Speed of Conduction of Impulses (AQA A Level Biology)

Factors that Affect the Speed of Conduction

• 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