The Action Potential Flashcards
Are passive properties of axons sufficient to conduct an AP?
No. Require V-G Na channel
3 key features of an AP
- There is a threshold for excitation
- All or none regardless of stimulus
- No attenuation
What determines threshold voltage?
The point @ which Na and K currents are equal and opposite
Define “threshold”
Threshold is the amount of depolarization required for a membrane to generate AP
Are all Na channels conducting @ threshold?
No, it takes tens of microseconds to open all Na channels
How/Why does repolarization occur?
- Permeability of Na declines back to resting level
2. Permeability of K undergoes a transient increase
When an axon transmits electrical signals, is attentuation observed?
No.
There is no gradual loss in signal strength/intensity seen when axons transmits signals.
What are the typical values for ENa and Ek? Vm?
ENa = 40mV EK = -90 mV
Vm = -70mV
At threshold, does an action potential occur?
No, the membrane potential must be "over" threshold, even if it is only slightly
At threshold, if K current flows outward slightly faster than Na current flowing inward, what happens to Vm?
Vm will be pulled back towards resting potential.
Na channels will close,
No AP will occu
Describe positive feedback of the rising phase of AP
- At threshold, not all Na channels are conducting yet.
- Na entering channels that are conducting will depolarize membrane further
- That depolarization will cause more Na channels more Na channels to open
- W/in a few microseconds, all Na channels are open
- –> Vm closer to ENa
At threshold, if Na current flows outward slightly faster than K current flowing inward, what happens to Vm?
More depolarization will occur
More Na channels open —> explosive diarrhea of positive charge influx = AP
The Na channel consists of what 2 gates?
- The activation or m gate
2. the inactivation or h gate
At rest the ___ gate is shut, while the ____ gate is open.
- activation gate (m)
- inactivation gate (h)
What would happen if both m and h gates are always open at same time? How is this avoided?
the channel could never conduct
the activation gate swings faster than the inactivation gate (h gate has delayed response)
- when axon is first depolarized, there is brief instant when both gates are open and sodium can run into the cell
When the membrane is depolarized, the ___ gate is open, while the ____ gate is closed
- activation gate (m)
2. inactivation gate (h)
At which point during the AP is the h gate of Na channel closing?
at the peak of the AP, (dont forget it is delayed)
- —> PNa promptly declines
- —> promotes repolarization
K channels open during an AP, in response to what?
in response to depolarization (but with delay)
K channels open as Na channel inactivation gates are closing.
- —> K+ promptly rushes out of cell
- —> Vm plummets (repolarized) towards resting level
What is the delay in K+ channel opening meant for?
- allow for maximum height of AP
2. rapid and faster repolarization
Why does Vm undershoot the resting potential after repolarization?
- result of increased Pk.
- closing of K channels are delayed
- (Vm wants to move closer to Ek than resting membrane pot)
is Na or K channel an example of negative feedback? How?
K channel behavior -> ex of negative feedback.
- —> depolarization caused them to open
- —> which causes repolarization
- —> which causes them to shut
Absolute Refractory period
period after an AP, in which an axon cannot generate another one for a few (~3) msecs
relative refractory period
follows the refractory period, in which an stronger than normal stimulus is required to generate another AP
What does positive feedback refer to with regards to initiation of an AP?
More Na+ channels open –> More depolarization –> more Na+ channels open –> more depol… etc
Leads to all or none AP
Where does AP generation start (physically on neuron)?
Axon hillock
Do Ek and ENa change during an AP? Why or why not?
They do not change.
This is because it requires a very small number of ions to make the change, so concentrations remain roughly the same. From Nernst, same [ ] indicates that E won’t change
Na/K pump doing something different during repolarization?
No. Constitutively active.
absolute refractory period
AP initiation impossible
Functional consequence of refractory period?
Limits maximum possible firing rate of cell
In hyperkalemia, are more APs generated? Why or why not?
No. It is more difficult to generate an AP because Na+ channels are inactivated. This is due to constant depolarization.
Summarize AP stages
- Small depol to threshold
- Large depol: V-G Na channels open
- Repolarization: Na channels inactivate; K channels open
- K channels close; concentrations return to baseline
conduction velocity depends what two things?
- myelination (= faster conduction velocity)
- axon width/diameter (wider = larger current = faster AP propagation faster conduction velocity)
- is directly proportional to conduction velocity
What 4 things are AP affected by?
- Ionic concentrations
- Membrane potential
- Relative membrane permeability
- Specialized voltage-sensitive channels
What is occurring at the ‘active locus’ during an action potential?
Na+ ions are rushing into the axon fiber, making the region positive with respect to the rest of the fiber.
Current will flow away from the active locus and create more active loci.
What do local anesthetics like novocaine do?
Blocks sodium channels, keeping them from opening, the AP could die.
Myelin increases/decreases conduction speed?
Give two reasons why.
Increases
- Myelin serves as an electrical insulator, reducing the ‘leaky’ cable properties of nerve fibers.
- Myelin also changes fiber diameter.
What happens when a “node of ranvier’ is depolarized to threshold?
Since the node has no myelin, Na channels in the nodal membrane open and the node becomes an active locus. The current then is able to spread to the next node.
