Module 3: Lecture 3

Question 1

what causes a graded potential?

Answer 1

• some triggering event allows the response to change the cells membrane permeability

Question 2

what is the action potential?

Answer 2

everything that occurs following the threshold potential being reached
- the depolarization and the repolarization phases back down to rest
- occurs very rapidly

Question 3

because the action potential has so much amplitude, what does it reach during depolarization?

Answer 3

0mV and even positive mV membrane potential
- note: membrane potential at rest is -70mV

Question 4

how fast is repolarization after a depolarization of action potential?

Answer 4

just as rapid as the depolarization

Question 5

what is an overshoot?

Answer 5

the reversion period (‘reversing’ the potential above 0 mV)

Question 6

how long is an action potential?

Answer 6

always the same duration
- 1 msec in a nerve cell

Question 7

what is another name for an action potential?

Answer 7

a spike

Question 8

when an excitable membrane is triggered to undergo an action potential, we say it?

Answer 8

‘fires’

Question 9

what is the key to an action potential?

Answer 9

the change in membrane permeability to ions

Question 10

which gated ion channel uses change of voltage/membrane potential as a mechanism of opening and closing triggering event?

Answer 10

voltage-gated channels

Question 11

what is the triggering event of a mechanically gated ion channel?

Answer 11

• open in response to mechanical deformation

Question 12

what is the triggering event of a chemically gated ion channel?

Answer 12

• open in response to some sort of molecule binding to the ion channel specifically neurotransmitters

Question 13

which gated ion channel opens to initiate action potential?

Answer 13

voltage gated ion channels

Question 14

what are the two voltage gated ion channels?

Answer 14

1. voltage gated K+ (potassium) channels
2. voltage gated Na+ (sodium) channels

Question 15

how does a voltage gated K+ (potassium) channel operate?

Answer 15

simple with one one activation gate that opens once the threshold potential has reached - senses voltage difference and it opens

Question 16

what are the two differences between Na+ and K+ voltage-gated channels?

Answer 16

1. Na+ channels are much faster to respond to change in the membrane voltage / potential
2. Na+ channels have in their cytosolic region an activation and an inactivation gates

Question 17

what happens when threshold potential is reached at the channel level?

Answer 17

• potassium and sodium gates open
• sodium inactivation gate begins to close
• all occurs at the same event once the threshold is reached
• only thing that differs is how quickly these events occur

Question 18

how fast do the gates of the sodium voltage gated channels open?

Answer 18

activation gate - very rapidly
inactivation gate - quite slow

Question 19

how fast is the potassium’s gate to open?

Answer 19

quite slow

Question 20

what has huge implications for the shape and physiology of the action potential?

Answer 20

the rate at which the voltage gated ion channels are opening

Question 21

what is the state of the voltage gated channels at rest (-70mV)?

Answer 21

• all voltage-gated channels are CLOSED
• Na+ channels are in the configuration with the inactivation gate open but the activation gate closed

Question 22

the electrical and concentration gradients favour sodium movement into or out of the cell?

Answer 22

into the cell

Question 23

what leads to greater depolarization of the membrane?

Answer 23

inward movement of Na+ ions

Question 24

what leads to the positive feedback cycle?

Answer 24

opening of more voltage-gated Na+ channels because of the sodium ions being entered from the already opened voltage gated sodium ion channels

Question 25

when we have the initial depolarization, are all the voltage gated sodium channels opening?

Answer 25

no, only some

Question 26

what does the positive feedback cycle esnure?

Answer 26

that once the threshold potential is reached, all the voltage gated sodium channels will open - this is what is responsible for the explosive increase in Na+ permeability

Question 27

there is so many voltage gated sodium ion channels that membrane permeability is how much more permeable to sodium than potassium when they are open?

Answer 27

membrane is 600 times more permeable to Na+ than to K+ when their is an active potential - this is why there is the explosive depolarization event

Question 28

why does depolarization stop very abruptly at +30mV in action potential?

Answer 28

because of the sodium voltage channel inactivation gate

Question 29

what allows the very rapid depolarization event to occur in action potential?

Answer 29

the rate at which the sodium channels open and then inactive

Question 30

how long is the ball and chain inactivation gate of a sodium channel in a closed and incapable of opening again state for?

Answer 30

until the membrane potential is back to its resting value

Question 31

what resets your sodium ion channels?

Answer 31

bringing the membrane potential to rest

Question 32

what happens if you want to generate multiple action potentials?

Answer 32

you need to first depolarize and repolarize the cell to ensure the sodium voltage gated ion channels are capable of opening again

Question 33

what does the potassium gated ions respond to?

Answer 33

threshold potential being reached - starts opening at the exact same time as your voltage gated sodium ion channels just occurs more slowly

Question 34

what repolarizes after a depolarization of action potential?

Answer 34

the potassium voltage gated ion channels - this makes the inside of the cell more negative because the potassium ions are leaving

Question 35

summary of what happens/gets activated when threshold potential is reached

Answer 35

1. rapid opening of voltage-gated Na+ channels --> Na+ influx until inactivation gate closes 2. slow closing of the Na+ channel inactivation gates --> Peak; prevents the action potential from increasing further and exceeding +30mV 3. slow opening of voltage-gated K+ channels (reverse polarity, returns membrane potential towards resting)

Question 36

when do the potassium voltage gates begin to fully open?

Answer 36

at peak during action potential - permeability increased to about 300x because the outward K+ movement is down its concentration AND electrical gradient at peak

Question 37

what is responsible for the rapid repolarization of the action potential?

Answer 37

the electrical and chemical gradient pushing potassium out of the cell using all the K+ gated ion channels

Question 38

how come there is a hyperpolarization after repolarization?

Answer 38

because the potassium ion gated channels take awhile to close again

Question 39

when does an action potential only occur?

Answer 39

ONLY when the triggering stimulus AND the current that is generated via the opening of Na+ channels (graded potentials) are sufficient to reach the threshold potential

Question 40

what is threshold stimuli?

Answer 40

if the strength of your graded potentials are sufficient to reach threshold

Question 41

what is the threshold of most excitable membranes be?

Answer 41

15 mV less negative than the resting potential - ex. resting of a neural cell is -70mV so its threshold is -55mV

Question 42

what happens if the initial depolarization is lower than the threshold?

Answer 42

- action potential will not be activated - ONLY graded potential - the positive feedback cycle does not start

Question 43

what is weak depolarization that does not activate an action potential called?

Answer 43

sub-threshold potential

Question 44

what is the stimulus that does not active an action potential called?

Answer 44

sub-threshold stimulus

Question 45

what happens if the initial depolarization reaches the threshold?

Answer 45

- an action potential is generated - all action potentials have similar amplitude regardless of how long it took to reach threshold (same magnitude and duration every time) - once the threshold is reached --> All-or-None law

Question 46

what is used for communication?

Answer 46

action potentials

Question 47

if all action potentials have the same magnitude, how do we integrate the magnitude of the stimulus? (how do we feel the different levels of touch/pain)

Answer 47

the frequency of action potentials - the rate at which they are firing