5.3.3: Nerve impulses - action potentials.

Question 1

When is a neurone at rest?

Answer 1

When it is not transmitting an action potential.

Question 2

How is the resting potential maintained?

Answer 2

• Sodium/potassium pumps use ATP to pump three sodium ions out for every two potassium ions in.
• The plasma membrane is more permeable to potassium ions so potassium ions tend to diffuse out of the cell.
• The cell cytoplasm contains large organic anions.

Question 3

Why is the membrane more permeable to potassium ions?

Answer 3

The gated sodium ion channels are kept however, some of the potassium ion channels are open.

Question 4

Describe the resting potential.

Answer 4

The interior of the cell is maintained t a negative potential compared with the outside. The cell membrane is said to be polarised. The potential difference across the membrane is -60 mV.

Question 5

Where is the concentration of Na and K ions higher?

Answer 5

Concentration of Na ions higher outside than inside.

Concentration of K ions higher inside than outside.

Question 6

When does a generator potential occur?

Answer 6

When a few gated channels open, they allow a few sodium ions into the cell and produce a small depolarisation. This is known a a generator potential.

Question 7

What happens when many gated channels are opened?

Answer 7

The generator potentials are combined to produce a larger depolarisation. If depolarisation reaches a particular magnitude it passes a threshold and will cause an action potential.

Question 8

What are voltage-gated channels?

Answer 8

Sodium ion channels that are opened by changes in the potential difference across the membrane.

Question 9

When do voltage-gated channels open?

Answer 9

When there are sufficient generator potentials to reach threshold potential, they cause the voltage gated channels to open.

Question 10

How is this is an example of positive feedback?

Answer 10

A small depolarisation o the membrane causes a change that increases depolarisation further.

Question 11

When will a neurone transmit the action potential?

Answer 11

When the voltage-gated sodium ion channels allow a large influx of sodium ions and the depolarisation reaches (+40 mV).

Question 12

Why are action potentials an ‘all or nothing’ response?

Answer 12

All action potentials are the same magnitude (+40 mV).

Question 13

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 1…

Answer 13

The membrane is in its resting state- polarised with the inside of the cell being -60 mV compared to the outside.
There is a higher concentration of sodium ions outside than inside and a higher concentration of potassium ions inside than outside.

Question 14

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 2…

Answer 14

Sodium ion channels open and some sodium ions diffuse into the cell.

Question 15

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 3..

Answer 15

The membrane depolarises- it becomes less negatively charged with respect to the outside and reaches the threshold value of -50 mV.

Question 16

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 4…

Answer 16

Positive feedback causes nearby voltage gated sodium ion channels to open and many sodium ions flood in. As more sodium ions enter the cell, the cell becomes more positively charges inside compared with outside.

Question 17

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 5…

Answer 17

The potential difference across the plasma membrane reaches +40 mV. The inside of the cell is positive compared with the outside.

Question 18

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 6…

Answer 18

The sodium ion channels close and the potassium ion channels open.

Question 19

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 7…

Answer 19

Potassium ions diffuse out of the cell bringing the potential difference back to negative inside compared with outside - this is called repolarisation.

Question 20

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 8…

Answer 20

The potential difference overshoots slightly making the cell hyperpolarised.

Question 21

use the graph on page 51 to describe what happens at stages 1 to 9.
Stage 9…

Answer 21

The original potential difference is restored so that the cell returns to its resting state,

Question 22

After an action potential, the sodium and potassium ions are in the wrong places . How is the concentrations of these ions inside and outside the cell restored?

Answer 22

By the action of the sodium/potassium ion pumps.

Question 23

What is the refractory period?

Answer 23

For a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential.

Question 24

Why is the refractory period important?

Answer 24

• It allows the cell membrane to recover after an action potential.
• It ensures that action potentials are transmitted only in one direction.