Lecture 19: Ion Channels & Action Potentials Flashcards
Are there more leakage or voltage-gated Na+ channels?
Voltage-gated
Are there more leakage K+ channels or Na+ channels?
K+
What is necessary for a cell to depolarize?
Incoming Na+ ions must exceed leaving K+ ions
What does the threshold voltage correspond to?
Voltage at which a cell will fire an action potential = voltage-gated Na+ channels are at equil with leakage K+ channels = critical number of voltage-gated Na+ channels opening to OVERCOME the K+ leakage channels
What is important to note about action potentials?
They are all of nothing events and are all of the same amplitude (except from poison action)
What are subthreshold depolarizations due to?
Upticks of depolarization due to a few voltage-gated Na+ channels opening but there is no response from the cell
How fast does the cell depolarize once threshold is reached? How come?
Very fast due to + feedback: the opening of Na+ channels induces more channels to open
What is the voltage of the resting membrane potential?
-70mV
What is the threshold for the first ion channel to open?
-50mV
What is the overshoot?
When the depolarization surpasses 0mV
What is the reversal/equilibrium potential of Na+?
+60mV
What regulates the Na+ channel to close? How does this affect permeabilities?
Time and voltage
- Na+ channels deactivate → decrease in GNa+
- V-gated K+ channels open → increase in GK+
What happens when the Na+ channels reach equilibrium potential?
They close and the K+ channels open (delayed rectifier) = repolarization
What is the undershoot? Why is it due to?
Membrane is hyperpolarized to a level below that of the RMP because the delayed rectifier K+ channels have a slower activation kinetics than the voltage-gated Na+ channels
What regulates the K+ channel to close?
Hyperpolarization to slightly below -70mV
What does poisonous fugu cause?
It poisons and irreversibly blocks Na+ channels with tetrodotoxin (TTX) = death because of respiratory failure
Can the equilibrium voltage = the RPM?
Yes, if there is only one ion that can go through the membrane channels
What is the convention to draw currents?
- Inward: drawn downward
- Outward: drawn upward
What are non-poisonous, reversible blockers of Na+ channels?
Lidocaine, cocaine, anti-arrhythmic drugs
What is the purpose of a voltage clamp experiment?
Voltage is held at a specific value and current is measured, which is basically measuring the membrane’s conductance which depend on their number of channels (permeability) for many different ion channels
What is the purpose of a current clamp experiment?
Keep current constant and measure voltage
What does poisining Na+ or K+ channels allow us to do?
Identify which channels are responsible for which parts of the action potential curve
What does Tetraethylammonium (TEA) do?
Blocks voltage-gated K+ channels
What do patch clamp experiments measure?
The activity of a single channel
Do all ion channels work similarly to the Na+ and K+ channels every time? How is this visible?
NOPE
Each channel displays a variety of open/closed channel patterns that can be visualized as an average curve that represents the global current resulting from the average behavior of each of the individual ion channels
Na+ channel:
- Activation speed?
- Number of conformations?
- Inward or outward current?
- Dependent on time and/or voltage?
- Fast activation/inactivation
- Open, closed, or inactive
- Begins outward and will reverse to inward near +60mV
- Dependent on both time and voltage
K+ channel:
- Activation speed?
- Number of conformations?
- Inward or outward current?
- Dependent on time and/or voltage?
- Slow activation
- Open or closed (do not inactivate as long as there is voltage)
- Outward current
- Dependent on voltage only
What is a genetic disorder associated with Na+ channels? What is it caused by? What is the result?
Hyperkalemic periodic paralasys or paramyotonia congenita: caused by several AA mutations which result in the disruption of the Na+ flow and muscle fatigue
What is the Na+ transmembrane domain 4 responsible for?
Voltage sensor of the Na+ channel
What part of the Na+ channel is the P-loop? What do mutations in this area cause?
Domain 5
Mutations ⇒ depolarization of RMP = epilepsy
What is the effect of Ca2+ on voltage-gated Na+ channels? How? Explain how it works.
It can shield their voltage sensors by altering what the sensor is reading:
- Excess extracellular Ca2+→ Lower perceived membrane potential → Decreased excitability ⇒ Threshold shifts to a more polarized value (more +)
- Shortage of extracellular Ca2+ → Larger perceived membrane potential → Increased excitability ⇒ Threshold shifts to a less polarized value (less +)
What is the absolute refractory period? What causes the absolute refractory period? When is this?
Time in which no action potential CANNOT be generated
Na+ channels are inactive and need to go back to closed state: when the axon is getting depolarized and hyperpolarized (bell of the curve) the cell is unable to fire another action potential
What is the relative refractory period? What causes the relative refractory period? When is this?
Time during which an action potential can be generated by a STRONGER STIMULUS
The need for the membrane to be repolarized back to RPM: from hyperpolarization to resting potential
What prevents the action potential from propagating in 2 directions?
Na+ channels being in the inactivated state after