Action potentials and Graded Potentials

Question 1

How can a cell be hyperpolarized using a probe?

Answer 1

By injecting negative charge into the cell

Question 2

T or F: one the threshold has been passes a graded potential can occur?

Answer 2

False - IF there are channels in that area, a full action potential will occur

Question 3

T or F: a cell that has a resting potential of +40 mV is hyperpolarized at +35 mV

Answer 3

True - hyperpolarization is just relative to resting potential

Question 4

What is typically the threshold potential?

Answer 4

10 - 15 mV more positive than resting Vm

Question 5

T or F: different cells have different action potential durations

Answer 5

True - for average nerve its about 1 second

Question 6

What happens when threshold potential is reached?

Answer 6

• only a slight depolarization needed (~-55mV)
• almost all Na+ open
• there is a chemical and electric pot. pulling Na+ into cell

Question 7

Why does the maximum membrane potential not reach +60 mV, which would be the potential that Na+ would want to be at using the nernst eqn?

Answer 7

Because the permeability of Na+ is not 100%

Question 8

What terminates the positive feedback loop where Na+ influx is greater than efflux?

Answer 8

The Na+ channels are inactivated

Question 9

What is the state of the activation gate at resting Vm?

Answer 9

closed and channel is non-conductive

Question 10

What happens in the rising phase of the action potential?

Answer 10

• Stimulus depolarization causes some Na+ to open causes Na+ influx
• Na+ influx causes more channels to open

Question 11

What happens in the falling phase of the action potential?

Answer 11

• Na+ channels inactivate

- there is now K+ efflux (K+ channels open and it leaves)

Question 12

What drives K+ out during the falling phase of the action potential?

Answer 12

The electrochemical gradient

Question 13

What happens after hyperpolarization in the action potential?

Answer 13

• K+ permeability is greater at the end of the action potential than at the start
• K+ channels slowly close stopping K+ flow

Question 14

What does TEA do to the action potential?

Answer 14

Blocks K+ channels so the cell is slow to depolarize and hyperpolarization can’t happen

Question 15

What does TTX do to the action potential?

Answer 15

Blocks Na+ channels and you get absolutely no action potential

Found in japanese puffer fish

Question 16

Can a second action potential occur in the rising or falling phase of a preceding action potential?

Answer 16

NO

Question 17

What can a second action potential occur?

Answer 17

After inactivation of the Na+ channels - relative refractory period

• to illicit a response the stimulus must be larger than usual

Question 18

Why is the relative refractory period important in the heart?

Answer 18

It allow a pause so that blood can be pumped in

Question 19

What causes rickets and its side effects of cramps and seizures?

Answer 19

• hypocalecemia means not enough Ca2+ is available to bind the extracellular site on the Na+ channel
• this causes improper folding of the gates which causes them to open haphazardly
• this leads to cramps and siezures

Question 20

How do cocaine and novocaine work?

Answer 20

drug binds to Na+ channel
Inhibit opening of channel
Reduced number of action potentials
lack of pain perception

Question 21

What determines the peak of the action potential?

Answer 21

• Sodium permeability (approaches 100%)

- Note: potassium determines resting potential

Question 22

What is the difference in fast rises in K+ (like KCl injection) and slow rises in K+ like Kout?

Answer 22

Fast rises generate crazy action potentials

Slow rises give you no action potential (see paper notes for why)

Question 23

How do action potentials propagate?

Answer 23

1. Electrode causes AP in nerve
2. AP reaches +40 mV
3. Adjacent area attracts + charge
4. Adjacent region becomes slightly more +
5. Voltage dependent Na+ gates open

Question 24

What is the only way to increase the maximum voltage of an action potential?

Answer 24

Increase the Na+ gradient

Question 25

What is a graded depolarization?

Answer 25

One that never reaches the threshold potential and fizzles out due to electronic spread

Question 26

What is the space constant lambda?

Answer 26

distance at which graded potential has decreased by 63%

Question 27

T or F: space constant is useful for comparing diseased nerves to healthy nerves.

Answer 27

True

Question 28

What affect does myelin have on the space constant?

Answer 28

Increases it by increasing membrane resistance

—>this can also be done by increasing n. diameter

Question 29

What is the equation for the space constant?

Answer 29

lambda = (dRm / 4Ri)^1/2

Question 30

What is the time constant?

Answer 30

Time needed to change Vm by 63% of final value

Question 31

What can a long time constant indicate?

Answer 31

Disease states

Question 32

What does the time constant depend on?

Answer 32

tau = Rm Cm

• membrane resistance
• capacitive abilites (to store charge)

Question 33

T or F: you want a high time constant when running from a cheeta?

Answer 33

False - high time const. means it take a lot of time to move the potential

Question 34

Why do MS patients and 1 year old babies show slow clumsy uncoordinated movement?

Answer 34

lack of myelin