3.4.3Additional lessons
Nerve Impulses: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 63 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Reading an Action Potential Trace
Follow the voltage trace from left to right and name each phase.
1
An oscilloscope trace plots membrane potential in millivolts on the vertical axis against time in milliseconds on the horizontal axis.
2
The flat line at about −70 mV before the spike is the resting potential, when the axon is polarised and no impulse is passing.
3
A stimulus that lifts the potential to the threshold (about −50 mV) opens voltage-gated sodium channels, and the trace rises steeply during depolarisation.
4
The peak near +30 mV marks the point where sodium channels close and voltage-gated potassium channels open, so the trace falls during repolarisation.
5
The trace often dips below −70 mV briefly (hyperpolarisation) before returning to rest, which is when the axon cannot easily fire again.
Part 2 of 6Factors Affecting Speed of Conduction
Myelination
- Myelination Myelin is a lipid-rich insulator made by Schwann cells,
- so ion movement occurs only at the nodes of Ranvier and impulses conduct far faster.
Saltatory conduction
- Saltatory conduction The local currents bridge each myelinated stretch so the impulse effectively jumps from one node of Ranvier to the next.
Axon diameter
- Axon diameter A wider axon offers less resistance to the flow of ions inside it, so the impulse travels faster than in a narrow axon.
Squid giant axon
- Squid giant axon This unmyelinated axon is up to about 1 mm wide,
- which compensates for the lack of myelin but is still slower than a thin myelinated mammalian axon.
Temperature
- Temperature Warmer conditions speed ion movement and channel opening, so impulses generally travel faster in a warmer axon within normal limits.
| Feature | Myelinated axon | Unmyelinated axon |
|---|---|---|
| Where ions cross | Only at nodes of Ranvier | Along the whole length |
| Type of conduction | Saltatory | Continuous, step by step |
| Relative speed | Fast | Slow |
| Typical example | Mammalian motor neurone | Squid giant axon |
Part 3 of 6Correlation and Causation
Correlation and Causation
Scatter graph
- Scatter graph Plotting speed against diameter shows whether points follow a pattern, such as a rising trend for a positive association.
Correlation coefficient r
- Correlation coefficient r A value from −1 to +1 where values near ±1 indicate a strong relationship and values near 0 indicate little or none.
Positive and negative
- Positive and negative A positive correlation means both variables rise together, whereas a negative correlation means one falls as the other rises.
Correlation is not causation
- Correlation is not causation Two variables can move together because of a third factor,
- so a high r alone does not prove one variable causes the other.
Establishing cause
- Establishing cause Cause is supported by controlled experiments that change one variable while keeping the rest constant,
- such as comparing the same axon before and after myelin damage.
Part 4 of 6Good Practice with Neurone Data
1
State the independent variable (diameter or myelination) and the dependent variable (conduction speed in m/s) before analysing any data.
2
Convert units consistently, because axon diameters are often given in micrometres while distances are given in millimetres or metres.
3
Calculate speed using distance divided by time, then compare values using the same units for every axon.
4
Describe the trend first, quote data with units, and only then explain it using insulation and resistance.
Part 5 of 6Worked Example
Question
- A myelinated axon is stimulated at one end and the impulse is detected 0.45 m away after 4.5 ms.
- Calculate the conduction speed.
- Convert time to seconds4.5 ms = 4.5 × 10⁻³ s = 0.0045 s.
- Use speed = distance ÷ time = 0.45 m ÷ 0.0045 s.
- Calculate0.45 ÷ 0.0045 = 100.
Answer 100 m/s
Part 6 of 6Exam Checklist & Tips
3.4.3 – Key Points for the Exam
- Conduction speed The distance travelled by an impulse along an axon per unit time, usually in m/s.
- Saltatory conduction Impulse transmission that jumps between nodes of Ranvier along a myelinated axon.
- Node of Ranvier An uninsulated gap between Schwann cells where ion channels are concentrated.
- Correlation coefficient A statistic between −1 and +1 describing the strength and direction of a linear relationship.
- Hyperpolarisation A period when the membrane potential is more negative than the resting potential.
Common Mistakes
- Saying the impulse is an electric current flowing along the axon like a wire;
- it is a wave of membrane potential reversal.
- Claiming a wider axon is always faster than a myelinated one; myelination can outweigh diameter.
- Concluding that a strong correlation proves causation without any controlled experiment.
- Writing that sodium ions jump between nodes;
- it is the depolarisation (local current) that spreads, not the ions themselves.
Last updated:
\n