Nerve Impulse: Generation, Transmission, and Propagation
A nerve impulse is not merely an electrical signal — it is a self-propagating electrochemical tsunami that races along the neuron at speeds up to 120 m/s. It is nature’s most elegant way of converting chemical information into pure electrical code and back again.
While every living cell maintains a membrane potential, only neurons and muscle cells have evolved the ability to rapidly and reversibly flip that potential. This ability is the foundation of thought, sensation, movement, and consciousness itself.
1. Resting Membrane Potential (RMP) – The Neuron’s Silent Charge
When a neuron is quiet, it is polarized. The inside is negative relative to the outside. The value of this difference is the Resting Membrane Potential, approximately –70 mV.
Four forces lock the neuron in this charged state:
- Fixed anions: Large negatively charged proteins and organic phosphates trapped inside the cell.
- Sodium-Potassium Pump: Actively pumps 3 Na⁺ out for every 2 K⁺ in, using ATP. Net loss of positive charge from the interior.
- Concentration gradients: K⁺ is ~30 times higher inside; Na⁺ is ~10 times higher outside.
- Selective permeability: At rest the membrane is far more permeable to K⁺ than to Na⁺. K⁺ continuously leaks out through leak channels, making the inside even more negative.
This –70 mV is not static — it is a dynamic steady state maintained by continuous pumping and leaking.
2. Generation of Action Potential – The Electrical Storm
When a stimulus depolarizes the membrane to threshold (usually around –55 mV), voltage-gated sodium channels explode open. What follows is a precise four-phase sequence:
| Phase | Key Ionic Event | Membrane Potential Change |
|---|---|---|
| Depolarization | Voltage-gated Na⁺ channels open → Na⁺ rushes in | –70 mV → +50 mV |
| Repolarization | Na⁺ channels inactivate; Voltage-gated K⁺ channels open → K⁺ rushes out | +50 mV → –70 mV |
| Hyperpolarization | K⁺ channels stay open slightly longer | Briefly goes below –70 mV |
| Recovery | Na⁺/K⁺ pump restores original gradients | Returns exactly to –70 mV |
During the absolute refractory period (most of depolarization + early repolarization), the neuron cannot fire another action potential. This guarantees unidirectional travel and limits firing frequency.
3. Propagation – How the Impulse Travels Without Dying
An action potential does not “move” like a ball rolling down a tube. It is regenerated at every point along the membrane:
- Local depolarization of one patch becomes the stimulus for the adjacent resting patch.
- The adjacent patch reaches threshold and fires its own action potential.
- The original patch recovers, ensuring the signal only travels forward.
This continuous regeneration keeps the amplitude of the action potential constant from axon hillock to axon terminal — a perfect digital signal.
4. Speed of Conduction – Why Some Axons Are Superhighways
Two structural features control velocity:
- Axon diameter: Larger diameter = lower internal resistance = faster conduction.
- Myelination: Myelin acts as electrical insulation. The action potential is forced to jump from one Node of Ranvier to the next. This is called saltatory conduction.
Speed comparison:
- Non-myelinated axons: 0.5 – 3 m/s
- Myelinated axons: up to 120 m/s (40–50 times faster)
Saltatory conduction is one of evolution’s greatest energy-saving and speed-enhancing innovations.
Quick Revision Points (Exam Gold)
- Resting Membrane Potential ≈ –70 mV
- Peak of Action Potential ≈ +50 mV
- Threshold ≈ –55 mV
- Saltatory conduction occurs only in myelinated neurons
- Refractory period ensures one-way travel
- Speed depends on axon diameter + presence of myelin
- Na⁺/K⁺ pump restores ionic gradients after the action potential
✅ 30 High-Quality MCQs + 10 Powerful FAQs Nerve Impulse – Class 12 Biology (Exam-ready • Concept-hacking • Board + Competitive level)
30 Multiple Choice Questions
1. The resting membrane potential of a typical neuron is approximately: A) +70 mV B) –70 mV C) 0 mV D) +30 mV Answer: B
2. Which of the following is primarily responsible for the negative charge inside a resting neuron? A) High concentration of Na⁺ inside B) Fixed negative anions (proteins & organic phosphates) C) Continuous influx of Cl⁻ D) High permeability to Na⁺ at rest Answer: B
3. The sodium-potassium pump transports: A) 2 Na⁺ out and 3 K⁺ in B) 3 Na⁺ out and 2 K⁺ in C) 3 Na⁺ in and 2 K⁺ out D) Equal number of Na⁺ and K⁺ Answer: B
4. During resting state, the membrane is most permeable to: A) Sodium ions B) Potassium ions C) Calcium ions D) Chloride ions Answer: B
5. The threshold potential for generating an action potential is usually around: A) –70 mV B) –55 mV C) 0 mV D) +30 mV Answer: B
6. Depolarization of the axon membrane is caused by: A) Outflow of K⁺ B) Inflow of Na⁺ C) Inflow of K⁺ D) Outflow of Na⁺ Answer: B
7. The peak of the action potential reaches approximately: A) –70 mV B) 0 mV C) +30 to +50 mV D) –90 mV Answer: C
8. Repolarization is mainly due to: A) Opening of Na⁺ channels B) Closing of K⁺ channels C) Opening of voltage-gated K⁺ channels D) Activity of Na⁺/K⁺ pump only Answer: C
9. During the absolute refractory period: A) A stronger stimulus can generate another action potential B) No new action potential can be generated C) The membrane is hyperpolarized only D) Na⁺ channels are fully open Answer: B
10. Hyperpolarization occurs because: A) Na⁺ channels remain open B) K⁺ channels close slowly C) Cl⁻ ions enter rapidly D) The pump stops working Answer: B
11. The all-or-none principle means: A) Action potential amplitude varies with stimulus strength B) Once threshold is reached, a full action potential is generated C) Weak stimuli produce small action potentials D) Action potentials decrease in size along the axon Answer: B
12. Propagation of action potential occurs by: A) Continuous movement of the same ions B) Local circuit current stimulating adjacent membrane C) Diffusion of neurotransmitters along the axon D) Flow of cytoplasm Answer: B
13. Saltatory conduction is characteristic of: A) Non-myelinated axons B) Myelinated axons C) Both myelinated and non-myelinated D) Dendrites only Answer: B
14. In saltatory conduction, the action potential jumps from: A) One myelin segment to another B) One Node of Ranvier to the next C) Dendrite to axon terminal D) Soma to axon hillock Answer: B
15. Maximum speed of nerve impulse in myelinated human axons is about: A) 1–3 m/s B) 10–20 m/s C) 50–70 m/s D) Up to 120 m/s Answer: D
16. Conduction velocity is higher in: A) Thin non-myelinated fibres B) Thick myelinated fibres C) Thin myelinated fibres D) All are equal Answer: B
17. The refractory period ensures: A) Bidirectional travel of impulse B) Unidirectional travel of impulse C) Continuous generation of impulses D) Decrease in amplitude Answer: B
18. Which of the following restores the original ionic concentration after an action potential? A) Voltage-gated Na⁺ channels B) Voltage-gated K⁺ channels C) Na⁺/K⁺ ATPase pump D) Leak channels only Answer: C
19. At the peak of action potential, the membrane is: A) Polarized B) Depolarized C) Hyperpolarized D) At resting potential Answer: B
20. Myelination increases conduction speed mainly by: A) Increasing axon diameter B) Decreasing membrane capacitance and increasing resistance C) Allowing continuous Na⁺ entry D) Increasing the number of ion channels Answer: B
21. The local currents that help in propagation flow: A) Only outside the axon B) Only inside the axon C) Both inside and outside the membrane D) Through the myelin sheath Answer: C
22. In non-myelinated fibres, conduction is: A) Saltatory B) Continuous C) Jumping D) Extremely fast Answer: B
23. The negative sign of resting potential indicates: A) Outside is negative B) Inside is negative relative to outside C) Equal charges on both sides D) Membrane is positively charged Answer: B
24. Which ion is more concentrated outside the neuron at rest? A) K⁺ B) Na⁺ C) Organic anions D) Both A and B Answer: B
25. During depolarization, the membrane potential changes from: A) Positive to negative B) Negative to positive C) Zero to positive D) Positive to zero Answer: B
26. The period during which a second stimulus can produce another action potential only if it is stronger is called: A) Absolute refractory period B) Relative refractory period C) Latent period D) Recovery period Answer: B
27. Nodes of Ranvier are: A) Myelinated regions B) Gaps in the myelin sheath C) Synaptic knobs D) Dendritic spines Answer: B
28. Energy for the Na⁺/K⁺ pump comes from: A) Diffusion of ions B) ATP hydrolysis C) Membrane potential itself D) Neurotransmitters Answer: B
29. Action potential amplitude remains constant along the axon because: A) It is regenerated at each point B) Myelin prevents loss of charge C) The pump works continuously D) Axon diameter is uniform Answer: A
30. Which of the following is correct about conduction velocity? A) Increases with decrease in diameter B) Decreases with myelination C) Increases with both larger diameter and myelination D) Independent of temperature Answer: C
10 Frequently Asked Questions (FAQs)
1. Why is the resting membrane potential negative? Because large negatively charged proteins and organic ions remain trapped inside the cell, and the continuous outward leakage of K⁺ ions makes the interior even more negative. The Na⁺/K⁺ pump further contributes by removing more positive charges than it brings in.
2. What is the difference between depolarization and repolarization? Depolarization is the rapid rise of membrane potential toward positive values caused by Na⁺ influx. Repolarization is the return toward negative values caused mainly by K⁺ efflux.
3. Why does the action potential travel in only one direction? Because the region that has just fired enters the refractory period and cannot be re-excited immediately. Therefore the impulse can only stimulate the resting membrane ahead of it.
4. What is saltatory conduction and why is it faster? In myelinated axons, the action potential jumps from one Node of Ranvier to the next instead of travelling continuously. This skips the insulated regions, making conduction up to 50 times faster and more energy-efficient.
5. Does the strength of the stimulus affect the size of the action potential? No. Once the threshold is reached, the action potential is always the same size (all-or-none principle). Stronger stimuli only increase the frequency of action potentials, not their amplitude.
6. Why do we need the sodium-potassium pump if channels can move ions? Channels allow passive movement down concentration gradients. After many action potentials, the gradients would slowly run down. The Na⁺/K⁺ pump actively restores the original concentration differences using ATP.
7. What would happen if myelin is damaged (as in multiple sclerosis)? Saltatory conduction is disrupted. The impulse slows down or may fail to propagate, leading to neurological symptoms such as weakness, numbness, and loss of coordination.
8. Why is conduction faster in thicker axons? Larger diameter reduces the internal resistance to the flow of local circuit currents, allowing the next region of membrane to reach threshold more quickly.
9. Is the action potential the same in all neurons? The basic mechanism is the same, but the exact peak value, duration, and speed can vary depending on the type of ion channels present and whether the axon is myelinated or not.
10. How can I remember the sequence of events easily for exams? Use this flow: Resting (–70 mV) → Stimulus → Threshold → Na⁺ in (Depolarization to +50 mV) → Na⁺ close + K⁺ out (Repolarization) → Slight undershoot (Hyperpolarization) → Pump restores resting state. Refractory period prevents immediate re-firing and ensures one-way travel.
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