4.1 The Action Potential

Question 0

What does ‘all or nothing’ mean?

Answer 0

An action potential will occur at the same intensity when the threshold level is reached. If the threshold is not reached, no action potential will occur.

Question 1

Define ‘Action Potential’

Answer 1

A change in voltage across a membrane.

Question 2

How does an action potential occur?

Answer 2

The membrane becomes temporarily more permeable to Na+ ions so lots of Na+ ions move IN to the cell. The membrane is depolarised.

Question 3

Explain the process of voltage clamping

Answer 3

Voltage clamping controls the membrane potential so that ionic currents can be measured.

Question 4

Explain what happens during an action potential

Answer 4

Na+ ions move in very rapidly causing depolarisation when the channels open. They then rapidly close. K+ channels open slowly and cause K+ to move out of the cell. They close slowly and remain open after repolarisation. When the potential becomes more negative than the resting, the K+ channels close.

Question 5

Define ‘Summative Conduction’

Answer 5

If a few impulses arrive at a cell body at the same time, the depolarisation accumulates to reach the threshold level.

Question 6

By what mechanism does an axon depolarise?

Answer 6

Positive feedback

Question 7

Define an ‘absolute refractory period’

Answer 7

All the Na+ channels are in the inactivated state so another action potential cannot be fired.

Question 8

Define the ‘relative refractory period’

Answer 8

Na+ channels are recovering from the inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases. An action potential still cannot be fired.

Question 9

Define Accommodation

Answer 9

The longer the stimulus, the larger the depolarisation necessary to initiate an action potential. The threshold becomes more positive.

Question 10

Describe the structure of an Na+ voltage-gated channel

Answer 10

6 transmembrane domains
4 units
Pore region
Voltage sensor (S4)

Question 11

Describe the structure of a K+ voltage-gated channel

Answer 11

6 transmembrane domains

4 units but all combine to make 1 channel

Question 12

Give an example of a local anaesthetic. Describe the action of local anaesthetics.

Answer 12

Procaine

Block Na+ channels

Question 13

In what order do local anaesthetics block axons

Answer 13

1. ) Small myelinated axons
2. ) Un-myelinated axons
3. ) Large myelinated axons