Neurones

Question 1

Describe a neurones resting state

Answer 1

The outside of the membrane is positively charged compared to the inside as there are more positively charged ions outside the cell than inside.

Question 2

Why is the membrane during its resting state described as polarised?

Answer 2

There is a difference in charge

Question 3

Define resting potential

Answer 3

The voltage across the membrane at rest

Question 4

How is resting potential maintained?

Answer 4

By sodium potassium pumps and potassium ion channels

Question 5

Describe how sodium potassium pumps and potassium ion channels work

Answer 5

1. Sodium potassium pumps move sodium ions out of the neurone. 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 than inside.
2. The sodium potassium pump moves potassium ions into the neurone but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.
3. This makes the outside of the cell positively charged compared to the inside

Question 6

What does a stimulus trigger?

Answer 6

Ion channels to open

Question 7

What happens if the stimulus is big enough?

Answer 7

If it is big enough it will trigger a rapid change in potential difference (action potential)

Question 8

What are the 5 stages of an action potential?

Answer 8

Stimulus
Depolarisation
Repolarisation
Hyperpolarisation
Resting potential

Question 9

Describe the stimulus stage of an action potential

Answer 9

This excites the neurone cell membrane causing sodium ion channels to open. The 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.

Question 10

Describe the depolarisation stage of an action potential

Answer 10

If the potential difference reaches the threshold more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.

Question 11

Describe the repolarisation stage of an action potential

Answer 11

At a potential difference of around +30mV the sodium ion channels close and potassium channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone getting the neurone back to resting potential

Question 12

Describe the hyperpolarisation stage of an action potential

Answer 12

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

Question 13

Describe the resting potential of an action potential

Answer 13

The ion channels are reset. The sodium potassium pump returns to the membrane to its resting potential and maintains it until the membranes get excited by another stimulus

Question 14

What causes a wave of depolarisation to travel along the neurone?

Answer 14

When an action potential happens some of the sodium ions that enter the neurone diffuse sideways.
This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part

Question 15

What happens during the refractory period?

Answer 15

Ion channels recover and cannot be opened

Question 16

What does the refractory period do?

Answer 16

Acts as a time delay between one action potential and the next

Question 17

What does the time delay as a result of the refractory period mean?

Answer 17

Action potentials don’t overlap but pass along as a discrete (separate) impulses
There is a limit to the frequency at which the nerve impulses can be transmitted
Action potentials are unidirectional

Question 18

Explain the all or nothing law

Answer 18

Once the threshold is reached an action potential will always fire the same charge in voltage no matter how big the stimulus is. If the threshold isn’t reached an action potential won’t fire.

Question 19

What will a bigger stimulus cause?

Answer 19

Action potentials to fire more frequently

Question 20

What 3 factors affect the conduction speed of an action potential?

Answer 20

Myelination
Axon diameter
Temperature

Question 21

What do myelinated neurones have?

Answer 21

Myelin sheath

Question 22

In the peripheral nervous system what is the sheath made from?

Answer 22

Schwann cells

Question 23

What are nodes of Ranvier?

Answer 23

Tiny patches of bare membrane located between Schwann cells

Question 24

What is concentration at the nodes of Ranvier?

Answer 24

Sodium ion channels

Question 25

Where does depolarisation happen in a myelinated sheath?

Answer 25

Nodes of Ranvier

Question 26

What happens in a myelinated neurone?

Answer 26

The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node so the impulse jumps from node to node. This is called saltatory conduction and it’s really fast

Question 27

What happens in a non-myelinated neurone?

Answer 27

The impulse travels as a wave along the whole length of the axon membrane.

Question 28

Where does depolarisation happen in a non-myelinated sheath?

Answer 28

Along the whole length of the membrane

Question 29

What happens to axons with bigger diameters and why?

Answer 29

Action potentials are conducted quicker as there is less resistance to the flow of ions than in the cytoplasm of a smaller axon. This means that depolarisation reaches other parts of the neurone cell membrane quicker

Question 30

What happens if you increase the temperature of a neurone and why?

Answer 30

The speed of conduction increase as ions diffuse faster. The speed only increase up to around 40 degrees and after that the proteins begin to denature and the speed decreases