Action Potentials

What it is.. when it happens and why.

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Stimulus

Stimulus; Any change in the environment of an organism that causes a response.

E.g. Waving, needle pricking a finger etc.

The stimulus excites the neurone cell membrane.

Causing sodium ion channels to open.

This makes it more permeable to sodium ions.   Sodium ions diffuse into the the neurone, down the sodium electrochemical gradient.   This makes the inside of the cell, less negative.

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Depolarisation and Repolarisation

Depolarisation

If the potential difference reaches threshold (-55mV) Voltage gated sodium ion channels open.

More sodium ions can diffuse into the neurone.

Repolarisation

When the PD is around +30 mV the voltage gated Na+ channels close and VG potassium channels open.

The membrane is more permeable to K+ ions, so more diffuse out of the neurone down the potassium ion concentration gradient.

This starts to get the membrane back to it's resting potential.

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Hyperpolarisation

 

The potassium ion channels are too slow to close so this leads to a slight 'overshoot'.

 

This is where too many potassium ions diffuse out of the neurone.

 

The potential difference becomes more negative than the resting potential.

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Resting Potential

'The potential difference across the neurone cell membrane while it is at rest.

The ion channels are 'reset'.

The sodium potassium pump returns the membrane to its resting potential an maintains it, until the membrane is excited by another stimulus.

After an action potential,the neurone cell membrane cannot be excited again straight away.

The ion channels are recovering and cannot be made to open.

Sodium ion channels are closed during repolarisation and potassium channels are open during hyperpolarisation.

The period of recovery is called the refractory period.

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