IB Biology HLNerves & SignallingPaper 1 & 2~10 min read
Interpreting Oscilloscope Traces
Half the marks in this part of the topic come from reading a graph properly. Once you know what each bit of the shape means, an oscilloscope trace stops being a squiggle and starts being a sentence.
📚 What you need to know
Membrane potential is measured by putting electrodes on each side of the membrane and displaying the reading on an oscilloscope.
An oscilloscope is an electronic test instrument that graphically displays varying voltages.
The display is a graph: time in milliseconds on the x-axis, membrane potential in millivolts on the y-axis.
A straight horizontal line at −70 mV means the neurone is at resting potential.
An action potential appears as a spike rising to between +30 and +40 mV.
The rising phase shows depolarisation; the falling phase shows repolarisation.
After the spike the trace often dips below −70 mV for a short period — hyperpolarisation.
The slow rise before threshold is often left off the graph, but it is there in real traces.
What you are actually looking at
You cannot see a membrane potential, so it has to be turned into something you can. Electrodes are placed on each side of the axon membrane and they measure the difference in charge between the two sides — the potential difference, or voltage.
That reading is fed into an oscilloscope, which draws it as a graph in real time. So every point on the trace is one moment, and the height of the trace at that moment is how negative or positive the inside of the axon is.
Reading the axes
x-axis = time / ms • y-axis = membrane potential / mV
How to read the shape
What you see
What it means
What is happening to the ions
Flat horizontal line at −70 mV
Resting potential — the neurone is polarised
Pumps working, K+ leaking out steadily
Rising part of the spike
Depolarisation
Voltage gated sodium channels open, Na+ floods in
Top of the spike, +30 to +40 mV
Peak of the action potential
Sodium channels closing
Falling part of the spike
Repolarisation
Potassium channels open, K+ leaves
Dip below the resting line
Hyperpolarisation
Potassium channels slow to close, too much K+ has left
Slow gentle rise before the spike
Sub-threshold depolarisation
Only a few sodium channels open, threshold not yet reached
Trace back from the top of the spike to the y-axis to get the peak, and drop straight down to the x-axis to get the timing. Those two habits pick up most of the marks.
Getting numbers off a trace
Skills questions usually want a calculation, not a description. There are only three that come up.
Read a value. Draw a line from the point on the trace across to the y-axis. Give the unit — mV.
Find a duration. Drop lines from the start and end of the event to the x-axis and subtract. Give the unit — ms.
Find a frequency. Count the spikes, note the total time in seconds, then divide. The unit is Hz (spikes per second).
Frequency of action potentials
frequency (Hz) = number of action potentials ÷ time (s)
Watch the units. The x-axis is in milliseconds but frequency is per second. Divide the time by 1000 before you do anything else and you will never lose that mark.
Traces from stimuli of different strengths
Because action potentials are all-or-nothing, a stronger stimulus does not give you a taller spike. It gives you more spikes in the same amount of time. On a trace that is very easy to see, and it is a favourite exam question.
If a question shows you two traces and asks which stimulus was stronger, count the spikes. Never compare their heights, because they are always the same.
Worked examples
WE 1
Calculate the frequency of action potentials
An oscilloscope trace recorded over 120 ms shows 6 action potentials, evenly spaced. Calculate the frequency of action potentials in Hz. (2 marks)
Step 1: convert the time
120 ms ÷ 1000 = 0.120 sStep 2: divide
frequency = 6 ÷ 0.120
50 Hz (50 action potentials per second)the unit Hz is usually worth its own mark. Never leave a frequency as a bare number
WE 2
Calculate a rate of depolarisation
On a trace, the membrane potential rises from −60 mV to +30 mV in 0.6 ms. Calculate the mean rate of depolarisation. (2 marks)
Step 1: find the change
+30 − (−60) = 90 mVStep 2: divide by the time
90 ÷ 0.6
150 mV per mssubtracting a negative catches people out. +30 minus −60 is 90, not 30
WE 3
Explain a feature of a trace
A trace shows the membrane potential falling to −80 mV immediately after an action potential, before returning to −70 mV. Explain this part of the trace. (3 marks)
Step 1: name it
This is hyperpolarisation — the membrane is more negative than resting potential.
Step 2: give the cause
The voltage gated potassium channels are slow to close, so K⁺ keeps diffusing out after the membrane has already repolarised.
Step 3: what happens next
The channels close and the sodium–potassium pumps restore resting potential; during this time the membrane is in its refractory period and cannot fire again.
Too much potassium leaves, so the inside overshoots to −80 mVlink it to the refractory period — that is often the third mark
💡 Exam tips
Label the axes in your head before you read anything: ms across, mV up.
Use a ruler edge or the side of your pencil to trace across to the axis. Estimating by eye loses marks.
Quote values with units and signs. "70" is wrong; "−70 mV" is right.
When asked to compare two traces, look at number of spikes and spacing, not height.
If a trace is flat, say why: threshold was not reached.
⚠ Common mistakes
Forgetting to convert ms to s in a frequency calculation. Every time.
Saying a taller spike means a stronger stimulus. Action potentials are all-or-nothing.
Calling the dip below the line repolarisation. Repolarisation is the fall to resting; the dip past it is hyperpolarisation.
Dropping the minus sign when reading a value below zero.
Describing rather than calculating. If it says "calculate", show the numbers and the working.
Reading the peak as exactly +40 mV every time. Read the actual axis; it may be +30.
Up next: Nerve Conduction Velocity — saltatory conduction, and the calculation that goes with it.
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