Lecture 4: Action Potentials

Question 1

Describe the process of an action potential, based on the diagram

Answer 1

Something happens to bring the membrane potential to threshold. This triggers the all or nothing action potential. The membrane potential becomes more positive (eventually passing above 0). Then the membrane potential falls towards zero and then there is hyperpolarisation

Question 2

When is no action potential possible?

Answer 2

the absolute refractory period

Question 3

When is action potential possible but it is more difficult to initiate?

Answer 3

the relative refractory period

Question 4

Action potentials are regenerative. What does this mean?

Answer 4

They are propagated without decreasing the amplitude

Question 5

When there is a depolarisation and the membrane potential becomes more positive, does it approach the equilibrium potential for Na+ or K+?

Answer 5

Na+

Question 6

What is the conductance of an ion and what two things does it depend on?

Answer 6

the amount of a particular charged ion that can flow across a membrane and it depends on

• permeability due to ion channels
• equilibrium potential (driving force)

Question 7

Describe the conductance of Na+ throughout an action potential

Answer 7

the conductance of Na+ rises rapidly at the beginning of an action, and falls rapidly back to zero

Question 8

Describe the conductance of K+ throughout an action potential

Answer 8

the conductance rises much more slowly and doesn’t get as high and then very slowly decreases

Question 9

Describe voltage gated Na+ channels (4)

Answer 9

• there is an activation gate which is opened when the membrane potential reaches a certain voltage
• selectively filter (specific for Na+)
• there is an inactivation gate
• blocked by chemicals such as TTX

Question 10

Describe the makeup of voltage gated Na+ channels in terms of the subunits

Answer 10

• 4 α subunits

- 2 β subunits

Question 11

What are the 4 parts of the voltage gated Na+ channel

Answer 11

• voltage sensing activation gate
• a pore with a selectivity filter
• an inactivation gate inside the cell
• a modulation function

Question 12

Describe the voltage gated Na+ channel when it is not activated

Answer 12

There are positive charges in the way which closes the pore and prevents Na+ from entering because like charges repel

Question 13

What are the three phases that the voltage gated Na+ channel can be in?

Answer 13

• not activated
• activated
• inactivated

Question 14

Describe the voltage gated Na+ channel when it is activated

Answer 14

there has been a depolarisation to bring the membrane potential to threshold which moves the positive charge out of the way, allowing Na+ to flow down its electrochemical gradient into the cell

Question 15

Describe the voltage gated Na+ channel when it is inactivated

Answer 15

the ball and chain part of the gate inside the cell closes to that the channel is open but blocked from the inside

Question 16

Which responds faster, the ball and chain or the movement of the charge out of the pore?

Answer 16

they both respond to change in the voltage but the charge movement responds faster

Question 17

Describe the voltage gated K+ channels (4)

Answer 17

• has an activation gate
• selectively filters (specific for K+)
• has an inactivation gate
• blocked by toxins

Question 18

Which is faster, the voltage gated Na+ channel or the voltage gated K+ channel?

Answer 18

voltage gated Na+ channel

Question 19

What is happening to the activation gates (both Na+ and K+) at resting membrane potential?

Answer 19

they are both closed so there is no conduction through the channels

Question 20

When the resting membrane potential is brought to the threshold, what voltage gated channels open?

Answer 20

the Na+ ion channels

Question 21

When the resting membrane potential is brought to the threshold, what voltage gated channels remain shut? What does this mean for the conductance of K+ ions?

Answer 21

K+ ion channels which means that the conductance is 0

Question 22

Just after the resting membrane potential occurs, the K+ voltage gated channels open. What happens to the K+ ions? What does this cause?

Answer 22

they flow out of the cell which causes repolarisation

Question 23

During repolarisation, which voltage gated ion channels close first?

Answer 23

the inactivation gate of the Na+ voltage gated channels close first

Question 24

What is the absolute refractory period? Including when it occurs and why it occurs

Answer 24

This is when no action potential can occur and it happens when the Na+ channel inactivation gate it closed but the K+ channel is still open. This means that K+ ions still flow out which pushes the membrane potential further from threshold (hyperpolarisation)

Question 25

Which gate opens after the absolute refractory period?

Answer 25

the inactivation gate for Na+ but the ions don't flow in because the activation gate is still closed

Question 26

What is the relative refractory period?

Answer 26

This occurs when the Na+ channel inactivation gate is open (the activation gate is still closed) but the K+ channel is still open. K+ ions still flow out and so the membrane potential becomes further from threshold so the neuron is less excitable

Question 27

When there is a repolarisation and the membrane potential becomes more negative, does it approach the equilibrium potential for Na+ or K+?

Answer 27

K+

Question 28

The inactivation gate of the voltage gated Na+ channel blocks the channel pore BECAUSE the inactivation gate is part of the pore

Answer 28

the first statement is true and the second statement is false because the inactivation gate is not part of the pore, it is part of the intracellular loop (ball and chain)

Question 29

When considering the speed of conduction of electricity (ions), what does Rm stand for?

Answer 29

membrane resistance

Question 30

When considering the speed of conduction of electricity (ions), what does Rin stand for?

Answer 30

axial (internal) resistance

Question 31

What are two things in the axon which affect the speed of conduction?

Answer 31

- internal (axial) resistance | - membrane resistance

Question 32

What is the internal (axial) resistance regulated by?

Answer 32

the cross-sectional area of the axon

Question 33

What are the two ways to increase the speed of conduction?

Answer 33

``` decreasing internal (axial) resistance increasing membrane resistance ```

Question 34

Explain how decreasing the internal resistance can increase the speed of conduction along the axon

Answer 34

decreasing the resistance increases the speed of conduction by allowing ions to flow faster which increases AP conduction

Question 35

Explain how increasing the membrane resistance increases the speed of conduction along the axon

Answer 35

increasing the membrane resistance stops the axon from being "leaky" which means that less ions are able to cross the membrane which forces more ions to flow and this increases the speed of conduction

Question 36

What modifications are made to the axon to increase the speed of conduction?

Answer 36

the Schwann cells in the PNS and the oligodendrocytes in the CNS make myelin which provides great electrical insulation to the axon and increases the speed of conduction.

Question 37

Explain what saltatory conduction is and why this increases the speed of conduction

Answer 37

- saltatory conduction is the propagation of an action potential from one node of ranvier to another - this increases the speed of conduction because if everything is insulated the whole way along, the AP fails because the ions won't travel far enough from the point of origin as they end up along the whole volume of the axon

Question 38

Where is there a high density of Na+ channels and where is there no channels?

Answer 38

- high density at the nodes of ranvier | - no channels at the internode

Question 39

Describe the voltage-gated K+ channels (4)

Answer 39

- activation gate - selectively filter (specific for K+) - inactivation (slower than for the voltage-gated Na+ channel - blocked by toxins