1.3.3 Action Potential Generation Flashcards
When both gates are open what will happen to gNa and INa?
When both gates open gNa and INa will both be increased.
When threshold is achieved what happens?
At threshold there is a net depolarizing current due to active sodium channels. This will move V more positive - thus opening more Na activation gates (increasing gNa)
What conformation are the voltage-gated sodium channels in when at normal resting potential (resting Na+ channel)?
At resting potential the inactivation gate is open and the activation gate is closed
C
What are symptoms of hypercalcemia?
Muscle weakness, tiredness, confusion, etc
As membrane potential becomes more positive, what happens to the potassium delayed rectifier channels?
As it becomes more positive, activation gates slowly open on the potassium delayed rectifier channels.
Depolarization of Na-gates acts as what kind of feedback loop?
Positive feedback loop
At the peak of the action potential, what will cause repolarization (V moving away from ENa)?
Even though all activation gates are opened, Na+ inactivation gates are closed. This will cause a decrease in gNa and thus depolarization.
Is the threshold value fixed within a cell?
NO - it depends of the balance between depolarizing and repolarizing currents
What is important to not about the kinetics of the activation and inactivation gates? Why might this be important?
The inactivation gate opens and closes slowly and the activation gate opens and closes rapidly. Because of this, if a cell depolarizes quickly enough activation gates can open before inactivation gates have closed completely.
What is conduction velocity?
Conduction velocity is how quickly an action potential propogates along a tissue. It is dependent on the magnitude of the depolarzing current during the upstroke of the action potential.
Summarize the process of a nerve action potential.
At the resting potential, both Na- and K-voltage gated channels are closed. The status of the Na-channel is in the resting state. The K-channel is also in the resting state. During the upstroke of the action potential, sodium channels move from the closed (resting) state to the open (active) state. At the peak of the action potential, most of the sodium channels are in the open (active) state. The membrane depolarization that occurs during the upstroke phase also cause K-channels to transition from the closed (resting) state to an open (active) state. With time, the membrane depolarization that activated the Na-channels also causes the open (active) state to transition to a closed (inactivated) state. The repolarization phase results from opening of K-channels. During the undershoot, the open voltage-gated K-channels cause K-permeability to be greater than the potassium permeability that occurs at the resting potential and this causes membrane potential to approach the potassium Nernst potential. With time, the open (active) K-channels transition to the closed (resting) state. This causes K-permeability to decrease and membrane potential moves back to the resting potential.
The magnitude of the depolarizing current during the upstroke of the action potential will determine?
What are the two gates associated with the Voltage-Gated Sodium Channel?
Activation and Inactivation gate
What is the relative refractory period?
During the time that inactive channels are being converted to resting channels, the cell can generate AP if a stronger depolarizing stimulus is given than normally required
D
The number of resting Na+ channels is dependent on what?
Having sufficient time during repolarization before the next depolarization stimulus occurs, and repolarizing to a sufficient extent.
If resting potential is moved more positive, how will this affect membrane excitability?
As membrane potential becomes more positive what will happen to Na-channels?
There will be an increased probability that their activation gates will be open
Increasing fgNa will do what to membrane potential? What formula is associated with that?
Increased fgNa will move membrane potential towards the sodium Nernst potential. The image is the associated formula
If the magnitude of the Na+ current during depolarization is decreased what will occur?
- Threshold potential is more positive (decreased excitability)
- Amplitude of AP is decreased
- Rate of Rise of AP is decreased
- Conduction velocity is decreased
E
What is the absolute refractory period?
The period where the cell cannot generate an AP regardless of the strength of the depolarizing stimulus (during the upstroke of the AP)
What is the effect of hypercalcemia on the sodium activation curve?
Hypercalcemia will reduce excitability by shifting sodium activation curve more positive
D
What is the threshold point determined by?
If enough active Na+ channels are produced to reach a point where INa (depolarizing current) just barely exceeds IK (repolarizing current), the cell has reached threshold
When gNa is increased what will this do to V?
V, the membrane potential, will be moved closer to ENa
What is the normal resting potential?
-70 to -80 mV
What will be the result of decreased number of resting Na+ channels?
This will diminish the cell’s ability to generate INa . This is the underlying problem with hypo and hyperkalemia
A
C
As the cell repolarizes what will occur to the sodium activation gates?
The activation gates will close first, then the inactivation gates will open again.
Depolarization of an action potential is caused by what?
Increasing gNa
As the membrane potential moves more positive what happens to the inactivation gates?
They begin to slowly close
