The Action Potential

Question 1

Are passive properties of axons sufficient to conduct an AP?

Answer 1

No. Require V-G Na channel

Question 2

3 key features of an AP

Answer 2

1. There is a threshold for excitation
2. All or none regardless of stimulus
3. No attenuation

Question 3

What determines threshold voltage?

Answer 3

The point @ which Na and K currents are equal and opposite

Question 4

Define “threshold”

Answer 4

Threshold is the amount of depolarization required for a membrane to generate AP

Question 5

Are all Na channels conducting @ threshold?

Answer 5

No, it takes tens of microseconds to open all Na channels

Question 6

How/Why does repolarization occur?

Answer 6

1. Permeability of Na declines back to resting level

2. Permeability of K undergoes a transient increase

Question 7

When an axon transmits electrical signals, is attentuation observed?

Answer 7

No.

There is no gradual loss in signal strength/intensity seen when axons transmits signals.

Question 8

What are the typical values for ENa and Ek? Vm?

Answer 8

ENa = 40mV
EK = -90 mV

Vm = -70mV

Question 9

At threshold, does an action potential occur?

Answer 9

No, the membrane potential must be "over" threshold, even if it is only slightly

Question 10

At threshold, if K current flows outward slightly faster than Na current flowing inward, what happens to Vm?

Answer 10

Vm will be pulled back towards resting potential.

Na channels will close,

No AP will occu

Question 11

Describe positive feedback of the rising phase of AP

Answer 11

1. At threshold, not all Na channels are conducting yet.
2. Na entering channels that are conducting will depolarize membrane further
3. That depolarization will cause more Na channels more Na channels to open
4. W/in a few microseconds, all Na channels are open
   
   • –> Vm closer to ENa

Question 12

At threshold, if Na current flows outward slightly faster than K current flowing inward, what happens to Vm?

Answer 12

More depolarization will occur

More Na channels open —> explosive diarrhea of positive charge influx = AP

Question 13

The Na channel consists of what 2 gates?

Answer 13

1. The activation or m gate

2. the inactivation or h gate

Question 14

At rest the ___ gate is shut, while the ____ gate is open.

Answer 14

1. activation gate (m)
2. inactivation gate (h)

Question 15

What would happen if both m and h gates are always open at same time? How is this avoided?

Answer 15

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

Question 16

When the membrane is depolarized, the ___ gate is open, while the ____ gate is closed

Answer 16

1. activation gate (m)

2. inactivation gate (h)

Question 17

At which point during the AP is the h gate of Na channel closing?

Answer 17

at the peak of the AP, (dont forget it is delayed)

• —> PNa promptly declines
• —> promotes repolarization

Question 18

K channels open during an AP, in response to what?

Answer 18

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

Question 19

What is the delay in K+ channel opening meant for?

Answer 19

1. allow for maximum height of AP

2. rapid and faster repolarization

Question 20

Why does Vm undershoot the resting potential after repolarization?

Answer 20

1. result of increased Pk.
2. closing of K channels are delayed
   
   • (Vm wants to move closer to Ek than resting membrane pot)

Question 21

is Na or K channel an example of negative feedback? How?

Answer 21

K channel behavior -> ex of negative feedback.

• —> depolarization caused them to open
• —> which causes repolarization
• —> which causes them to shut

Question 22

Absolute Refractory period

Answer 22

period after an AP, in which an axon cannot generate another one for a few (~3) msecs

Question 23

relative refractory period

Answer 23

follows the refractory period, in which an stronger than normal stimulus is required to generate another AP

Question 24

What does positive feedback refer to with regards to initiation of an AP?

Answer 24

More Na+ channels open –> More depolarization –> more Na+ channels open –> more depol… etc

Leads to all or none AP

Question 25

Where does AP generation start (physically on neuron)?

Answer 25

Axon hillock

Question 26

Do Ek and ENa change during an AP? Why or why not?

Answer 26

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

Question 27

Na/K pump doing something different during repolarization?

Answer 27

No. Constitutively active.

Question 28

absolute refractory period

Answer 28

AP initiation impossible

Question 29

Functional consequence of refractory period?

Answer 29

Limits maximum possible firing rate of cell

Question 30

In hyperkalemia, are more APs generated? Why or why not?

Answer 30

No. It is more difficult to generate an AP because Na+ channels are inactivated. This is due to constant depolarization.

Question 31

Summarize AP stages

Answer 31

1. Small depol to threshold 2. Large depol: V-G Na channels open 3. Repolarization: Na channels inactivate; K channels open 4. K channels close; concentrations return to baseline

Question 32

conduction velocity depends what two things?

Answer 32

1. myelination (= faster conduction velocity) 2. axon width/diameter (wider = larger current = faster AP propagation faster conduction velocity) - is directly proportional to conduction velocity

Question 33

What 4 things are AP affected by?

Answer 33

1. Ionic concentrations 2. Membrane potential 3. Relative membrane permeability 4. Specialized voltage-sensitive channels

Question 34

What is occurring at the 'active locus' during an action potential?

Answer 34

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.

Question 35

What do local anesthetics like novocaine do?

Answer 35

Blocks sodium channels, keeping them from opening, the AP could die.

Question 36

Myelin increases/decreases conduction speed? | Give two reasons why.

Answer 36

Increases 1. Myelin serves as an electrical insulator, reducing the 'leaky' cable properties of nerve fibers. 2. Myelin also changes fiber diameter.

Question 37

What happens when a "node of ranvier' is depolarized to threshold?

Answer 37

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.