Action Potential

Question 0

Describe events of an action potential

Answer 0

Membrane is depolarised to threshold
Voltage gated sodium channels open ➡️ sodium influx
Further depolarisation towards the Ena
Na channels close once a certain voltage has been reached
Voltage gated potassium channels are opened
Potassium efflux
Repolarisation of the membrane

Question 1

List some properties of the action potential

Answer 1

All or nothing
Propogated without the loss of amplitude
Depends on ionic gradients and relative permeability
Only occurs if threshold level is reached

Question 2

What is the absolute refractory period?

Answer 2

When nearly all sodium channels are in their inactive state so excitability is at 0.

Question 3

What is the relative refractory period?

Answer 3

Sodium channels are recovering from inactivation. Excitability returns towards normal as the number of deactivated channels decreases. Requires a larger stimulus than usual to trigger an action potential

Question 4

How many subunits/polypeptide chains are required for the function of a sodium channel?

Answer 4

1

Question 5

What is an action potential?

Answer 5

A change in voltage across a membrane

Question 6

Explain the ‘all or nothing’ principle G

Answer 6

The sodium channels are voltage gated.
As the membrane potential becomes more positive, positive feedback means that more channels will open until they all are.
Depolarisation cannot stop halfway, as this voltage is the voltage that wold open more sodium channels.

Question 7

Describe accommodation

Answer 7

The longer the stimulus is, the larger the depolarisation necessary to initiate an action potential. This is because sodium channels become inactivated.

Question 8

Describe structure of sodium and calcium channels

Answer 8

A pore forming subunit is made up of:
One polypeptide chain containing
Four homologous repeats
Each repeat consists of six transmembrane domains

Question 9

In a sodium or calcium channel, how many of its domains are voltage sensitive?

Answer 9

1

Question 10

Structure of a potassium channel?

Answer 10

Four peptides

Six transmembrane domains

Question 11

How many subunits does a functional potassium channel require?

Answer 11

4

Question 12

How do local anaesthetics act?

Answer 12

By binding to and blocking sodium channels

Question 13

In which order to local anaesthetics block conduction?

Answer 13

Small myelinated axons
Non-myelinated axons
Large myelinated axons

Question 14

Do anaesthetics tend to affect sensory or motor neurones first?

Answer 14

Sensory

Question 15

What state of sodium channel do anaesthetics have a highest affinity form

Answer 15

Sodium channels in their inactivated state

Question 16

How can an action potential be measured?

Answer 16

Negatively charged cathode used to stimulate an axon (or group of axons) to threshold and initiate an action potential.
Record changes in potential between the cathode and anode along an axon.

Question 17

How do you work out conduction velocity?

Answer 17

Measure the distance between the stimulating electrode and the recording electrode; and the time gap between the stimulus and the AP being registered by the recording electrode
CV = distance ➗ time

Question 18

Explain local circuit theory of propagation

Answer 18

Depolarisation of a small region of the membrane produces transmembrane currents in neighbouring regions.
Na channels are voltage gated, so this opens more channels, propagating the action potential.

Question 19

What is the relationship between the distance the local current spreads and the speed of the conduction velocity of the axon?

Answer 19

The further the local current spreads down the axon, the faster the conduction velocity of the axon

Question 20

What factors contribute to a higher conduction velocity?

Answer 20

High membrane resistance
Low membrane capacitance
Large axon diameter as this leads to low cytoplasmic resistance

Question 21

What does Ohm’s Law state?

Answer 21

The higher the resistance of the membrane, the higher the potential difference across it.

Question 22

What does Ohm’s Law mean for the effect of high membrane resistance?

Answer 22

More voltage across the membrane means more sodium channels are open. Easier to reach the threshold for an AP. Conduction velocity is increased.

Question 23

What does Ohm’s law mean for axon diameter?

Answer 23

States that the lower the resistance, the larger the current, so action potential will travel further. Conduction velocity is therefore increased with a larger diameter.

Question 24

What is capacitance?

Answer 24

The ability to store charge

Question 25

What effect does capacitance have on conduction velocity?

Answer 25

A high capacitance reduces conduction velocity, because it takes more current to charge it.

Question 26

What is the effect of myelination on conduction velocity?

Answer 26

Increases it

Question 27

Which axons are myelinated?

Answer 27

Large diameter ones such as motor neurones.

Question 28

What is salutatory conduction?

Answer 28

AP jumps between nodes of Ranvier.

The myelin sheaths acts as an insulator, causing local currents to depolarise the next node

Question 29

What is the distribution of sodium channels in a myelinated axon?

Answer 29

High concentration of sodium channels at nodes.

Question 30

What are peripheral axons and CNS axons myelinated by?

Answer 30

Peripheral - Schwann cells

CNS - oligodendrocytes

Question 31

What is a condition that strips axons of their myelin?

Answer 31

Multiple sclerosis

Question 32

What can MS lead to?

Answer 32

Affects the ability of previously myelinated axons to conduct action potentials
Decreased conduction velocity
Complete block or cases where only some action potentials are transmitted.

Question 33

When does myelination occur?

Answer 33

4 months after foetal development

Question 34

What is the effect of the myelin sheath on capacitance of a nerve fibre? Why?

Answer 34

Decreases capacitance.

Reduces dissipation of the local current within the fibre

Question 35

What can be done to treat de myelinated nerves?

Answer 35

Potassium channel blockers

Question 36

How does accommodation occur?

Answer 36

The progressive depolarisation gives time for some sodium channels to inactivate and for some potassium channels to open.

Question 37

Why, at small diameters, do non-myelinated neurones conduct faster than myelinated?

Answer 37

Smaller the diameter -> slower the conductance

At small diameters, the myelin sheath is included in the diameter so fractional space occupied by the myelin is high