L7-Graded and Action Potentials

Question 1

What factor establishes an Ek of -90mV?

Answer 1

a relatively large net diffusion of K+ outward

Question 2

How to calculate equilibrium potential?

Answer 2

Nerst Eq’n:

Eion = [61/z] log ([ion]out / [ion]in)

Question 3

The ____ the difference between [ion]out and [ion]in, the higher the voltage

Answer 3

The GREATER the difference between [ion]out and [ion]in, the higher the voltage

Question 4

Voltage-gated channels open/close in response to:

Answer 4

changes in membrane potential

Question 5

Chemically-gated channels (ligand-gated) change in conformation allosterically in response to:

Answer 5

the binding of a specific chemical messenger with a membrane receptor that is in close association with the channel

Question 6

Mechanically-gated channels respond to:

Answer 6

stretching or other mechanical deformation such as touch.

Question 7

How is current lost during a graded potential?

Answer 7

current is lost across the cell membrane as ions leak through uninsulated parts of the membrane

Question 8

Do conductors have high or low resistance?

Answer 8

low. Therefore, higher current flow

Question 9

Do insulators have high or low resistance?

Answer 9

high. Therefore greatly hinder movement of charge

Question 10

Are lipids conductors or insulators?

Answer 10

Insulators. Therefore, current cannot flow through lipids but can move across the membrane ONLY THROUGH CHANNELS.

Question 11

Define action potential

Answer 11

a brief, rapid, large (100mV) change in membrane potential.
Membrane potential reverses so that the inside of the excitable cell is more positive than the outside.
Nondecremental.

Question 12

List the basic steps of an AP (no need to describe channels yet, just describe what happens to the membrane potential)

Answer 12

1) From a resting potential of -70mV, depolarization proceeds slowly until threshold potential is reached (between -55 and -50mV).
2) Once threshold potential is reached, an explosive depolarization occurs and peaks around +30 to +40mV.
3) Just as rapidly, membrane repolarizes (hyperpolarization occurs when it drops below resting potential briefly)

Question 13

Which ion contributes most to establishing resting potential and why?

Answer 13

K+, because the membrane at rest is much more permeable to K+ than Na+

Question 14

How many gates does the Na+ voltage-gated channel have? What are they called?

Answer 14

Two gates.

ACTIVATION and INACTIVATION gates

Question 15

Which gate faces the ICF in the Na+ voltage-gated channel?

Answer 15

The inactivation gate

Question 16

When is the Na+ channel:

1) closed but capable of opening,
2) open or activated,
3) closed and not capable of opening?

Answer 16

1) closed but capable of opening:
- activation gate CLOSED, inactivation gate OPEN.

2) open or activated:
- both gates OPEN.

3) closed and not capable of opening:
- activation gate OPEN, inactivation gate CLOSED.

Question 17

How many gates does the K+ channel have?

Answer 17

One: activation gate

Question 18

In resting condition, which gate is open/closed in the Na+ channel?

Answer 18

In resting condition, activation gate is closed. inactivation gate is open.

Question 19

At threshold potential, Na+ is _____ times more permeable at resting potential

Answer 19

600 times

Question 20

Action potential is due to rapid (positive/negative) feedback. choose one

Answer 20

positive

Question 21

Explain the positive feedback system in Na+ channels at threshold

Answer 21

1. Passive spread of current from adjacent site already depolarized
2. depolarization
3. Opening of some voltage-gated Na+ channels
4. influx of Na+
   * back to step 2. Cycle.*

Question 22

Steps in action potential (including voltage-gated channels)

Answer 22

1. Resting potential: K+ channel closed. Na+ channel closed (inactivation gate open, activation gate closed).
2. At threshold: Na+ activation gate opens and permeability of Na+ rises
3. Na+ enters cell, causing explosive depolarization to 30mV, which generates rising phase of AP.
4. At peak of AP, Na+ inactivation gate closes and PNa+ falls, ending net movement of Na+ into cell. At the same time, K+ activation gate opens and PK+ rises.
5. K+ leaves cell, causes its repolarization to resting potential, which generates falling phase of AP
6. On return to resting potential, Na+ activation gate closes and inactivation gate opens, resetting channel to respond to another depolarizing triggering event .
7. Further movement of K+ through open channel briefly hyperpolarizes membrane (hyperpolarization)
8. K+ activation gate closes, and membrane returns to resting potential.

Question 23

Graded vs Action Potentials:

Coding of magnitude of triggering events

Answer 23

Graded: varies with magnitude of trigger
Action: all-or-none

Question 24

Graded vs Action Potentials:

Duration

Answer 24

Graded: varies with duration of trigger
Action: constant

Question 25

Do graded potentials experience refractory periods?

Answer 25

no