In neurobiology, nerve impulses are transmitted along axons in the form of electrical signals called **Action Potentials**. A-Level examiners repeatedly test the precise molecular dynamics of sodium and potassium channels across the neuronal membrane. Mastering the exact terminology is the key to securing full marks on these high-yield questions.
- Resting Potential (-70 mV): Maintained by the active Na+/K+ pump and K+ leak channels.
- Depolarisation (+40 mV): Stimulus exceeds threshold potential (-55 mV), opening voltage-gated Na+ channels.
- Repolarisation: Na+ channels close, voltage-gated K+ channels open, K+ flows out.
- Refractory Period: Hyperpolarisation prevents backward propagation, making impulses unidirectional and discrete.
- Boards: Heavily featured in AQA Paper 2 and OCR A (Section 5.1.3).
The Nerve Impulse Roadmap
The establishment, generation, and recovery of an action potential follows a strict molecular sequence:
Resting Potential (-70 mV)
The sodium-potassium pump actively transports 3 Na+ out of the axon for every 2 K+ transported in. Because the membrane is more permeable to potassium (due to open K+ leak channels), potassium diffuses back out, leaving the inside of the axon negatively charged relative to the outside.
Depolarisation & Threshold (-55 mV to +40 mV)
A stimulus causes a few voltage-gated Na+ channels to open. Na+ diffuses down its concentration gradient. If the membrane potential reaches the threshold potential (-55 mV), positive feedback triggers all voltage-gated Na+ channels to open. Na+ floods in, raising the potential to +40 mV.
Repolarisation
At +40 mV, the voltage-gated Na+ channels close and become inactivated. Simultaneously, voltage-gated K+ channels open. K+ diffuses out of the axon down its concentration gradient, carrying positive charge away and lowering the membrane potential.
Hyperpolarisation & Recovery
Voltage-gated K+ channels are slow to close, causing an excess outflow of K+. The potential drops below the resting level (e.g. -80 mV). The voltage-gated channels close, and the Na+/K+ pump restores the original resting potential of -70 mV.
Ion Channel State Summary Sheet
To avoid confusing channel activities, review the states of the channels during each phase:
| Phase | Membrane Potential | Na+/K+ Pump | Voltage-Gated Na+ Channels | Voltage-Gated K+ Channels |
|---|---|---|---|---|
| Resting State | -70 mV | Active (Uses ATP) | Closed | Closed |
| Depolarisation | Rises to +40 mV | Active | Open | Closed |
| Repolarisation | Falling | Active | Closed (Inactivated) | Open |
| Hyperpolarisation | -80 mV | Active | Closed | Closing (Slow) |
How Action Potentials Propagate
When an action potential occurs, Na+ ions entering the axon diffuse sideways (laterally). This neutralises adjacent negative charges, depolarising the next section of the membrane to threshold and opening its voltage-gated Na+ channels. This wave of depolarisation travels down the axon.
Common Pitfall: Confusing Pump vs. Channels
A frequent error is stating that the Na+/K+ pump is responsible for depolarisation or repolarisation. It is not! The pump runs continuously to maintain the concentration gradient. The rapid voltage shifts are caused entirely by passive facilitated diffusion through voltage-gated channels. Do not write that the pump 'stops' or 'starts' during the action potential.
Pro-Tip: Memorise the 3 Roles of the Refractory Period
Exams frequently ask why the refractory period is vital. Memorise these three exact keywords: 1. Unidirectional (impulses only travel forward as the region behind is inactive), 2. Discrete (impulses are separated and cannot overlap), 3. Limits Frequency (caps the maximum number of nerve impulses per second).
Frequently Asked Questions
If a stimulus does not depolarise the membrane to threshold (-55 mV), no action potential will fire. If it does reach threshold, a full action potential of +40 mV will fire. The size of the action potential is always the same regardless of stimulus strength; stronger stimuli simply increase the frequency of firing, not the amplitude.
Myelin acts as an electrical insulator. Depolarisation can only occur at gaps in the myelin sheath called Nodes of Ranvier. The local ionic currents jump from one node to the next (saltatory conduction), which is significantly faster than depolarising the entire continuous length of the axon membrane.
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