Nerve Conduction Velocity (HL) (HL IB Biology)

• By insulating the axon membrane, the presence of myelin increases the speed at which action potentials can travel along the neurone:
  • The myelin sheath is formed from Schwann cells
  • 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
  • There are small, uninsulated sections of the axon, called the nodes of Ranvier, which contain clusters of ion pumps and channels which allow the action potential to occur
  • As a result, the action potentials ‘jump’ from one node to the next, this is known as saltatory conduction
  • The local circuits of current that trigger depolarisation in the next section of the axon membrane exist between the nodes of Ranvier
  • Saltatory conduction allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter

Saltatory Conduction Diagram

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