Action Potential

Question 1

What is the equation for a passive membrane?

Answer 1

I(membrane) = I(capacitance) + I(resistance)
– capacitance and resistance in parallel

Question 2

What is capacitance?

Answer 2

ability to store equal and opposite charges across an insulator

Question 3

What is the voltage curve when current is injected for a passive membrane?

Answer 3

• at current injection = charge goes to capacitance to reduce charge number
• depolarize = some charges go through channel
• steady state = all charges through channel
  (voltage is slow to rise and slow to fall)

Question 4

What are the 5 properties of a passive membrane?

Answer 4

1) Graded (multiple amplitudes)
2) Depolarize or Hyperpolarize
3) Not self-generating
4) No refractory period
5) Linear

Question 5

What is an action potential?

Answer 5

• inject current with increasing intensities to meet threshold potential
• overshot after depolarize
• undershot after hyperpolarize
• amplitude constant

Question 6

What is an absolute refractory period?

Answer 6

no action potential generated immediately after at any current injection

Question 7

What is a relative refractory period?

Answer 7

action potential generated at a later time with a stronger current injection

Question 8

What are the 5 properties of an action potential?

Answer 8

1) All or none (fixed amplitude)
2) Depolarize –> Hyperpolarize
3) Self-generating
4) Absolute and Relative refractory period
5) Non-linear

Question 9

What is Hodgkin & Katz’s hypothesis?

Answer 9

the peak of an action potential is Na+ mediated
- vary external Na+ concentration (replaced with choline)
- lower peak amplitude

Question 10

What is the prediction by Nernst Equation?

Answer 10

plot E(Na) vs log Na(out)
- linear slope of 60mV/10 fold change of Na(out)
- peak mediated by Na+ permeability

Question 11

How does the conductance change after an action potential?

Answer 11

fast Na+ conductance at action potential (channels open)
delay K+ conductance at AHP

Question 12

How does action potential change over time?

Answer 12

Rest – low Na permeability, K permeability = 1
Peak – high Na permeability, K permeability = 1
AHP – reduced Na permeability (Na inactivation), K permeability elevated

Question 13

How was a voltage-clamp studied?

Answer 13

giant squid axon
- 2 electrodes = inject current + measure voltage

Question 14

What is a voltage-clamp?

Answer 14

• peak of current around voltage jump
• inward current –> outward current

Question 15

How to isolate K+ current alone in a voltage-clamp?

Answer 15

• replace Na with choline or block Na channel with TTX
• delayed rectifier current
• no inactivation

Question 16

How to isolate Na+ current alone in a voltage-clamp?

Answer 16

• subtract K+ current recorded with no [Na(out)] from total current OR block K+ current with TEA
• rapid raise (activate), early peak, fast decay (inactive)

Question 17

How does conductance change during an action potential?

Answer 17

fast Na+ conductance (Na channels open)
- gradual inactivation

delayed K+ conductance
- membrane potential changes – no inactivation

Question 18

What are the phases of an action potential?

Answer 18

Rise = depolarize, Na+ open, inward current (positive feedback)
Repolarization = depolarize, K+ open, outward current, hyper-polarize (negative feedback) OR Na channel inactivation
AHP = K+ open to drive to E(k)

Question 19

What happens to the channels during absolute and relative refractory period?

Answer 19

Absolute refractory = Na+ channel inactivation
Relative refractory = more stimulus needed to overcome elevated K+ channel, sufficient Na+ channels open

Question 20

What are channel blockers to study action potentials?

Answer 20

TEA or 4-AP block K+ channels = prolonged AP
TTX block Na+ channel = no AP generated, only passive membrane response