Resting potential, action potentials and saltatory conduction

The resting potential

At rest the inside of an axon is negative compared with the outside, at about -70 mV: the resting potential.

• The sodium-potassium pump actively transports 3 Na+ out for every 2 K+ in, using ATP.
• The membrane has many open K+ channels and few open Na+ channels, so K+ diffuses out down its concentration gradient more than Na+ diffuses in.

The result is more positive ions outside than inside: the membrane is polarised.

The action potential

1. A stimulus opens some voltage-gated Na+ channels. Na+ diffuses in, making the inside less negative.

2. If the membrane reaches the threshold (about -55 mV), many more Na+ channels open (positive feedback) and Na+ floods in: depolarisation, to about +40 mV.

3. The Na+ channels close and voltage-gated K+ channels open. K+ diffuses out: repolarisation.

4. Too many K+ leave, so the membrane briefly becomes more negative than resting: hyperpolarisation.

5. The K+ channels close and the pump restores the resting potential.

During the refractory period a new action potential cannot be produced. This makes impulses travel one way and keeps them separate. Action potentials are all-or-nothing: a stronger stimulus gives more impulses per second, not bigger ones.

Propagation and myelin

The influx of Na+ sets up local currents that depolarise the next section of membrane to threshold, so the action potential moves along the axon.

In myelinated neurones, the myelin sheath insulates the axon, so depolarisation happens only at the nodes of Ranvier. The impulse jumps from node to node: saltatory conduction, which is much faster. Wider axons also conduct faster.

Worked example

An impulse travels 0.9 m along a myelinated axon in 0.015 s. Calculate its speed.

1. Speed = distance / time
2. = 0.9 / 0.015 = 60

Answer: 60 m s-1

Check yourself

1. During an action potential the membrane potential changes from -70 mV to +40 mV. What is the total change in mV?
   Show answer

   110 mV. 40 - (-70) = 110 mV.

2. An impulse travels 1.2 m in 0.02 s. Calculate its speed in m s-1.
   Show answer

   60 m s-1. 1.2 / 0.02 = 60 m s-1.

3. The sodium-potassium pump moves 150 Na+ ions out of a cell. How many K+ ions does it move in?
   Show answer

   100. Ratio 3 Na+ : 2 K+, so 150 / 3 x 2 = 100.

4. What causes depolarisation during an action potential?
   • K+ diffuses into the axon
   • Voltage-gated Na+ channels open and Na+ diffuses into the axon
   • The sodium-potassium pump speeds up
   • Cl- ions leave the axon
   Show answer

   Voltage-gated Na+ channels open and Na+ diffuses into the axon. Na+ influx makes the inside positive.

5. What causes repolarisation?
   • Na+ channels open wider
   • Calcium ions enter
   • ATP is broken down outside the cell
   • Na+ channels close and K+ channels open, so K+ diffuses out
   Show answer

   Na+ channels close and K+ channels open, so K+ diffuses out. Loss of K+ makes the inside negative again.

6. What is one consequence of the refractory period?
   • Impulses become larger
   • Impulses travel in one direction only
   • The resting potential becomes positive
   • Myelin forms
   Show answer

   Impulses travel in one direction only. The membrane behind the impulse cannot fire again straight away.

7. Why is conduction faster in myelinated axons?
   • Myelin carries the impulse as a current of Na+
   • Myelinated axons have no Na+ channels anywhere
   • The impulse travels inside the Schwann cells
   • Depolarisation occurs only at nodes of Ranvier, so the impulse jumps between nodes
   Show answer

   Depolarisation occurs only at nodes of Ranvier, so the impulse jumps between nodes. Saltatory conduction skips the insulated sections.

8. How does the nervous system tell a strong stimulus from a weak one?
   • A strong stimulus produces larger action potentials
   • A strong stimulus slows the impulses
   • A strong stimulus produces more action potentials per second
   • A weak stimulus produces negative action potentials
   Show answer

   A strong stimulus produces more action potentials per second. Action potentials are all-or-nothing, so frequency carries intensity.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• Lumen Learning (OpenStax Biology 2e): How Neurons Communicate

Last updated 7 November 2026. Spotted a mistake? Tell us.