M&R Session 4- The Action Potential And Its Properties

Question 0

What is the all or nothing principle?

Answer 0

The idea that an AP will only occur if the threshold is reached. Opening of sodium channels once the threshold is reached sets off a cascade of sodium channel opening, meaning that depolarisation, once begun, never stops halfway.

Question 1

What are action potentials?

What are they dependent on? (2)

Answer 1

They are changes in voltage across a membrane.

They are dependent on ionic gradients and membrane permeability.

Question 2

Is amplitude lost when APs are propagated?

Answer 2

No.

Question 3

Which channels opening during depolarisation?

Answer 3

Voltage gated sodium channels

Question 4

Which channels open and close during repolarisation respectively?

Answer 4

Voltage gated potassium channels open.

Voltage gated sodium channels become inactivated.

Question 5

What is the absolute refractory period?

Answer 5

It is the period after an AP when nearly all of the sodium channels are inactivated and cannot be excited, no matter how much you try and excite them.

Question 6

What is the Relative Refractory Period?

Answer 6

It is the period when sodium channels that are inactivated are recovering.
As the number of inactivated sodium channels decreases, the excitability increases back to normal.

Question 7

What is accommodation?

What is the consequence for the AP peak and threshold value?

Answer 7

The longer a stimulus is, the larger the depolarisation necessary to initiate an AP. This is because, some sodium channels will open before the threshold is reached, and become inactivated.
The action potential peak is therefore smaller, and the threshold more positive.

Question 8

What are the similarities (1) and differences (2) in the structures of voltage gated sodium and potassium channels?

Answer 8

Similarities:
-each of the 4 units have 6 transmembrane domains, one of which detects voltage.
Differences:
-sodiums functional channel is one subunit, where as potassiums is 4 separate units.
-the 4 units of sodium are homologous, but are all different in potassium.

Question 9

What is an example of a local anaesthetic?

Answer 9

Procaine

Question 10

What is the action of local anaesthetics?

Answer 10

They bind to, and block sodium channels, hence preventing action potential generation.

Question 11

What is the order in which local anaesthetics affect axons?

Answer 11

• small myelinated axons
• unmyelinated axons
• large myelinated axons

Question 12

Which sodium channels do local anaesthetics have high affinity for?

Answer 12

Open and inactivated sodium channels