S+R Neurones

Question 1

Describe a neurone in its resting state.

Answer 1

1) In a neurone’s resting state (when it’s not being stimulated), the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside.

2) So the membrane is polarised-there’s a difference in charge (called a potential difference or voltage) across it.

3) The voltage across the membrane when it’s at rest is called the

resting potential—it’s about-70 mV (millivolts).

4) The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone’s membrane:

Question 2

Describe how the resting potential is created and maintained.

Answer 2

The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in.

This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside.

• The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.

This makes the outside of the cell positively charged compared to the inside.

Question 3

Describe the roles of the Na+ /K+ pump and the K+ ion channel.

Answer 3

Sodium-potassium pump-
These pumps use active 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.

Potassium ion channel-
These channels allow facilitated diffusion of potassium ions (K) out of the neurone, down their concentration gradient.

Question 4

What is an action potential (in a neurone)?

Answer 4

Where a stimulus triggers V.G. sodium channels to open.

If the stimulus is big enough it’ll trigger a rapid change in potential difference.

Question 5

Describe what happens to neurone cell membranes when they’re stimulated. (1)

Answer 5

1. Stimulus - 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.
2. Depolarisation - if the potential difference reaches the threshold (around -55 mV), more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.

Question 6

Describe what happens to a neurone cell membrane when it’s stimulated. (2)

Answer 6

Repolarisation-at a potential difference of around +30 mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.

Question 7

Describe what happens to a neurone cel membrane when it’s stimulated. (3)

Answer 7

Hyperpolarisation-potassium ion channels are 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 (i.e. less than -70 mV).

Resting potential—the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane’s excited by another stimulus.

Question 8

Diagram to represent the changes in potential difference during an A.P.

Answer 8

Question 9

What is the refractory period?

Answer 9

Where after an A.P the neurone cell membrane can’t be excited straight away.

Time delay between one A.P and another.

Question 10

What causes the refractory period?

Answer 10

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

Question 11

When do the different ion channels close during an A.P (when do they go into a refractory period)?

Answer 11

The Na+ ions close during repolarisation.

K+ ions close during Hyperpolarisation.

Question 12

What are the 3 purposes of the refractory period?

Answer 12

1. A.Ps can only go in one direction
2. produces discrete impulses - don’t all merge into one (overlap)
3. limits the no. of A.Ps

Question 13

How does an A.P move along a neurone?

Answer 13

An A.P moves along a neurone as a wave of depolarisation.

1. When an A.P happens, some of the sodium ions that enter the neurone diffuse sideways.
2. This causes the sodium ion channels in the next region of the neurone to open and sodium ions to 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 A.P

Question 14

Describe how A.Ps have an all or nothing nature.

Answer 14

1. Once teh threshold is reached, the A.P will always fire with the same change in voltage, no matter how big the stimulus is.
2. A bigger stimulus won’t cause a bigger A.P, but it will cause them to fire more frequently.
3. If the threshold isn’t reached, an A.P won’t fire. This is the all or nothing nature of A.Ps.

Question 15

What doe it mean if a neurone is myelinated?

Answer 15

they have a myelin sheath.

Question 16

How could you describe a myelin sheath?

Answer 16

• an elecrical insulator
• in the peripherhal NS, it’s made up of a type of cell called a schwann cell.

Question 17

Diagram of a myelinated motor neurone.

Answer 17

Question 18

Describe how a myelinated sheath leads to saltatory conduction.

Answer 18

1. Between the schwann cells aretiny patches of bare mebrane called the nodes of Ranvier.
2. In a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane).
3. The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node.

= saltatory conduction.

Question 19

How does an electrical impulse travel along a non-myelinated neurone.

Answer 19

In a non-myelinated neurone, the impulse travels along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane).

This is slower than saltatory conduction (although it’s pretty fast).

Question 20

What are the three factors that affect the sppeed of conduction of A.Ps?

Answer 20

1. Myelination
2. Axon diameter
3. Temperature

Question 21

How does axon diameter affect the rate of conduction of an A.P?

Answer 21

• quicke conduction along axons with larger diameters
• less resistance to the flow of ions than in the cytoplasm of a smaller axon.
• with less resistance depolarisation reaches other parts of the cell membrane quicker.

Question 22

How does temperature affect the rate of saltatory conduction?

Answer 22

• speed of conduction increases as saltatory conduction increases.
• as ions diffuse faster
• after 40°C proteins denature; speed decreases.

Question 23

The graph shows an A.P across a membrane following the application of a stimulus.

Explain what causes the changes in potential difference between point A and point B.

Answer 23

A stimulus causes sodium ion channels in the neurone cell membrane to open.

sodium ions diffuse into the cell, so the membrane becomes depolarised.

Question 24

The graph shows an A.P across a membrane following the application of a stimulus.

b. the same stimulus was applied consistently for over one hour. The next action potntail fired at 4.5m/s.

calculate how many action potentials fired in 1 hour. give your ans in standard form.

Answer 24

b. the first A.P fired at 0.5ms. If the second fired at 4.5ms, this means the action potentail is fired (4.5 - 0.5) 4ms.

the no. of ms in 1 hour = 60 x 60 x 1000 = 3 600 000

There is one A.P ever 4ms, so in one hour there will be:

3 600 000 / 4 = 9 000 000

= 9 x 10^5 A.Ps