Action Potential

Depolarization

• Depolarization: a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge
• In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon
• As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium
• The influx of sodium causes the membrane potential to become more positive (depolarization)

Repolarization

• Repolarization: the restoration of a membrane potential following depolarization (i.e. restoring a negative internal charge)
• Following an influx of sodium, potassium channels open within the membrane of the axon
• As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium
• The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarization)

Refractory Period

• Refractory period: the period of time following a nerve impulse before the neuron is able to fire again
• In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential)
• Following depolarization (sodium influx) and repolarization (potassium efflux), this ionic distribution is largely reversed
• Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump

Nerve Impulses

• Nerve impulses: action potentials that move along the length of an axon as a wave of depolarization
• Depolarization 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, depolarization at one point of the axon triggers the opening of ion channels in the next segment of the axon
• This causes depolarization to spread along the length of the axon as a unidirectional ‘wave’
• 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 depolarization
• If the overall depolarization 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

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