Nerve Impulses

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•  Nerve impulses are action potentials propagated along the axons of neurons

Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation

  • Depolarisation occurs when ion channels open and cause a change in membrane potential
  • The ion channels that occupy the length of the axon are voltage-gated (open in response to changes in membrane potential)
  • Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon
  • This causes depolarisation to spread along the length of the axon as a unidirectional ‘wave’

Propagation of a Nerve Impulse

nerve impulse

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•  Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to

    reach the threshold potential

Action potentials are generated within the axon according to the all-or-none principle

  • An action potential of the same magnitude will always occur provided a minimum electrical stimulus is generated
  • This minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels 
  • If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire

Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation

  • If the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section

Direction of a Nerve Impulse