9. Action Potential

Question 1

Action potential

Answer 1

Only occurs if threshold level reached
All or nothing
Propagated without loss of amplitude

Question 2

Axon hillock

Answer 2

Depolarisation to threshold initiates action potential at axon hillock
Contains lots of voltage gated ion channels

Question 3

Describe the action potential graph

Answer 3

Stimulus changes potential to above threshold
Sodium channels open, sodium rushes in, depolarises, membrane potential moves closer to Ena
Sodium channels become inactive, potassium channels open, potassium ions move into cell, membrane is repolarised
Hyperpolarisation occurs as potential moves closer to Ek
Potassium channels begin to close and potential moves back to resting

Question 4

Conductance

Answer 4

An increase in conductance for a particular ion represents and increase in number of open channels for that ion

Question 5

What does APR stand for?

Answer 5

Absolute refractory period
Nearly all sodium channels are in inactivated state
Slope down

Question 6

What does RRP stand for?

Answer 6

Relative refractory period
Sodium channels recovering from inactivation
Excitability returns towards normal
Sodium channels begin to reactivate
Number of open voltage gated K+ channels close

Question 7

Structure of voltage gated sodium channel

Answer 7

One alpha subunit consists of four similar sections or repeats
Has pore region, voltage sensor (S4) and inactivation particle
Each sodium channel is only one alpha subunit with pore in middle

Question 8

Structure of voltage gated potassium channel

Answer 8

One alpha subunit is 1/4 of channel
Channel consists of 4 subunits
Has pore region and voltage sensing region

Question 9

Local anaesthetics

Answer 9

Block sodium channels

Question 10

What order do local anaesthetics block in?

Answer 10

1. Small myelinated axons
2. Unmyelinated axons
3. Large myelinated axons

Question 11

What is capacitance and what structure has it?

Answer 11

Ability to store charge

Lipid belayer

Question 12

Membrane resistance

Answer 12

Depends on number of ion channels open

Lower the resistance, more ion channels open

Question 13

What happens at high capacitance?

Answer 13

Voltage changes more slowly in response to current injection

Question 14

What happens at high resistance?

Answer 14

Change in voltage spreads further along the axon

Question 15

Salvatory conduction

Answer 15

Myelin sheath acts as good insulator, causes local circuit currents to depolarise the next node above threshold and initiate action potential
Action potential jumps from node to node
Much faster conduction velocity

Question 16

How does myelination increase conduction velocity?

Answer 16

Large increase in membrane resistance
Large decrease in membrane capacitance
These increase length constant
Slight decrease in time constant

Question 17

Demyelination

Answer 17

Density of action current reduced because of resistive and capacitive shunting
Causes failure to reach threshold
Stops saltatory conduction