Action Potentials

Question 1

Describe some properties of action potentials

Answer 1

• Change in voltage across membrane
• Depends on ionic gradients and relative permeability of the membrane
• Only occurs if a threshold level is reached
• All or nothing
• Propagated without loss of amplitude

Question 2

Can you draw action potentials for different tissues?

Answer 2

Question 3

What is the relationship between conductance and membrane potential?

Answer 3

as conductance to an ion increases, membrane potential moves closer to equilibrium potential of that ion

Question 4

How can it be shown experimentally that sodium is responsible for action potential depolarisation?

Answer 4

The peak of the action potential changes in a manner parallel to the changes in sodium equilibrium potential. This shows that the upstroke of the action potential is due to a large increase in permeability to sodium ions.

Question 5

How many sodium ions flow into an axon to produce an action potential?

Answer 5

Each action potential increases [Na+] in the axon by only 40um. If the resting [Na+] is 10mM this represents an increase of 0.4%.

Question 6

What is inactivation?

Answer 6

• channels in recovery state (after depolarisation)
• Potassium channels don’t inactivate, rather close slowly

Question 7

How is an action potential started at an axon?

Answer 7

• Depolarisation to threshold at axon hillock (dense sodium channels)
• Sodium channels open
• Sodium enters
• Positive feedback

Question 8

What occurs during the downstroke of an axon action potential?

Answer 8

• Depolarised membrane
• Sodium channels inactivate
• Potassium channels open
• Potassium efflux
• Sodium influx stopped
• Repolarisation

Question 9

Describe recovery from an action potential

Answer 9

ARP - nearly all Na+ channels are in the inactivated state (during action potential)

RRP - Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases. (for a few milliseconds after the action potential)

Question 10

How do local anaesthetics work?

Answer 10

• Charged molecule blocks pore
• Entry into cell depends on pH (acidic state means they are charged)
• Hydrophobic pathway has no-use dependance
• Hydrophillic pathway is use dependent

Question 11

In what order do local anaesthetics work?

Answer 11

Local anaesthetics block in the following order:

1. small myelinated axons
2. un-myelinated axons
3. large myelinated axons

Question 12

Describe some diseases which affect conduction of an action potential

Answer 12

Central Nervous System:

• Multiple sclerosis – all CNS nerves
• Devic’s disease – optic and spinal cord nerves only

Peripheral Nervous System

• Landry-Guillain-Barre syndrome
• Charcot-Marie-Tooth disease

These diseases result from breakdown or damage to the myelin sheath.

Question 13

Explain the local circuit theory of propagation

Answer 13

The depolarisation of a small region of membrane produces transmembrane currents in neighboring regions.

As Na+ channels are voltage gated, this opens more channels, causing the propagation of the action potential. The further the local current spreads, the faster the conduction velocity of the axon.

Question 14

What is capacitance?

Answer 14

Capacitance, C, is the ability to store charge. This is a property of the lipid bilayer. Increased capacitance decreases the speed in which membrane potential changes

Question 15

How is action potential propagated in an unmyelinated axon?

Answer 15

• local currents
• inactivation means action potential moves forward only

Question 16

What is the relationship between resistance and potential difference of a membrane?

Answer 16

• the higher the resistance of the membrane, the higher the potential difference across it.
• more voltage across the membrane means more voltage gated Na+ channels are open; making it easier to reach the threshold to fire an AP.
• conduction velocity is therefore increased.

Question 17

How does a large axon diameter affect conduction velocity?

Answer 17

• the lower the resistance (In this case, lower cytoplasmic resistance from a large diameter), the larger the current, therefore the action potential will travel further.
• Conduction velocity is therefore increased.

Question 18

How does capacitance link to conduction velocity?

Answer 18

• Capacitance is the ability to store charge.
• a membrane with a high capacitance will take more current to charge (or a longer time for a given current).
• decrease in conduction velocity

Question 19

What is the consequence of demyelination on action potential conduction?

Answer 19

In regions of demyelination, current is reduced because of resistive and capacitive shunting.

Question 20

Explain the implications of myelination for conduction velocity

Answer 20

Myelination Reduces Capacitance and Increases Membrane Resistance of the axon - increasing conduction velocity.

Myelination also allows for Saltatory Conduction. This is because the myelin sheath acts as a good insulator, causing the local circuit currents to depolarise the next node above the threshold and generate an AP. These nodes have a high density of voltage gated Na+ channels. This is in contrast to unmyelinated axons, eg sensory neurones, which have an even distribution of channels.

Question 21

How is membrane resistance linked to the number of ion channels in a membrane?

Answer 21

The membrane resistance depends on the number of ion channels open. The lower the resistance the more ion channels are open.

Question 22

What types of neurones are myelinated?

Answer 22

Large Diameter Axons such as Motor neurones are myelinated, smaller ones such as sensory neurones are not.

Question 23

How is myelin formed?

Answer 23

Myelin is formed by special cells, which envelop axons in their plasmalemma.

Schwann Cells – Myelinate peripheral axons

Oligodendrocytes – Myelinate axons in the CNS