• 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

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, which in turn?increases the rate?at which?depolarisation?and?action potentials can occura