Block 1 - Membrane Potentials (L3-5)

Question 1

Cells are always low in —– and high in —–

Answer 1

low in Na+, high in K+

Question 2

There are way more —- channels than any other channel

Answer 2

K+

Question 3

Why does K+ create a membrane potential?

Answer 3

K+ is charged, so diffusion across the membrane establishes an electrical voltage and a membrane potential

Question 4

What forces affect the movement of K+ ions?

Answer 4

chemical force of the concentration gradient pushes K+ outward (since cellular concentration is high, ions want to move toward lower concentration)

electrical force of the membrane potential pulls K+ inward (Na+/K+ pumps create a negative charge inside the cell, pulls K+ in)

Question 5

In most cells, the —– force is balanced, which results in an overall ——-

Answer 5

the electrical force is balanced, resulting in an inside-negative membrane potential as K+ is pulled outward by the concentration gradient

Question 6

What is a typical cell membrane potential/resting potential?

Answer 6

-50 to -100 mV

Question 7

How do cells change their membrane potential?

Answer 7

they open or close ion channels

Question 8

How is a cell generally polarized or depolarized?

Answer 8

if K+ channels are open, membrane voltage is negative and the cell is polarized

if Na+ channels are open, the membrane voltage is positive and the cell is depolarized

Question 9

What are Wchem and Welec?

Answer 9

Wchem is the energy associated with moving N moles of an ion from concentration X1 to concentration X2

Welec is the energy associated with moving N moles of an ion in an electrical field of V volts

Question 10

What does the Nerst equation represent?

Answer 10

the equilibrium potential of an ion, or the membrane potential when an ion is at equilibrium
(shows that the voltage is a function of concentrations on one side vs. the other side)

Question 11

What does the Goldman equation reflect?

Answer 11

the real situation, where multiple ions contribute to the membrane potential and ion permeabilities determine the relative influence of each of those ions

Question 12

What are the two important parameters of the Goldman equation?

Answer 12

ion permeabilities and ion gradients

Question 13

If a membrane was only permeable to K+, the Goldman equation would also be —–

Answer 13

the Nernst equation

Question 14

What ion always has the highest permeability?

Answer 14

K+

Question 15

Why is Vm normally negative even though V Na+ is so positive?

Answer 15

becuase both Vcl and Vk are highly negative, and have greater influence on the Vm since there are very few Na+ channels and so many K+ channels

Question 16

What are the two critical factors of regulating ion permeability?

Answer 16

• establishing and maintaining Na+ and K+ gradients
• varying the activity of specific ion channels

Question 17

Describe the Cl- concentration in cells

Answer 17

normally close to equilibrium, resting potential tends to push Cl- out of the cell but it is balanced by the concentration gradient pushing Cl- into the cell

Question 18

How are voltage gated channels opened?

Answer 18

depolarization (when the membrane gets less negative inside)

Question 19

How are chemical/ligand gated channels opened?

Answer 19

when a signal molecule binds to the channel protein

Question 20

How are mechanically gated channels opened?

Answer 20

when the membrane gets stretched

Question 21

How do the strength of a stimulus and the size of the voltage response correlate?

Answer 21

stronger stimulus leads to a larger voltage response

Question 22

When a second stimulus closely follows the first stimulus, what happens?

Answer 22

the magnitude of voltage response may be greater (the responses are additive)

Question 23

Where do graded potentials occur?

Answer 23

dendrites and cell body of neurons

Question 24

What are the important characteristics of graded potentials?

Answer 24

• size varies with strength of stimulus
• multiple stimuli are additive
• opposing stimuli cancel each other out

Question 25

What happens when a membrane is depolarized to a threshold?

Answer 25

voltage gated Na+ channels open, Na+ enters the cell, more depolarization, more Na+ channels open, etc.
(positive feedback cycle)

right after Na+ channels open they snap closed, but the depolarization also slowly opens K+ channels

with these effects combined, the membrane repolarizes to the resting value

Question 26

What are the important characteristics of action potentials?

Answer 26

• all or none
• a second potential can not start until the refractory period is over
• not additive

Question 27

What are the absolute and relative refractory periods?

Answer 27

absolute: first refractory period, cannot be restimulated because sodium channels need to be reset

relative: second refractory period, takes a larger stimulation than normal because potassium channels need to be reset

Question 28

What is the significance of refractory periods?

Answer 28

• influence the max rate of action potential firing
• influence the characteristic of action potential propagation

Question 29

Do graded potentials spread?

Answer 29

Yes, but the magnitude diminishes with distance

Question 30

Because they propagate, action potentials can…

Answer 30

communicate signals over long distances

Question 31

What begins the propagation of an action potential?

Describe how it spreads.

Answer 31

entry of Na+ produces a local current that spreads and depolarizes nearby areas

if they depolarize to threshold, another action potential is triggered

as the voltage spreads it becomes weaker, and when it gets too weak it does not cause more channels to open (so those channels are not recruited by the depolarization)
but, the channels that were recruited will produce another signal that then further propagates and recruits more channels

Question 32

What 2 factors influence velocity of the action potential propagation and how?

Answer 32

1. size (diameter) of the axon = larger have a higher velocity because they can send signals faster, electrical resistance decreases as axon diameter increases
2. myelination speeds the conduction velocity

Question 33

How is an axon myelinated? Describe the structures.

Answer 33

• Schwann cell (in PNS) and Oligodendrocytes (in CNS) wrap around the axon
• the neurolemma of the cell develops into the myelin sheath
• the ends of the axon that poke out of the cell or in between multiple cells are called Nodes of Ranvier

Question 34

Describe continuous vs saltatory conduction?

Answer 34

Continuous:
1. Na channels open
2. electronic conduction flows down and opens more Na channels the entire way
3. signal moves more slowly

Saltatory:
1. Na channels at a node open
2. electronic conduction through the myelinated part of the axon
3. Na channels at the next node open
4. etc, electronic signal jumps from node to node and is faster

Question 35

What are some reasons that demyelination can occur? What do they do exactly?

Answer 35

multiple sclerosis (central) or Guillan-Barre (peripheral)

disrupt the myelin sheath

since there are only Na channels at the nodal regions but the myelin is now gone, the conduction gets blocked at the nodes and cant make it to depolarize the next node (so nerve conduction fails)