Nerve impulses

Question 1

Describe the resting potential

Answer 1

Extracellular concentration of positive ions greater than inside axon

This gradient created via Na+/K+ ATPase, which uses active transport to pump Na+ out of axon and K+ into the axon

Na+ ions can’t diffuse back in due to impermeability go membrane

K+ ions passively diffuse out via leak channels

Therefore inside is left negative in charge compared to outside

There is a potential difference across the membrane of around -70mV, which is the resting potential

Question 2

How is the action potential brought about?

Answer 2

When neurone is stimulated, the voltage increases, and the Sodium gates open consequently

Na+ ions diffuse down concentration gradient into the axon

This causes the potential difference across the membrane to reverse and reaches a value of +40mV

This is the action potential and is essentially caused by depolarisation

Question 3

How does repolarisation occur

Answer 3

Sodium voltage gates close after action potential

Potassium voltage gates open though and potassium ions diffuse out of axon

Therefore inside of axon becomes more negative in relation to outside once again

Then undershoot occurs before the resting potential is restored

Question 4

What is the threshold

Answer 4

Point when sufficient sodium ion channels open

Rush of sodium ions into axon greater than outflow of potassium ions

This results in an action potential

Question 5

What is the refractory period

Answer 5

Time taken for ionic movements to repolarise an area of the membrane and restore resting potential after action potential

Question 6

What is absolute refractory period

Answer 6

First millisecond after action potential when it is IMPOSSIBLE to to re-stimulate the fibre

The Na+ channels are closed and resting potential not restored

Question 7

What is relative refractory period

Answer 7

Several period of milliseconds after an action potential and the absolute refractory period when an axon may be re-stimulated, but only by a much stronger stimulus

During this period K+ ion channels are still open and they have to close before normal resting potential is restored

Question 8

What is importance of refractory period

Answer 8

Limits rate at which impulses flow

Ensures impulses only flow in one direction

This is because until resting potential has been restored, part of nerve fibre that impulse has just left cannot conduct another impulse

Therefore can only travel in 1 direction