lecture 11.3 main points

Question 1

what types of electrical signals are used in communication between neurons? which is short distance? which is long distance?

Answer 1

neurons are electrically excitable
graded potentials - short distance communication
action potentials - long distance communication

Question 2

what is necessary for the production of electrical signals in neurons

Answer 2

electrical signals in neurons depend on the existence of a resting membrane potential (RMP or Vrm)

Question 3

how is the RMP created

Answer 3

using ion gradients and a variety of ion channels that open or close in response to specific stimuli

Question 4

how does the plasma membrane contribute to Vrm?

Answer 4

because the lipid bilayer of the plasma membrane is a good insulator, ions MUST flow through ion channels

Question 5

why is RMP closer to the Nernst for potassium as opposed to sodium

Answer 5

because there are more potassium leak channels
there are also more negatively charged proteins that always remain in the cytosol

Question 6

how is RMP created and maintained? (what ions and ion transport mechanisms are important)

Answer 6

the Na and K leak channels are offset by Na/K ATPases (sodium potassium pumps) which pumps out Na as fast as it leaks in
RMP is due to Nernst values for Na and K as well as equilibrium in the Na/K pump and the K leak rates

Question 7

when is the cell primed for electrical signaling

Answer 7

a cell that exhibits an RMP is said to be polarized, meaning it is primed and ready to produce an action potential

Question 8

what type of potential, and what membrane voltage direction, is necessary for bringing the cell closer to the point of an action potential

Answer 8

a graded potential must depolarize the cell (make it more positive) to threshold

Question 9

how are graded potentials produced

Answer 9

a graded potential occurs whenever ion flow produce a current that is LOCALIZED, it spreads to adjacent regions for a short distance and then dies

Question 10

what types of ion channels are necessary for graded potentials

Answer 10

mechanically gated or ligand gated channels

Question 11

what does a depolarizing graded potential (excitatory) do to the Vm?

Answer 11

causes the intracellular membrane voltage to be less negatively charged with respect to the extracellular membrane voltage

Question 12

what does a hyper polarizing graded potential (inhibitory) do the Vm?

Answer 12

causes the intracellular membrane voltage to be more negatively charged with respect to the extracellular membrane voltage

Question 13

how are graded potentials named?

Answer 13

depending on what type of stimulus and where they occur

Question 14

where do graded potentials primarily occur

Answer 14

in the dendrites and cell body of a neuron

Question 15

contrast AP with graded potential

Answer 15

AP travels the length of an axon whereas graded is localized
graded can be excitatory or inhibitory

Question 16

what happens to the Vm during action potential?

Answer 16

the membrane potential reverses and then eventually is restored to its resting state

Question 17

what is threshold value

Answer 17

the minimum voltage required to elicit an action potential,

Question 18

what is a threshold stimulus ? what is a sub-threshold stimulus ?

Answer 18

threshold stimulus - full strength nerve impulse (AP) is produced and spreads down the axon of the neuron to the axon terminals

sub-threshold stimulus - stimulus is not strong enough, no nerve impulse will result and the membrane will go back to resting value

Question 19

what happens when a graded potential that result in depolarization of the neuron from RMP to threshold? (about -55mV in many neurons)

Answer 19

an action potential will occur

Question 20

according to the all or none principle, is a stimulus reaches threshold, is the action potential the same every time?

Answer 20

yes, a larger stimulus will not cause a larger impulse

Question 21

AP is a tug of war between Na and K, what determines the direction of Vm during an action potential

Answer 21

whichever has the highest current (I) at a particular time determines the direction the membrane coltage goes

Question 22

what is current dependent on?

Answer 22

current is dependent upon driving force and conductance (g)

Question 23

list the properties and states of voltage gated sodium channels

Answer 23

fast to open, fast to close
-deactivated : closed but can open
-activated: open
-inactivated: closed and cannot re-open until reset to deactivated state

Question 24

list the properties of voltage gated potassium channels

Answer 24

slow to open, slow to close

Question 25

three main phases of AP

Answer 25

depolarizing- increase in membrane potential
repolarizing - decrease membrane potential back toward RMP
after hyperpolarizing (hyperpolarization) - membrane potential drops below RMP value

Question 26

describe the conductance and current for sodium during depolarization (understand influx and efflux)

Answer 26

conductance (gNa) is increasing allowing Na to rush into the cell, current (I) increases and then decreases - makes the inside of the membrane more positive
Na influx > K efflux

Question 27

does the amount of sodium entering the cell during depolarization affect concentration? why is this important in terms of the sodium Nernst?

Answer 27

amount of Na entering the cell does not affect concentration gradient
Na driving force is increased because sodium wants to go into the cell where it is more negative

Question 28

does amount of sodium entering the cell during depolarizations affect membrane voltage?

Answer 28

causes a big change in membrane voltage

Question 29

describe the events for sodium and potassium current and conductance during early and late depolarization

Answer 29

as AP approaches peak, gNa increases however INa begins to decrease
gK begins to increase and IK begins to increase due to the increase in gK

Question 30

why does the sodium driving force and current decrease during late depolarization even though conductance is high

Answer 30

as membrane voltage gets closer to Nernst value (-55 for Na) the driving force for Na lessens, meaning current will decrease despite continued increase in conductance

Question 31

why does potassium driving force and current begin to increase during late depolarization

Answer 31

as membrane voltage gets more positive and This further away from the Nernt value for K, the driving force for K increases meaning that the current will increase IF the conductance increases

Question 32

describe current and conductance for sodium and potassium at the peak of AP

Answer 32

at the peak of AP, sodium current in and potassium current out are the same
Na influx = K efflux

Question 33

at the peak of AP, conductance (g) for Na is much greater than that of K, but the two currents are equal, why?

Answer 33

membrane is closer to Nernst for Na meaning that driving force is low therefore Na influx is low
membrane is further away from K Nernst meaning that driving force for K is high, is flux is as high as it can be based on K conductance at this point

Question 34

describe the events of sodium and potassium current and conductance during repolarization

Answer 34

voltage Na channels begin to undergo inactivation, gNa reaches it peal during onset of repolarization and progressively decreases as Na channels are closing
sodium current is greater during repolarization (more sodium flowing in) than during depolarization
however K efflux > Na influx
more slow voltage gated K channels continue to open - gK increases
Ik increases in first half of repolarization and then begins to decrease in the second half despite a continued increase in gK
as we begin to approach the Ek (Nernst) the driving force for potassium decreases leading to a decrease in current

Question 35

describe the events for hyperpolarization phase with respect to K current and conductance

Answer 35

slow voltage gated K channels remain open and the membrane potential becomes more negative than RMP
as voltage gated K channels close, the membrane potential returns to the RMP due to equilibrium in the rate of the Na/K ATPases and the Na and K leakage
potassium current - flows out of cell, starting to slow as approaching Nernst
conductance - still increasing but will eventually decrease

Question 36

why do we not discuss sodium during the hyperpolarization phase

Answer 36

most voltage gated Na channels have been closed and deactivated or are in the reset phase