Touch a hot pan and your hand pulls back before you feel the pain. The message travels along neurons as a wave of electrical and chemical change, and in the fastest neurons it covers up to 120 metres every second.
Chapter 15 of the PECTAA Biology 12 book is the Human Nervous System. After neurons and the reflex arc (section 15.2), section 15.3 explains the nerve impulse: how it is generated, how it moves along the neuron (15.3.1) and what sets its speed (15.3.2). This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Nerve impulse and membrane potential (15.3). A nerve impulse is a wave of electrochemical changes travelling along a neuron. Every cell has charges on both sides of its membrane: the inside (towards the cytoplasm) is negative and the outside (towards the extracellular fluid) positive. This difference is the membrane potential. It comes from unequal concentrations of ions and their movement across the membrane. In most cells it stays constant, but neurons and muscle cells can change ion movement, so their membrane potentials change.
1. Resting membrane potential (RMP). A neuron not conducting a signal is polarized, and its RMP is about −70 mV (inside negative). Three factors set it up:
- Fixed anions. Large negative molecules such as proteins and nucleic acids are more plentiful inside and cannot diffuse out.
- Sodium-potassium pumps. K⁺ is about 30 times more concentrated inside, and Na⁺ about 10 times more concentrated outside. The pumps use ATP to move these ions against their gradients: for every 2 K⁺ moved in, 3 Na⁺ are moved out, so the inside becomes more negative.
- K⁺ leakage. Some K⁺ leaks out through K⁺ gates.
The book's information box stresses that K⁺ leaving and negative proteins staying inside are the main factors.
2. Action potential. When a neuron becomes active, the potential at one region reverses (inside positive). This is the active membrane potential, or action potential:
| Stage | Event | Potential |
|---|---|---|
| Depolarization | Na⁺ gates open; Na⁺ rushes in | −70 mV → 0 → about 50 mV (book value), lasting about one millisecond |
| Repolarization | Na⁺ gates close; K⁺ gates open; K⁺ diffuses out | inside negative again |
| Hyperpolarization | K⁺ gates are slow to close | slightly below −70 mV; refractory period |
| Return to RMP | Na⁺-K⁺ pump restores ions | −70 mV |
During the refractory period the region cannot fire another action potential, so it is ready again only after the RMP is restored.
3. Propagation. An action potential does not really travel; it is reproduced in the next region. Each active region depolarizes the region ahead, which makes its own action potential, while the region behind repolarizes. The book compares this to a crowd doing the "wave" in a stadium: each person stands and sits in place, yet the wave moves round.
Velocity (15.3.2). The amplitude of the action potential stays the same along the axon; the last is as strong as the first. Speed varies: it is greater in axons of larger diameter and in myelinated axons. In myelinated axons action potentials form only at the nodes of Ranvier and jump from node to node, skipping the myelinated stretches. This is saltatory conduction, up to 50 times faster than the fastest non-myelinated axons.
| Neuron type (human) | Speed |
|---|---|
| Non-myelinated | 1 to 3 m s⁻¹ |
| Myelinated | up to 120 m s⁻¹ |
Key points
- RMP ≈ −70 mV, inside negative.
- Pump ratio: 3 Na⁺ out, 2 K⁺ in.
- Na⁺ in → depolarization; K⁺ out → repolarization.
- Refractory period during hyperpolarization.
- Myelin + wide axon = faster impulse.
Worked examples
Example 1. By how many millivolts does the potential change from rest to the peak of depolarization, using the book's values?
- From −70 mV to +50 mV.
- Change = 50 − (−70) = 120 mV.
Example 2. In one cycle the pump moves 3 Na⁺ out and 2 K⁺ in. What is the net charge moved out?
- Out: +3. In: +2.
- Net: one positive charge leaves per cycle, so the inside becomes more negative.
Example 3. A myelinated neuron conducts at 120 m s⁻¹. How long does an impulse take to cover 0.6 m?
- t = distance/speed = 0.6/120 = 0.005 s = 5 ms.
- At 2 m s⁻¹ (non-myelinated) it would take 0.3 s.
Common mistakes in the exam
- Saying K⁺ rushes in during depolarization. It is Na⁺ that enters.
- Saying the impulse weakens along the axon. Its amplitude stays constant.
- Getting the pump ratio backwards. 3 Na⁺ out, 2 K⁺ in.
- Saying action potentials form all along a myelinated axon. Only at the nodes of Ranvier.
- Leaving out hyperpolarization in the sequence of events.
Exam technique
- Draw and label the action potential graph: −70, 0, peak, dip below −70.
- Give each stage with the ion and the gate involved.
- Quote the book's speeds with units.
- Check your board's notice for the sections in your paper.
Practice questions
- What is a polarized neuron? Answer: A resting neuron, inside negative and outside positive, at about −70 mV.
- Which ions leave during repolarization? Answer: K⁺ ions.
- Name the gaps where action potentials form in a myelinated axon. Answer: Nodes of Ranvier.
- Give two factors that increase impulse speed. Answer: Larger axon diameter and myelination.
- What restores the ion distribution after an impulse? Answer: The sodium-potassium pump.
Ready to practise this? Continue in 2nd Year Biology.