Session 5: Action Potentials

Question 1

What is an action potential?

Answer 1

Change in voltage across a membrane

Question 2

What is conductance?

Answer 2

The number of channels for the ion that are open.

Question 3

TRUE OR FALSE: If the conductance to any ion is increased the membrane potential will move closer to the equilibrium potential for that ion.

Answer 3

True

Question 4

Which ion is responsible for AP depolarisation?

Answer 4

sodium

Question 5

How can it be experimentally proven that sodium is the ion responsible for AP depolarisation?

Answer 5

The peak of the action potential changes in a parallel manner to the changes in the equilibrium potential for a sodium ion.

Question 6

What is membrane capacitance?

Answer 6

The ability to store charge and determines how quickly the membrane potential can respond to a change in current.

Question 7

Why are voltage clamps useful?

Answer 7

They allow a membrane current to be measured at a set membrane potential

Question 8

What is the role of the Na/K pump in an AP?

Answer 8

Helps maintain the cells resting potential. (Not involved in the repolarisation!!!!!)

Question 9

Define absolute refractory period.

Answer 9

Regardless of how strong a stimulus is no AP can be achieved.

Question 10

Define relative refractory period.

Answer 10

A very strong stimulus has to potential to initiate a new AP

Question 11

What is meant by inactivated sodium channels?

Answer 11

The are blocked by an inactivation particle so ions can now longer flow through the channel. They will only recover once the cell has become hyperpolarised (more negative). Hence, until this happens no new AP can fire.

Question 12

Describe the molecular chemistry of a sodium channel.

Answer 12

• 1 subunit (single polypeptide)
• S4 voltage sensor (abundant positive charge)
• 4 alpha helical regions
• Pore

Question 13

What does the abundant positive charge on the S4 voltage sensor allow to happen?

Answer 13

Creates a voltage field across the membrane therefore causing a conformational change, hence opening the pore to allow ions through.

Question 14

Describe the molecular chemistry of a potassium channel.

Answer 14

• 4 subunits
• S4 voltage sensor (abundant positive charge)
• pore

Question 15

Describe local current theory.

Answer 15

Sodium moves into the cell causing positive charges to repel one another and negative changes to be attracted to the positive charge.

This causes depolarisation of adjacent areas.

The impact is smaller the further from the initial area of depolarisation.

Question 16

What is meant by the length constant?

Answer 16

The distance it takes for the potential to fall to 37% of its original value.

Question 17

What is the membrane resistance dependant on?

Answer 17

The number of ion channels open.

Question 18

What is the capacitance dependant on?

Answer 18

The property of the lipid bilayer.

Question 19

What is the result of a cell with a high capacitance?

Answer 19

Voltage changes more slowly in response to a current injection.

Question 20

What is the result of a cell with a high resistance?

Answer 20

change in voltage spreads further along the axon.

Question 21

Which cells are involved in myelination?

Answer 21

Schwann cells (peripheral nervous system)
Oligodenrocytes (CNS)

Question 22

What is saltatory conduction?

Answer 22

When the action potential jumps from node to node

Question 23

What property of myelin sheath makes it good for saltatory conduction?

Answer 23

Good insulator

Question 24

Name a demyelinating disease.

Answer 24

Multiple Sclerosis

Question 25

Describe a positive feedback loop associated with AP.

Answer 25

The opening of many voltage-gated sodium channels causes the cell to become less negative, causing yet further sodium voltage-gated channels to open.

Question 26

What causes the rapid upstroke of the AP?

Answer 26

The rapid influx of sodium ions

Question 27

What 2 things does depolarisation cause?

Answer 27

1. Inactivation of sodium channels

2 Opening of voltage-gated potassium channels