Action Potential Flashcards
What is the equation for a passive membrane?
I(membrane) = I(capacitance) + I(resistance)
– capacitance and resistance in parallel
What is capacitance?
ability to store equal and opposite charges across an insulator
What is the voltage curve when current is injected for a passive membrane?
- 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)
What are the 5 properties of a passive membrane?
1) Graded (multiple amplitudes)
2) Depolarize or Hyperpolarize
3) Not self-generating
4) No refractory period
5) Linear
What is an action potential?
- inject current with increasing intensities to meet threshold potential
- overshot after depolarize
- undershot after hyperpolarize
- amplitude constant
What is an absolute refractory period?
no action potential generated immediately after at any current injection
What is a relative refractory period?
action potential generated at a later time with a stronger current injection
What are the 5 properties of an action potential?
1) All or none (fixed amplitude)
2) Depolarize –> Hyperpolarize
3) Self-generating
4) Absolute and Relative refractory period
5) Non-linear
What is Hodgkin & Katz’s hypothesis?
the peak of an action potential is Na+ mediated
- vary external Na+ concentration (replaced with choline)
- lower peak amplitude
What is the prediction by Nernst Equation?
plot E(Na) vs log Na(out)
- linear slope of 60mV/10 fold change of Na(out)
- peak mediated by Na+ permeability
How does the conductance change after an action potential?
fast Na+ conductance at action potential (channels open)
delay K+ conductance at AHP
How does action potential change over time?
Rest – low Na permeability, K permeability = 1
Peak – high Na permeability, K permeability = 1
AHP – reduced Na permeability (Na inactivation), K permeability elevated
How was a voltage-clamp studied?
giant squid axon
- 2 electrodes = inject current + measure voltage
What is a voltage-clamp?
- peak of current around voltage jump
- inward current –> outward current
How to isolate K+ current alone in a voltage-clamp?
- replace Na with choline or block Na channel with TTX
- delayed rectifier current
- no inactivation
How to isolate Na+ current alone in a voltage-clamp?
- 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)
How does conductance change during an action potential?
fast Na+ conductance (Na channels open)
- gradual inactivation
delayed K+ conductance
- membrane potential changes – no inactivation
What are the phases of an action potential?
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)
What happens to the channels during absolute and relative refractory period?
Absolute refractory = Na+ channel inactivation
Relative refractory = more stimulus needed to overcome elevated K+ channel, sufficient Na+ channels open
What are channel blockers to study action potentials?
TEA or 4-AP block K+ channels = prolonged AP
TTX block Na+ channel = no AP generated, only passive membrane response
