Myelination & Saltatory Conduction (Edexcel International A Level Biology): Revision Note

Myelination & Saltatory Conduction

• In unmyelinated neurones the speed of conduction is relatively slow because depolarisation must occur along the whole membrane of the axon

• By insulating the axon membrane 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 cannot occur as the myelin sheath stops the diffusion of sodium and potassium ions

  • Action potentials can only occur at the nodes of Ranvier 

    • Nodes of Ranvier are the gaps between the Schwann cells that make up the myelin sheath

  • Sodium ions diffuse along the axon within the Schwann cells and the membrane at the nodes of Ranvier depolarises when the sodium ions arrive

    • The diffusion of sodium ions in this way is known as local currents, or local circuits

  • The action potential therefore appears to ‘jump’ from one node to the next; this is known as saltatory conduction

• Saltatory conduction allows the impulse to travel much faster than in an unmyelinated axon of the same diameter

Action potentials are transmitted along myelinated axons by saltatory conduction