High K⁺, low Na⁺.
Outside: High Na⁺, low K⁺.
Key channels:
The Na⁺/K⁺ ATPase pump maintains these gradients (3 Na⁺ out, 2 K⁺ in).
The membrane is more permeable to K⁺ due to leak channels → inside is negative.
action potential: sequence of ion movements across the neuronal (or muscle) cell membrane that generate and propagate the rapid electrical signal
Trigger: A stimulus depolarizes the membrane to reach threshold (around –55 mV).
Event:
Voltage-gated Na⁺ channels open.
Na⁺ rushes in (down its electrochemical gradient).
The membrane potential becomes positive (up to +30 to +40 mV).
Event:
Na⁺ channels inactivate (they close automatically after opening briefly).
Voltage-gated K⁺ channels open.
K⁺ exits the cell (outward current), restoring negative potential inside.
Event:
K⁺ channels remain open a bit longer → more K⁺ leaves than needed.
The membrane potential becomes more negative than resting (after-hyperpolarization).
Event:
Voltage-gated K⁺ channels close.
The Na⁺/K⁺ pump and K⁺ leak channels reestablish the resting ion gradients.
The membrane potential returns to –70 mV.
| Phase | Main Ion Movement | Channels Involved | Membrane Potential |
|---|---|---|---|
| Resting | K⁺ leak out | K⁺ leak channels | –70 mV |
| Depolarization | Na⁺ influx | Voltage-gated Na⁺ | +30 mV |
| Repolarization | K⁺ efflux | Voltage-gated K⁺ | Back to negative |
| Hyperpolarization | Continued K⁺ efflux | K⁺ channels (slow to close) | Below –70 mV |
| Return to Rest | Na⁺/K⁺ pump restores | Na⁺/K⁺ ATPase | –70 mV |
Would you like me to include a diagram or labeled graph of the action potential showing these ionic changes over time?