The Action Potential
Neurones transmit information as electrical impulses called action potentials. Understanding how these are generated, propagated, and regulated is fundamental to A-Level Biology. This lesson covers the ionic basis of the resting potential and action potential, the all-or-nothing principle, and the refractory period.
Key Definition: An action potential is a rapid, temporary reversal of the electrical potential difference across the axon membrane, caused by the sequential opening and closing of voltage-gated ion channels.
The Resting Potential
When a neurone is not transmitting an impulse, it is said to be at rest. The inside of the axon is negatively charged relative to the outside. This potential difference across the membrane is called the resting potential, typically around −70 mV.
How the Resting Potential is Maintained
The resting potential is established and maintained by several mechanisms:
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The sodium-potassium pump (Na⁺/K⁺-ATPase):
- This is an active transport protein in the axon membrane.
- It pumps 3 Na⁺ ions out of the axon for every 2 K⁺ ions pumped in, using one molecule of ATP per cycle.
- This creates a net movement of positive charge out of the cell, contributing to the negative interior.
- The pump maintains the concentration gradients: high Na⁺ outside, high K⁺ inside.
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Potassium leak channels:
- The axon membrane contains potassium ion channels that are open at rest, allowing K⁺ to diffuse out of the axon down its concentration gradient.
- This outward movement of positive K⁺ ions makes the inside of the axon more negative.
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Sodium channels are closed at rest:
- Voltage-gated Na⁺ channels are closed when the membrane is at −70 mV, preventing Na⁺ from entering.
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Large organic anions:
- Negatively charged proteins and other organic molecules inside the axon are too large to cross the membrane, contributing to the overall negative charge inside.
Exam Tip: The resting potential is mainly due to K⁺ leaking out through open potassium channels. The Na⁺/K⁺ pump is essential for maintaining the concentration gradients but contributes only a small direct electrogenic effect.
Generating an Action Potential
When a neurone is stimulated, the following sequence of events occurs:
1. Stimulus and Depolarisation
- A stimulus causes sodium ion channels (voltage-gated Na⁺ channels) to open at the point of stimulation.
- Na⁺ ions flood into the axon down their electrochemical gradient (high concentration outside, attracted by negative charge inside).
- This influx of positive charge causes the membrane potential to become less negative (depolarisation).
- If the depolarisation reaches the threshold (approximately −55 mV), an action potential is triggered.
2. Rapid Depolarisation
- Once threshold is reached, more voltage-gated Na⁺ channels open in a positive feedback loop.
- Massive Na⁺ influx causes the membrane potential to rapidly rise, reaching approximately +30 mV (the inside becomes positively charged relative to the outside).
- This phase is extremely rapid, taking less than 1 millisecond.
3. Repolarisation
- At approximately +30 mV, the voltage-gated Na⁺ channels close (they become inactivated).
- Voltage-gated K⁺ channels open (these are slower to respond to the initial depolarisation).
- K⁺ ions rush out of the axon down their concentration gradient, carrying positive charge out of the cell.
- The membrane potential falls back towards the resting potential.
4. Hyperpolarisation (Undershoot)
- The voltage-gated K⁺ channels are slow to close, so K⁺ continues to leave the axon even after the resting potential is reached.
- The membrane potential temporarily becomes more negative than −70 mV (e.g., −80 mV).
- This is called hyperpolarisation or the undershoot.
5. Restoration of the Resting Potential
- The voltage-gated K⁺ channels eventually close.
- The Na⁺/K⁺ pump restores the original ion distribution (3 Na⁺ out, 2 K⁺ in).
- The resting potential of −70 mV is re-established.
Summary of Ion Movements During an Action Potential