CIE A Level Biology (9700) exams from 2022

Revision Notes

15.1.7 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

Myelination

  • 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

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

Author:

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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