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3.5.6Inquiry Process

Interpreting Oscilloscope Traces- Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 137 of 160

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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.
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Part 1 of 5Reading the Display

Vertical axis
  • Vertical axis Shows membrane potential, and the volts-per-division setting converts the number of squares into millivolts.
Horizontal axis
  • Horizontal axis Shows time, and the time-base setting gives milliseconds for each division across the screen.
Baseline
  • Baseline The flat line before the spike corresponds to the resting potential, so measure amplitudes from or relative to this line.
Spike
  • Spike Each narrow upward peak is one action potential, and its steepness shows how quickly depolarisation occurs.

Part 2 of 5Measuring from a Trace

1
Check the scales first, noting mV per division and ms per division before taking any measurement.
2
Measure the amplitude as the vertical distance from the resting potential or baseline to the peak, multiplied by the vertical scale.
3
Measure the duration of one spike horizontally and multiply by the time base.
4
Measure the interval between two successive peaks, then take its reciprocal to find the frequency of impulses.
5
Compare traces under different conditions, and look for changes in amplitude, duration, interval and baseline.

Part 3 of 5Typical Patterns

Typical Patterns
Single spike
  • Single spike A single action potential is a rise, a fall and a small undershoot, lasting a couple of milliseconds.
Repeated spikes
  • Repeated spikes Evenly spaced identical spikes show a constant stimulus, and closer spacing means a stronger one.
No spike
  • No spike A flat trace after a stimulus indicates that the stimulus failed to reach threshold or conduction was blocked.
Two electrodes
  • Two electrodes Two recording points show the same shape delayed in time, which allows conduction velocity to be calculated.
MeasurementCalculationUnit
AmplitudeDivisions × mV per divisionmV
DurationDivisions × ms per divisionms
Frequency1 ÷ interval in secondsHz

Part 4 of 5Worked Example

Question

  • A trace has a baseline at −70 mV and a peak at +40 mV.
  • The vertical scale is 20 mV per division and the time base is 1 ms per division.
  • Successive peaks are 4 divisions apart.
  • Calculate the amplitude and the frequency.

  1. Amplitude = 40 − (−70) = 110 mV, which is 110 ÷ 20 = 5.5 divisions.
  2. Interval = 4 divisions × 1 ms = 4 ms = 0.004 s.
  3. Frequency = 1 ÷ 0.004 = 250 Hz, that is 250 impulses per second.

Answer

  • Amplitude 110 mV;
    • frequency 250 impulses per second.

Part 5 of 5Exam Checklist & Tips

3.5.6 – Key Points for the Exam

  • Oscilloscope Instrument that displays voltage against time, used to record nerve impulses.
  • Amplitude Size of a signal measured from baseline to peak.
  • Time base Setting that states how much time each horizontal division represents.
  • Frequency Number of events per second, equal to 1 divided by the interval.

Common Mistakes

  • Reading divisions without applying the scale factors.
  • Giving the interval in ms in a frequency calculation instead of converting to seconds.
  • Measuring the amplitude from the zero line rather than from the baseline when asked for change.

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