Session 4

Question 0

Define action potential

Answer 0

The change in voltage across a membrane

Question 1

What are the properties of action potentials?

Answer 1

Depends on ionic gradients and relative permeability, need to reach threshold for it to occur, all or nothing, propagated without loss of amplitudes

Question 2

How are action potentials generated?

Answer 2

Increased permeability to Na+, bringing the membrane closer to the Na+ equilibrium potential

Question 3

Describe the structure of Na+ channels

Answer 3

Main pore is one peptide consisting of 4 peptide chains. Each repeat consists of 6 transmembrane spanning domains

Question 4

Give an example of a local anaesthetic and how it works

Answer 4

Procaine. It works by binding to and blocking Na+ channels, thereby stopping generation of action potentials

Question 5

How are nerve fibres blocked by anaesthetics?

Answer 5

Small, myelinated axons
Non-myelinated axons
Large, myelinated axons

Question 6

What is accommodation?

Answer 6

The longer the stimulus, the larger the depolarisation necessary to initiate an action potential therefore the threshold becomes more positive. Causes an accumulation of Na+ channels in the inactive state.

Question 7

Define absolute refractory period

Answer 7

When nearly all Na+ channels are in the inactivated state

Question 8

Define relative refractory period

Answer 8

When Na+ channels are recovering from inactivation, excitability returns to normal as the number of channels in the inactivated state decreases

Question 9

How can you record electrical stimulation of a cell?

Answer 9

Stimulate them under a cathode, excitability is reduced under the anode

Question 10

Why would we need an extracellular recording of an action potential?

Answer 10

It can give information about conduction velocity under various conditions

Question 11

What is conduction velocity?

Answer 11

Nerve conduction velocity is the speed at which an electrochemical signal propagates down a neural pathway.

Question 12

How do we calculate conduction velocity?

Answer 12

Distance between electrodes/time taken

Question 13

How is an action potential conducted along an axon?

Answer 13

Change in membrane potential spreads down the axon due to local current spread
Conduction velocity is determined by how far along the axon these local currents spread
If depolarisation occurs –> ap is initiated.

Question 14

Which properties give rise to a high conduction velocity?

Answer 14

High membrane resistance
Low membrane capacitance
Large axon diameter (leading to low cytoplasmic resistance)

Question 15

Define capacitance

Answer 15

Ability to store charge.

Question 16

What does a high capacitance mean?

Answer 16

It takes more current to charge (or a longer time) and can cause a decrease in spread of local current, especially with brief current pulses

Question 17

What does a lower resistance mean for ion channels?

Answer 17

More ion channels are open, meaning that there is more loss of the local current effect

Question 18

What is the purpose of the refractory period?

Answer 18

Allows unidirectionality because Na+ channels are inactive for this period of time

Question 19

What effect does myelination have on the conduction velocity?

Answer 19

Conduction velocity is increased considerably by myelination of axons

Question 20

What effect does myelination have on capacitance?

Answer 20

Decreases capacitance

Question 21

What effect does myelination have on resistance?

Answer 21

Increases resistance

Question 22

What is myelin formed by?

Answer 22

Schwann cells in peripheral axons

Oligodendrocytes in axons in the CNS

Question 23

Describe the relationship between velocity and diameter in myelinated axons

Answer 23

Velocity is proportional to the diameter

Question 24

Describe the relationship between velocity and diameter in non-myelinated axons

Answer 24

Velocity is proportional to the square root of the diameter

Question 25

What is the cause of multiple sclerosis?

Answer 25

Autoimmune disease in which myelin is destroyed in certain areas of the CNS, leading to decreased conduction velocity, complete block or partial transmission of action potentials.

Question 26

What does activation mean?

What does inactivation mean?

Answer 26

Activation means that the voltage favours the opening of the channel and inactivation means the voltage is not sufficient for it to be active (open)

Question 27

Define accommodation when applied to nerves

Answer 27

Over a period of time there are too many inactivated Na channels and thus even if the threshold is exceeded there will be no action potential.

Question 28

What is the function of myelin?

Answer 28

Reduces capacitance and increases the resistance of the axons