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
