Action Potentials

Question 1

Where can action potentials occur

Answer 1

Excitable cells such as neurons, muscle cells and endocrine tissue

Question 2

How can permeability depend on the conformational state of the ion channels

Answer 2

Opened by membrane depolarisation
Inactivated by sustained depolarisation
Closed by membrane hyperpolarisation

Question 3

What are the 5 phases of an action potential

Answer 3

1. Resting membrane potential
2. depolarising stimulus
3. upstroke
4. repolarisation
5. After-hyperpolarisation

Question 4

Describe phase 1 of an AP

Answer 4

Resting potential where potassium potential>sodium potential . Potential is nearer to the K equilibrium

Question 5

Describe phase 2 of an AP

Answer 5

Stimulus depolarises the membrane potential and moves it towards a +ve potential

Question 6

Describe phase 3 of an AP

Answer 6

Large increase in sodium potential as voltage-gate sodium channels open.
Smaller increase in potassium potential due to voltage-gated channels opening more slowly.
The membrane potential moves towards the sodium equilibrium potential

Question 7

Describe phase 4 of an AP

Answer 7

Decrease in sodium potential as the voltage-gated channels become inactivated
Increase in potassium potential as the potassium voltage-gated channels remain open
Membrane potential moves towards the potassium equilibrium potential

Question 8

What is the absolute refractory period

Answer 8

The inactivation gate is close and new action potential cannot be triggered even with a strong stimulus

Question 9

Describe phase 5 of an AP

Answer 9

Voltage-gated potassium channels remain open and potassium continues to leave the cell
Membrane potential moves closer to potassium equilibrium
Some voltage-gated channels close
Membrane potential returns to resting

Question 10

What is the relative refractory period

Answer 10

Inactivation gate is open so a stronger than normal stimulus is required to trigger an action potential

Question 11

What does the term threshold mean

Answer 11

The potential where once reached an action potential will be triggered

Question 12

What is an action potential

Answer 12

Once triggered, a full sized action potential will occur

Question 13

What is the refractory state

Answer 13

Unresponsiveness to threshold depolarisation

Question 14

What feedback system do action potentials follow

Answer 14

Positive as a small increase in sodium influx results in a bigger one

Question 15

When will the cycle stop

Answer 15

When the voltage-gated sodium channels become inactive

Question 16

How is resting potential restored

Answer 16

Through ion channels as ion pumps are too slow

Question 17

When will the membrane exit the refractory state

Answer 17

When voltage gated sodium channels recover form inactivation

Question 18

Describe passive propagation

Answer 18

A gradient response at one end of the neurone will be much smaller at the other end due to exponential decline

Question 19

Describe active propagation

Answer 19

At the area of depolarisation, a local current depolarises the adjacent area. If this reaches threshold then another local current will be established

Question 20

How does AP propagation maintain unidirectionality

Answer 20

The refractory period means the area before the new action potential is in refractory and cannot be depolarised

Question 21

What is the function of the Nodes of Ranvier

Answer 21

They are gaps in insulating myelin sheath that allows the action potential to “jump” in a process known as saltatory conduction

Question 22

What is the average velocity in mammalian axons with a large diameter + myelination

Answer 22

12 m/s

Question 23

What is the average velocity in mammalian axons with a short diameter + no myelination

Answer 23

1 m/s

Question 24

Why is velocity greater in axons with a greater diameter

Answer 24

The greater the diameter, the less resistance to current flow

Question 25

What can reduce conduction velocity

Answer 25

Axon diameter e.g. re-growth after injury

Reduced myelination e.g. MS and diphtheria