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The facilitated diffusion of sodium and potassium ions through neurone membranes is through voltage gated channel proteins. These proteins are specific, so sodium and potassium move independently through different proteins.

The action potential is a temporary depolarisation to a point of a neurone membrane. This can then spread across the membrane surface. The events that make up an action potential usually all take place within one millisecond. The depolarisation that is the action potential, either takes place or does not. There is no such thing as half an action potential. Action potential are said to be 'all or none'.

The two main phases of an action potential are called depolarisation and repolarisation. Table 4, below, explains the events occurring during stages A (depolarisation), B and C (repolarisation) in figure 7.

Figure 7 : The two main phases of an action potential (depolarisation and repolarisation)
Figure 3 : The two main phases of an action potential (depolarisation and repolarisation)



A

Depolarisation

  • Voltage gated sodium channels open.
  • Sodium ions flood into the neurone down a charge and concentration gradient.
  • This makes the inside of the neurone more positive.

Table 3 : The two main phases of an action potential (depolarisation and repolarisation)


B


  • Sodium ions stop entering neurone and voltage gated sodium channels close


C

Repolarisation

  • Potassium ion channels open.
  • Potassium ions flood out down a change gradient.  Exam Advice
  • This makes the inside of the neurone more negative again.

Table 3 : The two main phases of an action potential (depolarisation and repolarisation)


After repolarisation takes place the resting potential is restored by the active transport and facilitated diffusion. A new action potential cannot occur until the resting concentrations of the sodium and potassium ions are re-established. The amount of time it takes this to happen is called the refractory period.



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