1. Neurones

What is the voltage across the membrane when it is at resting potential

-70mV

What is the voltage across the membrane at the point it reaches the threshold?

-55mV

What is the voltage across the membrane when the neurone is depolarised

+30mV

What is the voltage across the membrane when the impulse is in its refractory period

-80mV

What 2 things create and maintain the resting potential across the membrane?

• sodium-potassium pumps

• potassium ion channels

Draw and label a diagram of the membrane at resting potential.

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Describe the movement of sodium and potassium ions across the membrane at resting potential - 4

1. Sodium-potassium pumps used active transport to move 3 sodium ions out of the neurone for every two potassium ions moved in, using ATP.

2. Potassium ion channels allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient.

3. The membrane isn’t permeable to sodium ions, so they can’t diffuse back in. This creates a sodium ion electrochemical gradient as there are more positive sodium ions outside the cell than inside.

4. Even though positive ions are moving in and out of the cell, overall more positive ions move out of the cell than enter, making the outside of the cell positively charged compared to the inside.

electrochemical gradient

a concentration gradient of ions.

Draw and label an action potential graph

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Describe what happens when there is a stimulus. State the name of all the stages (5)

1. Stimulus excites the pressure-mediated membrane, causing sodium ion channels to open, allowing some Na+ ions to diffuse into the neurone down the gradient, making the inside less negative.

2. Depolarisation - If PD (Potential Difference) reaches the threshold, -55mV, more Na+ channels open. More Na+ will diffuse into channel.

3. Repolarisation - At a PD around +30mV. the Na_ channels close and the K+ channels open, The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down its gradient. This starts to bring the membrane back to its resting potential.

4. Hyperpolarisation - K+ channels are slow to close due to slight overshoot where too many K+ ions diffuse out of the neurone. The PD becomes more negative than the resting potential.

5. Resting potential - Ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, maintaining the resting potential until the membrane’s excited by another stimulus.

What is a refractory period?

• the time after an action potential where a neuron's membrane is unable to generate a new action potential.

• This is because the voltage-gated sodium channels are temporarily inactivated and cannot re-open, and the membrane is hyperpolarised.

• During repolarisation: The voltage-gated potassium ion channels remain open longer than needed to restore the resting potential, leading to a temporary state of hyperpolarisation where the membrane potential is more negative than the resting potential.

Why do we have refractory periods? - 3

• Acts as time delay between one action potential and the next so that action potentials don’t overlap but pass individually.

• Causes a limit tot he frequency at which the nerve impulses can be transmitted

• Make sure that the action potentials only travel in one direction.

Draw and label where the refractory period is

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Explain how the waves of depolarisation are formed - 2

• When an action potential happens, some of the Na+ ions that enter the neurone diffuse sideways, causing the Na+ channels to diffuse into that part, causing a wave of depolarisation to travel along the neurone.

• The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.

Describe the All or nothing principle.

• Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.

• If the action potential isn’t reached, an action potential won’t fire.

What is a myelin sheath?

An electrical insulator

What type of cell is the myelin sheath made up of?

Schwann Cells

Where are the Na+ ion channels concentrated at on a neurone?

• Tiny patches between the Schwann cells called the nodes of Ranvier.

Draw and Label a neurone

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