M&R Session 4 (Lecture 4.1)

Question 1

Describe 5 general characteristics of action potentials.

Answer 1

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

Question 2

What are the voltage changes during an action potential in the following tissues: Axon, Skeletal muscle, SAN, Cardiac ventricle.

What are the durations of each?

Answer 2

1) Axon : -70 to +30mV 0.5ms
2) Sk.M : -90 to +40mV 0.5ms
3) SAN : -60 to +30mV 100ms
4) C.V : -90 to +30mV 100ms

Question 3

What is conductance of the membrane for an ion?

Answer 3

Dependant on the no. of channels for the ion that are open. The higher the conductance; the more channels open for an ion

Question 4

What occurs to the membrane potential if the conductance for an ion is increased?

Answer 4

The membrane potential will move closer to the equilibrium potential for that ion.

Question 5

What occurs to the peak of an action potential if the extracellular [Na+] is decreased?

Answer 5

Peak reduced so supports idea that the upstroke of the A.P. is due to a large increase in permeability to Na+ ions

Question 6

For a axon of diameter 1 micro m, what is the net influx of Na+ ions into the axon?

Answer 6

40 micro M

Question 7

What does voltage-clamping involve? What can it assess?

Answer 7

Introduces two electrodes into the cell which can control the voltage and clamp it to investigate the ionic current movements.

Question 8

What are the two key differences between VGNCs and VGKCs?

Answer 8

VGNCs open and close quicker (dependant on rate of depol) and become inactive after a period of time.

VGKCs open slowly and and have no inactivation

Question 9

Where is the A.P. initiated?

Answer 9

Axon hillock

Question 10

Why is an A.P described as ‘all or nothing’ ?

Answer 10

Either you get an A.P or not. If the threshold level is reached then an A.P is generated.

Question 11

Describe the depolarisation events of the AP for the VGNCs (positive feedback loop).

Answer 11

1) Depolarisation to threshold
2) Na+ channels open
3) Na+ influx
4) Membrane depolarises

Repeat steps 2-4 until peak is reached

Question 12

What happens during the repolarisation of the AP?

Answer 12

1) Depolarisation
2) Opens K+ channels and inactivates Na+ channels
3) K+ efflux and Na+ influx stopped
4_ Repolarisation

Question 13

Define the ARP and RRP?

Answer 13

Absolute refractory period = all Na+ channels are inactivated so no stimuli can generate another AP.

Relative RP = Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases.

Question 14

What can occur to open VGNCs?

Answer 14

They are prone to inactivation

Question 15

Explain accommodation of nerves.

Answer 15

Decreasing the rate of depolarisation causes the A.P threshold to become more positive so an AP is less likely to be generated the longer the stimuli.

Useful in blocking out repeated stimuli e.g. watch or repetitive noise

Question 16

Describe the structure of a VGNC.

Answer 16

4 subunits each containing 6 transmembrane domains.
One pore region in each subunit
One voltage sensor (S4) containing positive AAs which undergoes a conformational change when the voltage changes
Only one alpha subunit is the functional channel
Contains an inactivation particle

Question 17

Describe the structure of a VGKC.

Answer 17

Similar to VGNCs
One pore region in each subunit providing pore selectivity for K+
Four alpha subunits are functional

Question 18

How does procaine and other local anaesthetics act on nerves?

Answer 18

Procaine for example can act via the hydrophobic/non - use-dependant pathway or the hydrophilic/use dependant pathway to block VGNCs in pain sensing C fibres.

Question 19

Describe the hydrohpobic/non-use dependant pathway for anaesthetics.

Answer 19

Anaesthetic diffuses into the bilayer and within the bilayer, diffuses into VGNCs and blocks them regardless of their use.

Question 20

Describe the hydrohphilic/use dependant pathway for anaesthetics.

Answer 20

Anaesthetic diffuses through bilayer and is protonated in the cytoplasm. When the VGNC is opened, the protonated anaesthetic blocks the channel. The more channels open = the more channels bloacked

Question 21

In which order do local anaesthetics block nerve fibres?

Answer 21

1) Small myelinated axons
2) Un-myelinated axons
3) Large myelinated axons