Depolarization, repolarization and oscilloscope traces

Cells (Interaction and interdependence) · Neural signalling · note 3 of 7

Spec C2.2.8, C2.2.9, C2.2.10

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Depolarization, repolarization and oscilloscope tracesSpec C2.2.8, C2.2.9, C2.2.10

In short

An action potential starts when the membrane reaches the threshold potential, about −55 mV, and voltage-gated sodium channels open. Na⁺ diffuses in (depolarization), then voltage-gated potassium channels open and K⁺ diffuses out (repolarization). Local currents of Na⁺ bring the next region to threshold. An oscilloscope trace plots these changes in membrane potential against time.

Depolarization and repolarization

A stimulus or local current makes the membrane potential less negative. If it reaches the threshold potential (about −55 mV), voltage-gated sodium channels open. Below threshold they stay closed and no action potential occurs.
Na⁺ diffuses in down its concentration gradient. The entry of Na⁺ depolarizes the membrane further, opening more sodium channels, until the inside reaches about +30 mV (depolarization).
The sodium channels close, and voltage-gated potassium channels open.
K⁺ diffuses out, making the inside negative again (repolarization). The potential briefly falls below the resting potential before the potassium channels close.
Sodium–potassium pumps maintain the concentration gradients, so the neuron can carry further impulses.

Propagation by local currents

Na⁺ entering a depolarized region diffuses along the inside of the axon towards the next, still polarized region. Outside the membrane, Na⁺ diffuses from the next region towards the depolarized region. These local currents make the potential in the next region less negative until the threshold potential is reached and its sodium channels open.

The region just behind the action potential is still repolarizing and its sodium channels cannot reopen yet, so the impulse travels in one direction only.

Local currents of Na⁺ bring the next region of the axon to the threshold potential.

Oscilloscope traces

An oscilloscope trace plots membrane potential (mV) on the y-axis against time (ms) on the x-axis. You should link each part of the trace to events in the membrane.

Reading an oscilloscope trace of one action potential
Part of trace	Membrane potential	Cellular event
Flat line before the spike	About −70 mV	Resting potential maintained by pumps and K⁺ leak
Small rise to threshold	−70 to −55 mV	Local currents or stimulus depolarize the membrane
Steep rise	−55 to about +30 mV	Voltage-gated Na⁺ channels open; Na⁺ diffuses in
Steep fall	+30 mV to below −70 mV	Na⁺ channels close; voltage-gated K⁺ channels open; K⁺ diffuses out
Return to resting level	Back to about −70 mV	K⁺ channels close; resting potential restored

Oscilloscope trace of one action potential: membrane potential / mV against time / ms, with resting potential at −70 mV, a small rise to the threshold potential at −55 mV, depolarization to a peak of +30 mV as Na⁺ channels open and Na⁺ diffuses in, repolarization as K⁺ channels open and K⁺ diffuses out, a brief fall to about −80 mV and a return to −70 mV. — Oscilloscope trace of one action potential (typical values): −70 mV resting, −55 mV threshold, peak about +30 mV.

Oscilloscope trace over 1.0 s showing eight action potentials of equal height, each rising from −70 mV to +30 mV, numbered for counting. — Counting impulses: eight action potentials in 1.0 s is a frequency of 8 impulses per second (illustrative trace).

Number of impulses per second

An oscilloscope trace shows 18 action potentials in a 0.30 s recording. Calculate the frequency of impulses.

frequency = number of impulses ÷ time
= 18 ÷ 0.30 s

Answer: 60 impulses per second (60 Hz)

Practical skill:

Read the time scale before counting spikes. A stronger stimulus gives more impulses per second, not taller action potentials: each action potential reaches the same peak.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is the resting potential negative?

The resting potential is negative because sodium–potassium pumps move three Na⁺ out for every two K⁺ in, the membrane lets K⁺ leak out much faster than Na⁺ leaks in, and negatively charged proteins stay inside the cytoplasm. Together these leave the inside of the neuron about 70 mV more negative than the outside.

How does a nerve impulse cross the synapse?

The impulse depolarizes the presynaptic membrane, so calcium ions enter and cause vesicles to release neurotransmitter by exocytosis. The neurotransmitter, such as acetylcholine, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Positive ions enter and depolarize it; if threshold is reached, a new action potential starts.

Why do myelinated neurons conduct impulses faster?

HL only Myelin insulates the axon, so action potentials can only occur at the nodes of Ranvier, where ion pumps and channels are clustered. Local currents spread under the myelin to the next node, so the impulse jumps from node to node. This saltatory conduction is much faster than depolarizing every part of the membrane in turn.

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