Interpreting Action-Potential Graphs

Revision notes

Read the axes first

An action-potential graph usually plots membrane potential in millivolts against time in milliseconds. Zero potential does not mean zero ions: it means no electrical potential difference between the measured sides at that instant. Negative values mean that the inside is negative relative to the outside under the stated convention.

A typical teaching trace starts near −70 mV, crosses a threshold near −55 mV, rises to roughly +30 mV, falls, briefly undershoots rest and then recovers. These values vary between cell types and are not universal constants. The labelled original graph uses illustrative values.

Match phases to mechanisms

The steep upward section is depolarisation, associated with increased sodium permeability and sodium influx. The downward section is repolarisation, associated with sodium-channel inactivation and increased potassium permeability. A region below the resting potential is hyperpolarisation, often reflecting delayed potassium-channel closure.

The threshold line indicates the level at which regenerative channel opening becomes sufficient to trigger a spike. A small subthreshold depolarisation need not produce a full action potential. The time interval during which a second spike cannot be produced is the absolute refractory period; it should not be confused with the entire resting baseline.

Quantitative interpretation

Voltage change is final potential minus initial potential. A rise from −70 to +30 mV is a change of +100 mV. To estimate an average rate of change, divide voltage change by elapsed time. An instantaneous slope uses a tangent, not two widely separated points across a curved trace.

To find frequency from repeated spikes, measure the period between equivalent points, such as successive peaks. Convert milliseconds to seconds before using f = 1/T. If counting spikes over a defined recording window, use number divided by duration and state the counting convention.

Compare recordings cautiously

More frequent equal-height spikes may represent a stronger stimulus. A wider spike does not automatically prove a higher conduction speed. Speed requires a known distance and time delay between recording sites. Do not infer ion concentrations directly from a voltage graph without additional data.

Worked application and data skills

A trace rises from −70 to +30 mV in 0.5 ms. Change = 30 − (−70) = 100 mV. Mean gradient = 100/0.5 = 200 mV ms⁻¹. Peaks 25 ms apart have period 0.025 s and frequency 1/0.025 = 40 Hz. A signal recorded 0.12 m farther along the axon after 3 ms has speed 0.12/0.003 = 40 m s⁻¹.

Common misconceptions

The difference between −70 and +30 is 100, not 40. Milliseconds must be converted for hertz. Spike width is not conduction speed.

Examination tips

Quote coordinates or intervals and connect each phase to the correct ion movement.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 A3.4.3. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.