AQA A Level Biology复习笔记6.2.5 Speed of Conduction of Impulses

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

     

Saltatory-conduction-1

 

Saltatory-conduction-2

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

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