Exam 1 Week 2: ppt 6 Action Potentials Flashcards
6 events (with 4 Phases) of AP
Dr. Lake might just referr to all these as phases (that’s how the powerpoint looked), but in the book only the middle four were phases
- –Threshold reached
- –Rising phase (depolarizing)
- –Overshoot phase (membrane potential reverses)
- –Falling phase (repolarizing) - slide said “hyperpolarizing” but this doesn’t make sense and the book said it was repolarizing during this phase.
- –Hyperpolarization phase (membrane Undershoot)
- –Membrane Repolarization to resting membrane potential
What are Voltage gated Na+ Channels?
Neural membranes have Voltage Gated Na+ Channels which are Complex ionic channels with Proteins that sense polarity changes across the membrane (voltage sensor). These channels are activated by depolarization (increase + charge) along the inside of the membrane
what happens at Voltage Gated Na+ channels?
In these Voltage Gated Na+ Channels
Depolarization (must be to threshold) causes the channels proteins to open and allow Na+ to flood the cell (Na+ is passively travelling [diffusing] down it’s electrochemical gradient). Activation of voltage-gated pores initiates an action potential.
(I think other voltage gated channels operate similarly, but with other ions, for example K+)
From PPt: “there is Movement of channel proteins under the influence of depolarization which opens in this specific case Na+ channels that are fully open and let Na+ flood into the cell. Activation of voltage-gated pores initiates an action potential”
What is “threshold depolarization” and how does it relate to Voltage Gated Na+ Channels?
“threshold” depolarization is the amount of depolarization required to open these channels
The activation of an _______________ is a sequence of opening of voltage gated Na+ channels & then voltage gated K+ channels, which both occur with the membrane reaching the “threshold” level for both of these types of channels
action potential
What two important voltage-gated channels are opened durnig an action potential?
voltage gated Na+ channel
voltage gated K+ channel
Voltage gated N+ channels:
How much depolarization needs to occur for them to activate (how much depolarization to threshold)?
How long is their activation period?
How much does activation increase Na+ permeability?
- Activated with voltage change of 15-20 mV (in positive direction - aka - becoming less negative, as in going from -70mV to -55mV)
- Activation period as short as .5-.8 msec
- Activation increase Na+ permeability 500-5000 fold
Voltage gated N+ channels:
How much depolarization needs to occur for them to activate (how much depolarization to threshold)?
Activated with voltage change of 15-20 mV (in positive direction - aka - becoming less negative, as in going from -70mV to -55mV)
Voltage gated N+ channels:
How long is their activation period?
Activation period as short as .5-.8 msec
Voltage gated N+ channels:
How much does activation increase Na+ permeability?
Activation increases Na+ permeability 500-5000 fold
How does the speed of opening of voltage gated K+ channels compare to voltage gated Na+ channels?
They open slowly so peak increase at about the time voltage gated Na+ channels are closing (which causes repolarization of the cell as the positive K+ ions leave)
(the channel is Open for 1.5-2 msec - longer than Na+)
How long are voltage gated K+ channels open compared to voltage gated Na+ channels?
K+ channels are open for 1.5-2 msec
Na+ channels are open as short as .5-.8 msec
what returns Na+ and K+ ions to their resting gradients and concentrations inside and outside of the cell?
The Na+-K+ pump
(sometimes called Na+-K+ ATpase pump)
It also maintains the cell membrane resting potential
what sets (determines) the threshold level of depolarization?
Threshold is set by sensitivity of the voltage gated Na+ channels
The Lower the threshold of these voltage-gated Na+ channels, the easier it is to excite action potential production
Explain “All or None” when it comes to action potentials
The action potential is ”All or None” meaning that if there is Depolarization less than threshold of the Na+ voltage-gated pores will not open and no action potential is produced – NONE. If Depolarization greater than threshold of the Na+ voltage-gated channels, the channels will open and the same amount of Na+ will rush in before the channels again close producing an action potential of a consistent amplitude – ALL
Details about Rising Phase and Overshoot in an action potential
–Rising Phase and Overshoot
- §Produced by the rapid influx of Na+ into the cell through voltage-gated Na+ pores
- §Produces a rapid depolarization
- §Membrane potential moves toward the Na+ equilibrium potential
- Inside of cell becomes more positive than the outside
When sufficient Na+ permeability occurs as a result of the opening of the voltage-gated Na+ channels, the Rising Phase of the action potential completely depolarizes the membrane and produces a positive Overshoot of 20-40 mV as the membrane potential is heading for the very positive Na+ equilibrium potential. It does not get all the way to Na+ equilibrium because the voltage-gated Na+ channels close and the membrane starts to repolarize as increasing amount of K+ begins to leave the neuron.
Details about the Faling Phase in an action potential
- Rapid deactivation of the voltage-gated Na+ channels – closed tight shut
- Slow activation of the voltage-gated K+ channels
- Stops the influx of Na+ and increases the efflux of K+ producing repolarization
The Falling Phase begins because of the Rapid deactivation of the voltage-gated Na+ channels combined with the fact that the slowly activated voltage-gated K+ channels are now fully open so K+ is able to rush out of the cell at full speed. These two actions stops the influx of Na+ and increases the efflux of K+ at the same time. This produces repolarization.
Details about the Undershoot (after - hyperpolarization) in an action potential
- Produced by combination of continued closure of voltage-gated Na+ channels & slow deactivation of K+ channels
- Continued efflux of K+ with no balancing Na+ influx produces movement of the membrane potential toward the K+ equilibrium potential
- The undershoot phase ends when the membrane potential returns to its resting state
The falling phase (repolarization) with the combination of continued closure of Na+ channels & slow deactivation of K+ channels eventually produces an after hyperpolarization phase where the membrane potential Undershoots the resting membrane potential to become more negative than resting as the membrane is headed for the quite negative K+ equilibrium potential. Note at this poing the voltage-gated Na+ channels can be opened but they are still closed because the membrane potential is below that of threshold. This phase corresponds to the relative refractory period, where Na+ channels could be opened to generate another AP, but it would require a stronger depolarizing stimulus to overcome the hyperoplarization of the undershoot phase and reach threshold.
What is a refractory period?
A period of time where there is decreased excitability of the neuron
can be:
- –Absolute – cannot re-excite AP at all
- –Relative – greater depolarization force is required to re-excite AP
When a section of membrane has initiated and action potential it is Refractory to the development of another action potential at that exact location, due to Decreased excitability of the neuron at that position along the membrane. There are two forms of refractoriness:
Absolute – cannot re-excite AP
Relative – need > depolarization to re-excite AP
Two types of refractory periods
- –Absolute – cannot re-excite AP at all
- –Relative – greater depolarization force is required to re-excite AP
Absolute Refractory Period
- From opening of voltage-gated Na+ channel through the end of voltage-gated Na+ channel closure
- – when Na+ pores cannot open further or reopen
During this period voltage-gated Na+ pores are either fully open so cannot open any more or closed tighly shut and cannot open.
Relative refractory period
When
- Throughout the after-hyperpolarization period
- (when it would take a stronger depolarizing stimulus to reach threshold because the membrane is in a hyperpolarized state (it is more polarized than normal resting membrane polaization)
- Ends when membrane potential returns to resting level
Relative refractory period is throughout the after hyperpolarization period. Remember the voltage-gated Na+ pores can now reopen but the membrane potential is below resting, so it takes relatively more depolarization for the membrane to reach threshold than it did from resting membrane potential. So it isharder to reach threshold and the membrane is relatively refractory. Of course this ends when the membrane potential returns to its resting level
