Speed of the nerve impulse

• If the threshold value is not reached, no action potential is generated
• All action potentials are the same size, the size of a stimulus can be detected by the frequency of impulses
• The speed of propagation can vary and depends on myelination, axon diameter and temperature
• Action potentials travel faster in myelinated neurones because the action potential doesn't need to depolarise the whole length of the neurone
• Larger axon diameters result in faster travel because they have a smaller surface area to volume ratio so there is less leaking of ions
• At higher temperatures, there is more kinetic energy which increases the diffusion of ions across the membrane
• There is a refractory period because voltage-gated sodium ion channels are closed
• The purpose of the refractory period

  • ensures that action potentials are only propagated in one direction
  • ensures that action potentials are discrete
  • limits the number of action potentials in a given time