Resting Potential and Action Potential

The action potential is the single most important event in neuronal physiology. In roughly two milliseconds, a neurone flips its membrane voltage from −70 mV to +40 mV and back again, in a precisely choreographed dance of sodium and potassium ions. Understanding every phase of this cycle — and why it works the way it does — is essential for OCR A-Level Biology A specification module 5.1.3(e)–(g).

Key Definitions:

• Resting potential — the steady voltage across the membrane of an unstimulated neurone, typically about −70 mV (inside negative relative to outside).
• Action potential — a rapid, transient reversal of membrane potential from −70 mV to approximately +40 mV, followed by repolarisation.
• Threshold — the potential (~−55 mV) at which voltage-gated Na⁺ channels open en masse, initiating an action potential.
• All-or-nothing — principle that action potentials either occur fully or not at all, irrespective of stimulus strength beyond threshold.
• Refractory period — the time after an action potential during which another cannot fire (absolute) or can only fire with a stronger stimulus (relative).

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The Resting Potential — How It Is Set Up

At rest, the inside of a neurone is about 70 mV more negative than the outside. This requires two things: (1) unequal distributions of ions across the membrane, and (2) differential membrane permeability.

The Sodium–Potassium Pump

Embedded in the membrane is a protein pump called the Na⁺/K⁺ ATPase. It uses ATP to export three Na⁺ ions for every two K⁺ ions it imports. This:

• Establishes a Na⁺ concentration gradient: Na⁺ is ~10× more concentrated outside.
• Establishes a K⁺ concentration gradient: K⁺ is ~30× more concentrated inside.
• Contributes a small direct negative charge to the inside (since three positive ions leave for every two that enter).

Differential Permeability

A second factor — equally important — is that the membrane at rest is roughly 100 times more permeable to K⁺ than to Na⁺. This is because:

• There are many K⁺ leak channels open at rest, allowing K⁺ to diffuse out down its gradient.
• There are very few open Na⁺ channels at rest; voltage-gated Na⁺ channels are closed until threshold is reached.

As K⁺ leaks out, positive charge leaves the cell faster than it enters, generating the negative interior. An equilibrium is reached when the negative interior is sufficient to electrostatically oppose further K⁺ loss. This is the resting potential.

Impermeant Anions

Inside the neurone are large, negatively charged proteins and organic phosphates that cannot cross the membrane. These contribute to the negative interior charge and help set up the electrical gradient.

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The Action Potential — Phase by Phase

When a stimulus depolarises the membrane to threshold (around −55 mV), a chain of events unfolds that OCR expects you to describe in detail.

1. Depolarisation

Once threshold is reached, voltage-gated Na⁺ channels open. Na⁺ rushes into the cell down both its concentration gradient and its electrical gradient. The inside of the cell rapidly becomes more positive, then more positive than the outside, reaching about +40 mV. This is an example of positive feedback: opening Na⁺ channels depolarises the membrane further, opening yet more Na⁺ channels, and so on.

2. Repolarisation

At the peak of the action potential, two things happen:

• Voltage-gated Na⁺ channels close (they inactivate — a second gate called the inactivation gate swings across the pore).
• Voltage-gated K⁺ channels, which open more slowly, have now opened. K⁺ flows out of the cell down its electrochemical gradient.

The combination of stopping Na⁺ entry and starting K⁺ exit rapidly restores the interior negative potential.

3. Hyperpolarisation

Voltage-gated K⁺ channels close slowly. During the delay between the cell reaching −70 mV and the channels fully closing, K⁺ continues to leave the cell, making the interior briefly even more negative than the resting potential — typically to about −80 mV. This dip is called hyperpolarisation (or the "undershoot").

4. Return to Rest

Once the voltage-gated K⁺ channels close and the Na⁺/K⁺ pump has had time to work, the membrane returns to −70 mV and the normal ion gradients are restored. The cell is ready to fire again.

flowchart TB
    A[Resting: -70 mV<br/>Voltage-gated channels closed] -->|Stimulus reaches threshold| B[Depolarisation<br/>Na+ channels open<br/>Na+ rushes in]
    B -->|Reaches +40 mV| C[Peak]
    C -->|Na+ channels inactivate<br/>K+ channels open| D[Repolarisation<br/>K+ flows out]
    D -->|K+ channels slow to close| E[Hyperpolarisation: -80 mV]
    E -->|Na+/K+ pump restores gradients| A

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A Graph of the Action Potential

If you plotted membrane voltage against time, you would see a characteristic spike: