Action Potentials

Question 1

Define the threshold for an action potential.

Answer 1

The point at which sodium and potassium currents are exactly equal and opposite. (Na influx = K efflux)

Question 2

What are the letter abbreviations for the two gates on the NaV channel?

Answer 2

m = activation gate (extracellular side)
h = inactivation gate (cytoplasmic)

Question 3

What is the resting position of the gates of NaV?

Answer 3

m gate = closed

h gate = open

Question 4

What is the resting position of the KV gate?

Answer 4

k gate = closed

Question 5

What is the position of all three gates during depolarization (upward slope of AP)

Answer 5

all gates are open (m, h, k)

Question 6

What is the position of all three gates during repolarization (downward slope of AP?)

Answer 6

m gate = open
h gate = closed
k gate = open

Question 7

What is the position of all three gates during hyper-repolarization?

Answer 7

m gate = closed
h gate = closed
k gate = open

Question 8

To what is the refractory period due? (2)

Answer 8

1. h gate remains closed (inactivation - takes time to reopen)
2. k gate remains open (again, takes time to close)

Question 9

What are the two subdivisions of the refractory period?

Answer 9

Absolute (impossible to have another AP)

Relative (difficult b/c KV remains open, must have stronger impulse to make Na influx = K efflux

Question 10

There exists a second gate in the sodium channel, which we will call the inactivation, or h gate, which is _____ at rest, and closes upon ______.

Answer 10

open; depolarization

Question 11

When the membrane is depolarized, the activation gate ___ and the inactivation gate ____.

Answer 11

opens; closes

Question 12

The activation gate swings ____- than the inactivation gate, so that when the axon is first depolarized there is a brief instant when both gates are open and sodium can then rush into the cell.

Answer 12

faster

Question 13

The K+ channel gates are ____ at rest, and ____ in response to depolarization.

Answer 13

closed; open

Question 14

What is the positive feedback regulation of the NaV channels?

Answer 14

Once threshold is exceeded, a miniature ‘explosion’ occurs. Remember, at threshold, not all sodium channels are conducting yet. But the sodium entering the channels that are conducting will depolarize the membrane further, and that depolarization will cause more sodium channels to start conducting, which will cause more sodium entry, which will cause more channels to open…. This is called ‘positive feedback.’ In a few tens of microseconds, the sodium channels are all open, and Vm is well on its way to ENa..

Question 15

What properties of nerves make APs necessary (3)?

Answer 15

Essentially, nerves are terrible conductors. Impulses can’t be propagated more than ~1mm. There are three reasons:

1. Internal resistance is high (Ri)
2. Membrane resistance is low (Rm)
3. Membrane capacitance is high (Cm)

This means that signal “leaks” through the membrane and is transmitted slowly through the axon.