Term
| What are the characteristics of an electrotonic potential? |
|
Definition
No threshold
Passive, decremental conduction - gradually decreases
Size graded
Chemical, mechanical stimulus (ex - pressure)
Polarity inside negative
Other features
Not self-regenerating
Does not propagate
No refractory period
Temporal & spatial summation
(just for reference for difference from AP) |
|
|
Term
| What are the characteristics of an action potential? |
|
Definition
Threshold
Active, all-or-none conduction
All-or-none, same amplitude
Electrical stimulus - flow of ions
Polarity inside positive
Other features
Self-regenerating
Propagates
Refractory period
No Temporal & spatial summation |
|
|
Term
| What is an action potential? |
|
Definition
| An action potential is a change in the voltage of the membrane potential that causes it to go from its negative resting state to a positive value for a very brief time. |
|
|
Term
| Why doesn't the action potential reach its theoretical values based on the conc's of Na+? |
|
Definition
| K+ channels open in response to the depolarization and prevent the AP from reaching its full theoretical value |
|
|
Term
| What is the cause of the membrane potential reaching its threshold? |
|
Definition
Anything that causes a depolarization will cause the membrane potential to reach its threshold.
Na+ flowing in through leak channels or co-transport could be examples but not voltage-gated Na+ channels (they are not open yet) |
|
|
Term
| What is going during the rising phase of the action potential? |
|
Definition
The voltage-gated sensitive channels are opening and Na+ is flowing in.
There is a positive feedback mechanism because as more Na+ flows in, the cell becomes more depolarized which causes more voltage-gated channels to open.
It is important to realize that voltage sensitive K+ channels start to open here in response to the depolarization but a lot more Na+ channels are open and they have a greater effect. |
|
|
Term
| What is going on during the overshoot phase of the action potential? |
|
Definition
Overshoot is when the membrane potential is depolarized to the point where it is now a positive value.
When it overshoots all the way to the peak, this is when gNa (Na+ conductance) is the highest and all the voltage-gated Na+ channels that will open during the AP have opened. |
|
|
Term
| What happens after the action potential reaches its peak? |
|
Definition
Some of the voltage-gated Na+ channels start to close (this is based on time, the ones that opened 1st will close 1st)
More voltage K+ channels are opening and gK (K+ conductance) is increasing. This will cause repolarization. |
|
|
Term
| What occurs during the repolarization phase? |
|
Definition
| During this phase there are now more voltage-gated K+ channels open than voltage-gated Na+ channels. This fact is what causes the repolarization. |
|
|
Term
| What happens in the afterhyperpolarization phase? |
|
Definition
| After the cell membrane potential is hyperpolarized (more negative than the normal RMP) voltage-gated K+ channels close and the cell goes back to its normal RMP. |
|
|
Term
| What are the underlying ionic mechanisms of the different stages of the action potential. |
|
Definition
Increase in gNa (Na+ conductance) causes depolarization
decrease in gNa and increase in gK causes repolarization
decrease in gK causes the cell to go back to its RMP after hyperpolarization |
|
|
Term
| What does tetraethylammonium (TEA) do? |
|
Definition
It prevents voltage-gated K+ channels from opening.
This causes the repolarization phase of the AP to be longer because normally voltage-gated K+ channels opening are what causes repolarization.
In this case inactivation of voltage-gated Na+ channels and leaky K+ channels are what causes repolarization. |
|
|
Term
| What do saxitoxin and tetrodoxin (TTx) do? |
|
Definition
They block the voltage-gated Na+ channels from opening but they do not affect the leaky channels.
Since the voltage-gated Na+ channels are the driving force behind depolarization, it does not occur.
It is possible hyperpolarization can occur because the voltage-gated K+ channels can still open. |
|
|
Term
| How does the voltage-gated Na+ channel work/change during the action potential? |
|
Definition
At resting state the activation gate is closed and the inactivation gate is open.
After threshold, the activation gate opens and Na+ can flow into the cell (the inactivation gate is still open)
During repolarization the inactivation gate closes and prevents more Na+ from entering. |
|
|
Term
| How does the voltage-gated K+ channel change/work during an action potential? |
|
Definition
Unlike the Na+ channel, the K+ channel only has an activation gate.
At the resting state the activation gate is closed and no K+ can flow out.
The activation gate opens once the cell starts to be depolarized. This activation is slower than the Na+ channels and this is why the cell continues to depolarize while K+ channels are starting to open.
The activation gate will close once the membrane potential becomes hyperpolarized below its threshold. |
|
|
Term
| Explain the voltage-gated channels at each of different phases. |
|
Definition
At resting state the channels are closed.
During the rising phase, Na+ channels are opening faster than K+ channels (remember slow activation)
During the overshoot phase up to the peak, Na+ channels are at max capacity (right at the peak) and more K+ channels are opening.
During repolarization, more K+ channels are opening and the inactivation gate of the Na+ channels start to close. (this occurs right after the peak)
During hyperpolarization, the Na+ channels are closed and the K+ channels are starting to close so that cell can return to RMP. |
|
|
Term
| What is the absolute refractory period? |
|
Definition
| It overlaps with almost the entire duration of the action potential. During this time another action potential cannot be generated no matter how great the stimulus is. |
|
|
Term
| What is the relative refractory period? |
|
Definition
This period is after the absolute refractory period and overlaps primarily with the hyperpolariztion period.
An action potential can be generated but only if a greater than normal depolarizing current is applied (to get to the threshold) |
|
|
Term
| During what phase is the membrane potential Vm the closest to E-Na? |
|
Definition
| At the peak because this is where the action potential reaches its most positive value. remember the peak is where the max. amount of voltage-gated Na+ channels will be open. |
|
|
