3.5.3Inquiry Process
Nerve Impulses- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 134 of 160
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.
Section 1 of 5Reading Membrane Potential Graphs
1
Start by reading the axes: membrane potential in millivolts on the vertical axis and time in milliseconds on the horizontal axis.
2
Identify the resting potential as the flat starting line, typically near −70 mV for a mammalian neurone.
3
Find the threshold, the point of about −55 mV at which the trace suddenly rises steeply.
4
Read the peak, usually around +30 to +40 mV, then the fall back towards and slightly below the resting potential.
5
Measure the duration of the impulse as the time between the start of the rise and the return to resting level, usually about 1 to 2 ms.
Section 2 of 5Experimental Treatments
Tetrodotoxin
- Tetrodotoxin This toxin blocks voltage-gated sodium channels, so depolarisation fails and no action potential is produced.
Sodium-poor solution
- Sodium-poor solution Lowering the extracellular Na⁺ reduces the concentration gradient, giving a smaller peak and slower rise.
Raised potassium outside
- Raised potassium outside Extra extracellular K⁺ reduces the K⁺ gradient and makes the resting potential less negative.
Metabolic poison
- Metabolic poison Poisoning ATP production stops the pump, and over time the gradients run down and impulses fail.
| Treatment | Effect on trace | Reason |
|---|---|---|
| Sodium channel blocker | No spike | Na⁺ cannot enter |
| Low external Na⁺ | Smaller peak | Smaller gradient for Na⁺ entry |
| Potassium channel blocker | Slow repolarisation | K⁺ cannot leave quickly |
Section 3 of 5Stimulus Strength
Stimulus Strength
Below threshold
- Below threshold A weak stimulus gives only a small local depolarisation that fades, and no action potential is formed.
Above threshold
- Above threshold Any stimulus reaching threshold produces a full action potential of the same size.
All-or-nothing
- All-or-nothing The size of the spike does not change with stimulus strength, so bigger stimuli cannot make bigger impulses.
Frequency coding
- Frequency coding Stronger stimuli instead produce more impulses per second, which is how intensity is signalled.
Section 4 of 5Worked Example
Question
- A trace shows the potential rising from −70 mV to a peak of +35 mV in 1.5 ms.
- Calculate the total change in potential and the mean rate of change.
- Change = (+35) − (−70) = 105 mV.
- Time = 1.5 ms.
- Rate = 105 ÷ 1.5 = 70 mV per ms.
Answer The potential changes by 105 mV at a mean rate of 70 mV per ms.
Section 5 of 5Exam Checklist & Tips
3.5.3 – Key Points for the Exam
- Threshold potential The membrane potential, around −55 mV, at which an action potential is triggered.
- All-or-nothing Principle that an action potential either occurs fully or not at all.
- Tetrodotoxin A poison that blocks voltage-gated sodium channels.
- Peak The most positive value reached by the membrane potential in an action potential.
Common Mistakes
- Reading time from the wrong axis, or forgetting to subtract a negative value correctly.
- Saying a stronger stimulus makes a larger action potential; it makes more of them.
- Quoting the threshold as the resting potential, which is lower, around −70 mV.
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