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A-level Biology > 6B - Neurones > Flashcards

6B - Neurones Flashcards

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1
Q

Are neurone cell membranes polarised or not at rest?

A

Polarised

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2
Q

Briefly how are neurone cell membranes polarised at rest?

A

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

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3
Q

What does it mean that the membrane is polarised?

A

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

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4
Q

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

A

Resting potential.

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5
Q

What is the value of the resting potential?

A

-65mv (-70mv)

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6
Q

What is resting potential?

A

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

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7
Q

What does the sodium-potassium pump do?

A

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.

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8
Q

How does the sodium potassium pump maintain resting potential?

A
  • 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.
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9
Q

What do leakage channels allow?

A

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

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10
Q

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

A

They become depolarised.

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11
Q

Describe how an action potential occurs

A
  • 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.
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12
Q

Explain the first stage of action potential (stimulus)

A
  • 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.
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13
Q

Explain the second stage of action potential (depolarisation)

A
  • 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).
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14
Q

Explain the third stage of action potential (repolarisation)

A
  • 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.
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15
Q

Explain the fourth stage of action potential (hyperpolarization)

A
  • 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.
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16
Q

Explain the fifth stage of action potential (resting potential)

A
  • 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.
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17
Q

What is action potential?

A

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.

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18
Q

What is the refractory period?

A

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.)

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19
Q

What happens after an action potential?

A

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.)

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20
Q

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

A

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

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21
Q

When are the sodium ion channels closed?

A

During repolarisation

22
Q

When are the potassium ion channels closed?

A

During hyperpolarisation

23
Q

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

A

Diffuse sideways.

24
Q

Explain how an action potential moves along the neurone

A

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.

25
Q

What does the refractory period produce?

A

Discrete impulses.

26
Q

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

A

They are recovering so cannot be opened.

27
Q

What does the refractory period act as?

A

A time delay between one AP and the next.

28
Q

What 3 things does a refractory period do/ensure?

A
  • 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).
29
Q

What type of summation do APs have?

A

All or nothing

30
Q

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

A

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.

31
Q

What does a bigger stimulus cause in terms of AP?

A

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

32
Q

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

A

1) Myelination
2) Axon diameter
3) Temperature

33
Q

What does it mean by a neurone being myelinated?

A

They have a myelin sheath - an electrical insulator.

34
Q

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

A

A type of cell called a Schwann cell.

35
Q

What are between the Schwann cells?

A

Tiny patches of bare membrane called the nodes of Ranvier.

36
Q

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

A

Nodes of Ranvier.

37
Q

What are the nodes of ranvier?

A

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

38
Q

What are concentrated at the nodes of Ranvier?

A

Sodium ion channels.

39
Q

Where are sodium ion channels concentrated in the neurone?

A

At the nodes of Ranvier.

40
Q

Where does depolarisation happen in a myelinated neurone?

A

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

41
Q

What is the role of the neurones cytoplasm in conduction?

A

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

42
Q

What is saltatory conduction?

A

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

43
Q

What is the speed of saltatory conduction like?

A

Really fast.

44
Q

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

A
  • 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.
45
Q

How is an impulse conducted in a myelinated neurone?

A
  • 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.
46
Q

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

A

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

47
Q

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

A

Slower but still pretty quick.

48
Q

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

A
  • 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.
49
Q

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

A
  • 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.
50
Q

How does temperature affect the speed of conduction of APs?

A
  • 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).
51
Q

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

A
  • 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).
52
Q

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

A

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

A-level Biology (42 decks)

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