6B - Neurones

Question 1

Are neurone cell membranes polarised or not at rest?

Answer 1

Polarised

Question 2

Briefly how are neurone cell membranes polarised at rest?

Answer 2

Inside the cell is more negative as there are more positively charged ions outside the neurone.

Question 3

What does it mean that the membrane is polarised?

Answer 3

There’s a difference in charge (a potential difference/voltage) across it.

Question 4

What is the voltage across the membrane when it’s at rest called?

Answer 4

Resting potential.

Question 5

What is the value of the resting potential?

Answer 5

-65mv (-70mv)

Question 6

What is resting potential?

Answer 6

The voltage across the membrane when it’s at rest.

Question 7

What does the sodium-potassium pump do?

Answer 7

They actively transport to move three sodium ions (Na+) out of the neurone for every two potassium ions (K+) moved in. ATP is needed to do this.

Question 8

How does the sodium potassium pump maintain resting potential?

Answer 8

• Actively transport Na+ ions out of the cell and K+ ions into the cell. As Na+ and K+ ions are transported against the conc. gradient, ATP is required.
• As there’s a higher conc. of Na+ outside the cell than inside, sodium should diffuse into the cell. Most of the sodium leakage channel proteins are closed preventing Na+ ions diffusing into the cell (the cell membrane is impermeable to Na+ ions) (sodium ion electrochemical gradient created).
• As there’s a higher conc. of K+ in the cell than outside, K+ should diffuse out. Most of the potassium leakage channel proteins are open, allowing K+ ions to diffuse out of the cell (cell membrane permeable to K+ ions).
  (Facilitated diffusion.)
• This makes the outside of the cell positively charged compared to the inside.

Question 9

What do leakage channels allow?

Answer 9

Facilitated diffusion of K+ out of the neurone, down their concentration gradient.

Question 10

What happens to neurone cell membranes become when they’re stimulated?

Answer 10

They become depolarised.

Question 11

Describe how an action potential occurs

Answer 11

• Stimulus causes (excites the neurone cell membrane) some voltage gated sodium channels to open.
• Membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient.
• This makes the inside of the neurone less negative => Depolarisation.
• Depolarisation; if enough voltage gated Na+ channels open, enough Na+ ions enter cell, the potential difference (voltage) reaches the threshold (-55mv).
• Allows more voltage gated sodium channels to open, more sodium ions diffuse rapidly into the neurone and the action potential occurs (the membrane reaches +40mv).
• Repolarisation; at around +40mv the voltage gated sodium channels close and the voltage gated potassium channels open.
• The membrane is more permeable to potassium so K+ ions diffuse out of the neurone down the potassium ion conc. gradient.
• Causes the membrane to become more negative, back to its resting potential.
• Hyperpolarization; voltage gated potassium ion channels are too slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone.
• The potential difference becomes more negative than the resting potential.
• Resting potential; voltage gated K+ ion channels shut and the sodium-potassium pump restores the resting potential, maintaining it until the membrane’s excited by another stimulus. Axon is depolarised.

Question 12

Explain the first stage of action potential (stimulus)

Answer 12

• Stimulus causes (excites the neurone cell membrane) some voltage gated sodium channels to open.
• Membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient.
• This makes the inside of the neurone less negative => Depolarisation.

Question 13

Explain the second stage of action potential (depolarisation)

Answer 13

• Depolarisation; if enough voltage gated Na+ channels open, enough Na+ ions enter cell, the potential difference (voltage) reaches the threshold (-55mv).
• Allows more voltage gated sodium channels to open, more sodium ions diffuse rapidly into the neurone and the action potential occurs (the membrane reaches +40mv).

Question 14

Explain the third stage of action potential (repolarisation)

Answer 14

• Repolarisation; at around +40mv the voltage gated sodium channels close and the voltage gated potassium channels open.
• The membrane is more permeable to potassium so K+ ions diffuse out of the neurone down the potassium ion conc. gradient.
• Causes the membrane to become more negative, back to its resting potential.

Question 15

Explain the fourth stage of action potential (hyperpolarization)

Answer 15

• Hyperpolarization; voltage gated potassium ion channels are too slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone.
• The potential difference becomes more negative than the resting potential.

Question 16

Explain the fifth stage of action potential (resting potential)

Answer 16

• Resting potential; voltage gated K+ ion channels shut and the sodium-potassium pump restores the resting potential, maintaining it until the membrane’s excited by another stimulus. Axon is depolarised.

Question 17

What is action potential?

Answer 17

The sequence of events; a stimulus triggers other ion channels, called sodium ion channels, to open. If the stimulus is big enough, it’ll trigger a rapid change in potential difference.

A massive depolarisation.

Question 18

What is the refractory period?

Answer 18

The stages of repolarisation and hyperpolarisation (decrease in potential difference).

After an action potential the neurone cell membrane can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open - sodium ion channels are closed during repolarization and potassium ion channels are closed during hyperpolarization. (This is the refractory period.)

Question 19

What happens after an action potential?

Answer 19

The neurone cell membrane can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open - sodium ion channels are closed during repolarization and potassium ion channels are closed during hyperpolarization. (This is the refractory period.)

Question 20

Why can’t the neurone cell membrane be excited again straight away?

Answer 20

Because the ion channels are recovering and they can’t be made to open.

Question 21

When are the sodium ion channels closed?

Answer 21

During repolarisation

Question 22

When are the potassium ion channels closed?

Answer 22

During hyperpolarisation

Question 23

What do some sodium ions that enter the neurone do when an action potential happens?

Answer 23

Diffuse sideways.

Question 24

Explain how an action potential moves along the neurone

Answer 24

1) When an AP happens, some of the sodium ions that enter the neurone diffuse sideways.
2) This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
3) This causes a wave of depolarisation to travel along the neurone.
4) The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an AP.

Question 25

What does the refractory period produce?

Answer 25

Discrete impulses.

Question 26

What is happening to the voltage gated channels during the refractory period?

Answer 26

They are recovering so cannot be opened.

Question 27

What does the refractory period act as?

Answer 27

A time delay between one AP and the next.

Question 28

What 3 things does a refractory period do/ensure?

Answer 28

- Ensures action potentials don't overlap, but pass along as discrete (separate) impulses. - There's a limit to the frequency at which the nerve impulses can be transmitted (limits the number of APs). - APs are unidirectional (they only travel in one direction).

Question 29

What type of summation do APs have?

Answer 29

All or nothing

Question 30

Explain what is meant by APs having an all or nothing nature

Answer 30

1) Once the threshold is reached, an AP will always fire with the same change in voltage, no matter how big the stimulus is. 2) If the threshold isn't reached, an AP won't fire. 3) A bigger stimulus won't cause a bigger AP, but it will cause them to fire more frequently.

Question 31

What does a bigger stimulus cause in terms of AP?

Answer 31

A bigger stimulus won't cause a bigger AP, but it will cause them to fire more frequently.

Question 32

What factors affect the speed of conduction (rate of impulse transmission) of APs?

Answer 32

1) Myelination 2) Axon diameter 3) Temperature

Question 33

What does it mean by a neurone being myelinated?

Answer 33

They have a myelin sheath - an electrical insulator.

Question 34

In the peripheral nervous system, what is the myelin sheath made of?

Answer 34

A type of cell called a Schwann cell.

Question 35

What are between the Schwann cells?

Answer 35

Tiny patches of bare membrane called the nodes of Ranvier.

Question 36

What are the patches of bare membrane on a neurone called?

Answer 36

Nodes of Ranvier.

Question 37

What are the nodes of ranvier?

Answer 37

Tiny patches of bare membrane between the Schwann cells on a neurone.

Question 38

What are concentrated at the nodes of Ranvier?

Answer 38

Sodium ion channels.

Question 39

Where are sodium ion channels concentrated in the neurone?

Answer 39

At the nodes of Ranvier.

Question 40

Where does depolarisation happen in a myelinated neurone?

Answer 40

Only at the nodes of Ranvier (where sodium ions can get through the membrane).

Question 41

What is the role of the neurones cytoplasm in conduction?

Answer 41

The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node.

Question 42

What is saltatory conduction?

Answer 42

The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node.

Question 43

What is the speed of saltatory conduction like?

Answer 43

Really fast.

Question 44

How is an impulse conducted in a non-myelinated neurone?

Answer 44

- Stimulus causes voltage gated sodium channels to open, the threshold is reached and an AP occurs. - The localised electric current is established by the influx of sodium ions causing voltage gated sodium channels to open further along the axon causing an AP to occur. - AP continues to be passed on.

Question 45

How is an impulse conducted in a myelinated neurone?

Answer 45

- Depolarisation only occurs at the nodes of Ranvier where the voltage gated ion channels are. - An AP occurs. - The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node. - This is called saltatory conduction.

Question 46

How does an impulse travel in a non-myelinated neurone?

Answer 46

As a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane) - saltatory conduction.

Question 47

What is the speed of saltatory conduction like in a non-myelinated membrane compared to a myelinated neurone?

Answer 47

Slower but still pretty quick.

Question 48

How does axon diameter affect the speed of conduction of APs?

Answer 48

- APs conducted quicker along axons with bigger diameters - This is because there's less resistance to the flow of ions than in the cytoplasm of a smaller axon. - With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker. - Also less leakage of ions.

Question 49

Why is the speed of conduction quicker in an axon with a bigger diameter?

Answer 49

- Because there's less resistance to the flow of ions than in the cytoplasm of a smaller axon. - With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker. - Also less leakage of ions.

Question 50

How does temperature affect the speed of conduction of APs?

Answer 50

- The speed of conduction increases as the temperature increases. - This is because ions diffuse faster. - The speed only increases up to around 40 degrees celcius though (after that the proteins begin to denature and the speed decreases).

Question 51

Why is the speed of conduction quicker when the temperature is higher?

Answer 51

- Because ions diffuse faster. - The speed only increases up to around 40 degrees celcius though (after that the proteins begin to denature and the speed decreases).

Question 52

At what temperature does the speed of conduction no longer increase proportionally to the temperature increasing and why?

Answer 52

The speed only increases up to around 40 degrees celcius though (after that the proteins begin to denature and the speed decreases).