6.2.5 Speed of Conduction of Impulses

• The speed of conduction of an impulse refers to how quickly the impulse is transmitted along a neurone
• It is determined by multiple factors:
  • The presence or absence of myelin (ie. whether or not the axon is insulated by a myelin sheath)
  • The diameter of the axon
  • Temperature

Myelination

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

• By insulating the axon membrane, the presence of myelin increases the speed at which action potentials can travel along the neurone:
  • The myelin sheath is formed from Schwann cells
  • In sections of the axon that are surrounded by a myelin sheath, depolarisation (and the action potentials that this would lead to) cannot occur, as the myelin sheath stops the diffusion of sodium ions and potassium ions
  • Action potentials can only occur at the nodes of Ranvier (small uninsulated sections of the axon)
  • The local circuits of current that trigger depolarisation in the next section of the axon membrane exist between the nodes of Ranvier
  • The presence of Schwann cells means the action potentials ‘jump’ from one node to the next, this is known as saltatory conduction
  • Saltatory conduction allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter

Transmission of an action potential in a myelinated axon by saltatory conduction

Diameter

• An impulse will be conducted at a higher speed along neurones with thicker axons compared to those with thinner axons
• Thicker axons have an axon membrane with a greater surface area over which the diffusion of ions can occur
  • This increases the rate of diffusion of sodium ions and potassium ions through protein channels, which in turn increases the rate at which depolarisation and action potentials can occur

• Axons with a greater diameter also possess a greater volume of cytoplasm (which contains ions). This reduces their electrical resistance so that an action potential can push into the next section faster

Temperature

• Some animals, such as mammals, maintain very stable body temperatures. Temperature does not usually affect the speed of nerve impulses in these animals
• The body temperature of other animals can vary with the environment
  • For example, cold-blooded reptiles

• Colder conditions can slow down the conduction of nerve impulses
• The colder temperatures mean there is less kinetic energy available for the facilitated diffusion of potassium and sodium ions during an action potential