1.5.8 action potential

Question 1

potential difference

Answer 1

t resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.
This is because there are more positive ions outside the cell than inside.
The difference in charge is called a potential difference.

Question 2

sodium-potassium pumps

Answer 2

The resting potential is maintained by sodium-potassium pumps in the neurone membrane.
Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.
This leads to a build-up of positive ions outside the cell.

Question 3

potassium ion channels

Answer 3

There are potassium ion channels in the neurone membrane. This means it is permeable to K+ ions.
When K+ ions are transported into neurones, they can diffuse back out.
The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.

Question 4

resting potential

Answer 4

Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane.
This potential difference is called the resting potential.
The neurone is said to be polarised.
Resting potential is about −70mV.

Question 5

stimulation

Answer 5

Na+ ion channels in the cell membrane open when a neurone is stimulated.
Na+ ions flood into the neurone.
The potential difference across the membrane changes to become more positive inside the neurone.

Question 6

depolarisation

Answer 6

If the potential difference increases above the threshold value (about −55mV) then the membrane will become depolarised.
More sodium channels open and there is a sharp increase in potential difference to about +30mV.

Question 7

all-or-nothing

Answer 7

Depolarisation is an all-or-nothing response.
If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same.
If the stimulus is stronger, action potentials will produced more frequently but their size will not increase.

Question 8

repolarisation

Answer 8

After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.
Na+ ions are transported back out of the neurone and the potential difference becomes more negative.
This is called repolarisation.

Question 9

hyperpolarisation

Answer 9

here is a short period after depolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.
This is called hyperpolarisation.
Hyperpolarisation prevents the neurone from being restimulated instantly. This is called the refractory period.

Question 10

resting potential

Answer 10

After the refractory period, the potassium ion channels close and the membrane returns to its resting potential.
The process where a neurone is depolarised and returns to resting potential is called an action potential.

Question 11

sodium ions

Answer 11

When an action potential is generated, there are more Na+ ions inside the neurone than outside.
Some of these Na+ ions diffuse sideways along the neurone axon.

Question 12

sodium ion channels

Answer 12

The presence of Na+ ions creates a change in potential difference further along the neurone membrane.
If this reaches the threshold value, sodium ion channels at this part of the membrane open.
Na+ ions diffuse into the neurone.
This part of the neurone now becomes depolarised.

Question 13

wave depolarisation

Answer 13

Na+ diffuse all along the neurone in this way.

Depolarisation takes place along the neurone membrane. This creates a wave of depolarisation.

Question 14

refractory period

Answer 14

The period of hyperpolarisation in an action potential is called the refractory period.
The ion channels are recovering during the refractory period. This means an action potential cannot be stimulated again instantly.
This ensures that the wave of depolarisation travels in one direction.