Speed of Conduction of Impulses (AQA A Level Biology): Revision Note

Factors that affect speed of impulse conduction

• The speed at which nerve impulses are conducted along an axon is determined by multiple factors, including:

  • the presence or absence of myelin

  • axon diameter

  • temperature

Myelination

• Nerve impulses are conducted more quickly in myelinated neurones than in unmyelinated neurones

  • In unmyelinated neurones:

    • depolarisation must occur along the whole membrane of the axon; this is relatively slow

  • In myelinated neurones:

    • depolarisation only occurs at the nodes of Ranvier, and transmission relies on the diffusion of sodium ions within the axon; this is very fast

    • action potentials appear to jump from one node to the next; this is saltatory conduction

Axon diameter

• Nerve impulses are conducted more quickly in axons with a wider diameter; this is because:

  • axons with a larger diameter have a higher volume of cytoplasm; this reduces the resistance to the flow of ions

  • larger axons lose fewer ions at the membrane due to leakage, meaning that membrane potential is maintained more easily

Temperature

• Nerve impulses are conducted more quickly at higher temperatures because molecules have more kinetic energy; this means that:

  • diffusion of ions across membranes and along axons is faster

  • respiration happens faster, providing more ATP for active transport of ions

• At a whole organism level the impact of temperature will depend on whether the organism regulates their body temperature, e.g.:

  • mammals maintain stable body temperatures, so environmental temperatures have a limited effect on the speed of nerve impulse transmission

  • the body temperature of reptiles fluctuates with their environment, so nerve impulse transmission will be affected by external temperatures