Speed of Conduction of Impulses (Cambridge International A-Level Biology)

Spec point — 15.1.7 · describe and explain the rapid transmission of an impulse in a myelinated neurone with reference to saltatory conduction

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 (i.e., whether or not the axon is insulated by a myelin sheath)
  • the diameter of the axon

Myelination and saltatory conduction

• 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

Transmission of an action potential in a myelinated axon by saltatory conduction

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
  • This in turn increases the rate at which depolarisation and action potentials can occur