Factors affecting the speed of conductance: myelination and saltatory…

The idea

Three factors affect nerve impulse conductance speed. Myelination provides electrical insulation. In myelinated neurones, depolarisation only occurs at the nodes of Ranvier, allowing the action potential to jump from node to node via saltatory conduction. This is much faster than continuous depolarisation along a non-myelinated axon. Axon diameter also affects speed; a wider axon, like the squid giant axon, offers less resistance to the flow of local currents in the cytoplasm, so depolarisation spreads faster. Finally, higher temperatures increase the kinetic energy of ions. This leads to a faster rate of passive diffusion of Na⁺ and K⁺ through voltage-gated channels, increasing conduction speed up to an optimum before membrane proteins denature.

What earns the marks

• Myelination provides electrical insulation, allowing saltatory conduction where the action potential jumps between nodes of Ranvier.
• In non-myelinated axons, depolarisation must occur across the entire membrane length, resulting in slower conduction.
• A greater axon diameter reduces resistance to the flow of ions in the cytoplasm, increasing the speed of conduction.
• Higher temperatures increase the kinetic energy of ions, leading to a faster rate of passive diffusion of Na⁺ and K⁺ through voltage-gated channels.
• Temperatures above the optimum cause voltage-gated channel proteins to denature, preventing action potentials and stopping conduction.

Common mistakes and how to put them right

• Stating myelin speeds up the impulse without explicitly naming saltatory conduction or the nodes of Ranvier.

  Put it right: Specify that the impulse jumps between nodes of Ranvier via saltatory conduction.

• Explaining demyelinating diseases (like multiple sclerosis) as the destruction of the axon itself.

  Put it right: Explain that loss of myelin means depolarisation must now occur along the whole membrane, slowing the impulse.

• Claiming higher temperatures speed up conduction because the sodium-potassium pump works faster.

  Put it right: The pump maintains resting potential; faster conduction is due to increased kinetic energy and faster passive diffusion of Na⁺ and K⁺ through channels.

Exam tips

• In questions about multiple sclerosis or demyelination, explicitly state that saltatory conduction is lost and depolarisation must occur over the whole membrane.
• When evaluating data on conduction speeds, always make comparative statements (e.g., 'axon A has a wider diameter than axon B, so offers less resistance').