GRQ #3 - Action Potential

Question 1

what is an action potential?

Answer 1

It is a uniform, rapid depolarization and repolarization of the membrane potential of a cell

Question 2

This change in the membrane potential causes a __

Answer 2

response/action

Question 3

only __ generate action potentials and __ and ___ generate local potentials only

Answer 3

axons; dendrites and cell bodies

Question 4

where are action potentials generated? What is this location called?

Answer 4

action potentials are generated in the initial segment of the axon; it is called the trigger zone

Question 5

voltage-gated channels do what?

Answer 5

they allow ions to move and change the membrane potential of the neuron

Question 6

what two types of voltage gates are involved in the depolarization and repolarization of the action potential

Answer 6

one for sodium ions and one for potassium ions

Question 7

where are voltage-gated channels found most abundantly?

Answer 7

in the axolemma of the neuron, which is why only axons have action potentials

Question 8

the voltage gated potassium ion channel has __ states. What are they?

Answer 8

two states - resting and activated - in resting state the channel is closed and open in the activated state, allowing potassium ions to cross the axolemma

Question 9

a sodium ion channel has __ states. what are they?

Answer 9

3 states; resting state, activated state, inactivated state

Question 10

what happens during resting state for a sodium ion channel?

Answer 10

the inactivation gate is open and the activation gate is closed. the neuron is not being stimulated and the no sodium ions cross the membrane when the channel is in the resting state

Question 11

what happens during activated state for a sodium ion channel?

Answer 11

both the activation and inactivation gates are open. when an action potential is initiated the voltage channel opens the activation gates and the channel is in its activated state, allowing sodium ions to cross the axolemma

Question 12

what happens during the inactivated state for a sodium ion channel?

Answer 12

the inactivation gate is closed and the activation gate is open. the channel in this state no longer allows sodium ions to pass through, but activation gate remains open. when the action potential is finished, the channel returns to the resting state

Question 13

neuronal action potentials have __ general phases. What are they?

Answer 13

3 general phases; the depolarization phase, the repolarization phase, and the hyperpolarization phase

Question 14

in the depolarization phase..

Answer 14

the membrane potential rises toward zero and then becomes briefly positive

Question 15

in the repolarization phase…

Answer 15

the membrane potential returns to a negative value

Question 16

in the hyperpolarization phase…

Answer 16

the membrane potential becomes temporarily more negative than resting potential

Question 17

each phase occurs because of…

Answer 17

the selective opening and closing of specific voltage gated ion channels

Question 18

before the action potential, when the membrane is at rest, both the sodium and potassium ion channels are __

Answer 18

in the resting state

Question 19

step 1 of the action potential:

Answer 19

a local potential depolarizes the axolemma of the trigger zone to threshold. the action potential begins when the VG sodium ion channels in the axolemma of the trigger zone enter activated stated (are opened)

Question 20

VC channels will only become activated if the membrane is already ___ to a level called ___

Answer 20

depolarized to a level known as threshold (usually -55 mV)

Question 21

what is the source of the depolarization of the membrane?

Answer 21

it generally comes from local potentials that arrive from the cell body

Question 22

step 2 of the action potential:

Answer 22

voltage-gated sodium ion channels activate, sodium ions enter, and the axon section depolarizes. sodium ions rush into the neuron with their electrochemical gradient and as the membrane potential becomes more positive, more VC sodium ion channels are activated

Question 23

the more the axon depolarizes, the more ___ are activated

Answer 23

voltage-gated sodium ion channels

Question 24

the influx of positive charged from the sodium ions causes ___ to about ___

Answer 24

rapid depolarization to about +30 mV

Question 25

how is this an example of a positive feedback loop?

Answer 25

the initial input (the activation of sodium ion channels and depolarization) amplifies the output (more sodium ion channels are activated and the axolemma depolarizes even more)

Question 26

step 3 of the action potential:

Answer 26

sodium ion channels inactivate and voltage gated potassium ion channels activate and repolarization begins. when the axolemma is fully depolarized (about +30 mV) the inactivation gates of the sodium ion channels close and the sodium ions stop entering the axon. at the same time, the voltage gated potassium ion channels slowly open and potassium ions flow out of the axon along their electrochemical gradient causing the axolemma of the trigger zone to lose positive charges and to begin repolarization

Question 27

step 4 of the action potential:

Answer 27

sodium channels return to their resting state and repolarization continues. as potassium ions exit the axon and repolarization continues, the activation gates of the sodium ion channels close and the inactivation gates open, returning sodium ion channels to their resting state

Question 28

step 5 of action potential:

Answer 28

the axolemma may hyperpolarize before potassium ion channels return to the resting state; after this, the axolemma returns to the resting membrane potential - the outflow of potassium ions usually continues until the membrane potential of the axolemma hyperpolarizes (can become as negative as -90 mV)

Question 29

the axolemma hyperpolarizes why?

Answer 29

because the gates of the potassium ion channels are slow to close, allowing additional potassium ions to leak out of the cell

Question 30

hyperpolarization finishes as…

Answer 30

the voltage gated potassium ion channels return to their resting state

Question 31

after the action potential,

Answer 31

the potassium leak channels and sodium/potassium pumps re-establish the resting membrane potential

Question 32

does a lot change throughout the preceding steps of a single action potential?

Answer 32

no - very little change occurs in the intracellular or extracellular concentration of sodium or potassium ions and as a result the gradient isn’t too disturbed

Question 33

what is the exception to there being little change?

Answer 33

with repetitive action potentials, the gradient will eventually deplete and the neuron relies on the sodium potassium pumps in the axolemma to restore it