CIE A Level Biology (9700) 2019-2021

Revision Notes

15.1.6 Speed of Conduction of Impulses

Speed of Conduction of Impulses

  • The speed of conduction of an impulse refers to how quickly the impulse is transmitted along a neurone
  • It is determined by two main factors:
    • the presence or absence of myelin (ie. whether or not the axon is insulated by a myelin sheath)
    • the diameter of the axon


  • In unmyelinated neurones, the speed of conduction is very slow
  • By insulating the axon membrane, the presence of myelin increases the speed at which action potentials can travel along the neurone:
    • In sections of the axon that are surrounded by a myelin sheath, depolarisation (and the action potentials that this would lead to) cannot occur, as the myelin sheath stops the diffusion of sodium ions and potassium ions
    • Action potentials can only occur at the nodes of Ranvier (small uninsulated sections of the axon)
    • The local circuits of current that trigger depolarisation in the next section of the axon membrane exist between the nodes of Ranvier
    • This means the action potentials ‘jump’ from one node to the next
    • This is known as saltatory conduction
    • This allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter


  • The speed of conduction of an impulse along neurones with thicker axons is greater than along those with thinner ones
  • Thicker axons have an axon membrane with a greater surface area over which diffusion of ions can occur
  • This increases the rate of diffusion of sodium ions and potassium ions, which in turn increases the rate at which depolarisation and action potentials can occur

The refractory period

  • Very shortly (about 1 ms) after an action potential has been generated in a section of the axon membrane, all the sodium ion voltage-gated channel proteins in this section close. This stops any further sodium ions diffusing into the axon
  • Potassium ion voltage-gated channel proteins in this section of axon membrane open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
  • This gradually returns the potential difference to normal (about -70mV) – a process known as repolarisation
  • Once the resting potential is close to being reestablished, the potassium ion voltage-gated channel proteins close and the sodium ion channel proteins in this section of membrane become responsive to depolarisation again
  • Until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive
  • This is known as the refractory period
  • The refractory period is important for the following reasons:
    • It ensures that action potentials are discrete events, stopping them from merging into one another
    • It ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential, as the region behind is ‘recovering’ from the action potential that has just occurred
    • This means that the impulse can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurones
    • This also means there is a minimum time between action potentials occurring at any one place along a neurone
    • The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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