The Nerve Impulse

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The Nerve Impulse

Causes

  • A stimulus excited a neurone cell membrane, opening Na+ channels
  • The potential difference reaches -55mV (depolarisation) so voltage-gated sodium ion channels open .
  • Na+ channels close and K+ channels open.
  • K+ ion channels are slow to close
  • Ion channels are reset and the Na/K channels continue to pump ions in and out of the neurone.

Effects

  • Na+ ions diffuse into the neurone, down its electrochemical gradient
  • More sodium ions diffuse into the neurone, until the potential difference reaches around +30mV.
  • K+ ions leave the cells, lowering its potential difference.
  • Potential difference bypasses -70mV (hyperpolarisation)
  • The cell's potential difference is restored to its resting potential, until it is excited by another stimulus.

Overall summary

An action potential is generated if a stimulus is strong enough for the generator potential to reach the threshold value. A stronger stimulus after this point does not produce to greater action potential, but more frequent impulses. After an action potential has passed and the membrane's potential difference is back to -70mV, the refractory period follows, meaning the neurone cell membrane cannot  be immediately excited again. This ensures that action potentials don't overlap, and that they are unidirectional. 

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