Lesson 2 - Membrane Resting Potential

Question 1

what do you get when you calculate the equilibrium potential for K+ using the nernst equation

Answer 1

• If you calculate using the K+ ion you get:
• EK+ = (RT/F) ln([K+]o /[K+]i) = -90 mV (equilibrium potential for K+)
• This is what the MP would be if only K+ ions was involved (ideal situation)

Question 2

if the equilibrium potential for K+ ions is calculated to be -90 mV, why is the membrane potential actually -70 mV to -80 mV? What is the other equation used to calculate the real MP?

Answer 2

• Membrane is most permeable to K+ at rest, but Na+ and Cl- ions are also
  diffusing somewhat even though the membrane is not as permeable to this.
• So we have another equation that puts all of the ion species into consideration, which include K+, Na+ and Cl- with their relative permeability (remember that K+ is the major player at rest = has the greatest permeability whereas Na+ and Cl- have low permeability)
• Actual membrane potential can be calculated from an expanded equation (goldman equation) containing a term for each diffusable ion species

Question 3

what is the goldman equation

Answer 3

it is an expanded version of the nernst equation where it calculates the membrane potential of a cell by considering not only the permeability of K+, but also Na+ and Cl-

calculates the MP to be -70 mV

Question 4

what is the ideal situation for Na+ ions and how does it defer to reality? if this ideal situation exists what can we calculate and which direction will the Na+ ions move

Answer 4

• if we find a situation where the membrane is very permeable to Na+, more than K+ (not typical) then the MP can change drastically
• we can use the nernst equation to calculate the Na+ equilibrium potential in this ideal situation
• if this ideal situation comes to be then there will be a net Na+ movement into the cell (because there are more Na+ on the outside - Sod.Pot pump)
• aka net cation accumulation on the inside

Question 5

describe the Na+ influx in the cell, when it stops, and what the calculated value is

Answer 5

• Na+ is more concentrated on the outside of the membrane, thus it will move inward.
• the Na+ will diffuse its positive charge in the membrane until it is lined with so much positive charge that more Na+ influx will be electrically repelled (equilibrium)
• when we plug the values in we get the equilibrium potential for Na+ via the nernst equation to be +60 mV.
• it is a positive number since positive charge is being distributed inwards

Question 6

describe the movement of Cl- ions. Does it go out or in? Why?

Answer 6

• Inside the cell, we have large proteins (which are basically trapped, they can only get across to the outside using exocitosis), and since they tend to have “-” charges, the Cl- ion is pushed out of the cell (electrical repulsion)
• Therefore, the Cl- ions tend to be more concentrated on the outside in the extracellular space
• This is due to anion proteins present on the inside and not due to an active pump

Question 7

how do we use Na+ channels to generate an action potential

explain what an AP and the threshold potential is in your explanation

Answer 7

• the membrane increases its conductance to generate a signal by opening a voltage gated Na+ ion channel
• In normal resting MP, this Na+ channel is shut!
• To open this Na+ channel we need to depolarize the membrane by a certain amount (make the inside of the membrane more positive)
• exactly, the membrane is depolarized from -70 mV to the threshold potential of -55 mV (MP needed to open the ACTIVATION gates and start a chain of events that will lead to rapid depolarization of the MP)
• this will allow the s4 segments to move upwards and allow the Na+ ions to move through

Question 8

what is the rapid depolarization of the membrane for Na+ channels

Answer 8

• after the MP moves past the threshold potential of -55 mV, rapid depolarization begins
• here more depolarization = more open voltage gated sodium channels = more Na+ in the cell = more positive mV
• this is a cycle – now more Na+ channels will open because of rapid depolarization

Question 9

explain why rapid depolarization and an influx of sodium cannot go on forever

explain the time it takes to stop the influx and what cannot happen now to the Na+ channels because of so

Answer 9

• no, rapid depolarization is halted due to the inactivation gate (or ball and chain) closing
• the inactivation gate is an appendage on the inside of the membrane channel, that reaches its final position where it closes the gate
• this position is reached 1/2 a millisecond after rapid depolarization thus unabiling
  the MP to reach the Na+ equilibrium potential of +60 mV (instead reaches around +30 mV)

Question 10

summary: what are the two gates in the Na+ channel and how are they triggered

Answer 10

• activation gate: triggered when MP goes from -70 mV to -55 mV (threshold potential)
• inactivation gate: triggers half a millisecond after the activation gate is opened and rapid depolarization takes place

Question 11

what has to happen after rapid depolarization is halted due to the inactivation gate closing

Answer 11

• the MP needs to drop below the threshold potential of -55 mV for the inactivation gate to release, allowing for more Na+ to enter if signaled.

Question 12

what is an action potential?

what does the spacing between APs mean

which membranes/cells can only make APs. why?

Answer 12

• An AP is essentially an impulse, a very short lived, change in the MP, an AP is used as a signal
• the spacing between APs can be for different signals
• You can only produce an AP in membrane that contains the voltage-gated Na+ channels
• By definition, the presence of voltage-gated Na+ channel is what makes the membrane ‘excitable’ and thus is found in high density within excitable membranes

Question 13

draw and annotate the graph of an action potential

Answer 13

check answer L2 notes one note

Question 14

frequency coding: what are the three types of threshold stimuli

Answer 14

the threshold for generating an AP corresponds to the level of depolarization necessary to produce the chain of events that starts with opening up the voltage gated sodium channels

subthreshold stimulus: a stimulus that causes depolarization that is less than the 15 mV difference (-70 mV to -55 mV)
-> opens some Na voltage channels, but not enough to overcome the outflow of K+ channels that happens in the back all the time – not enough to start the chain of events that leads to an AP

threshold stimulus: causes enough depolarization to result in the production of an AP.
-> enough Na voltage gated channels to start the chain of events that fires for an AP

suprathreshold stimulus: causes more than enough depolarization and will also result in the production of an AP

Question 15

what is the all or none principle for APs

Answer 15

Even if the suprathreshold stimulus generates an AP and is of greater stimulus strength, it will have no effect on the magnitude of the AP

This is the all of none principle, you either fire an action potential or not. if you go above the threshold, it will fire an AP of the same magnitude.

Question 16

if the all or none principle exists, how are we able to distinguish between stimulus intensity

Answer 16

frequency coding: Information pertaining to stimulus intensity is coded by the changes in the frequency of the Action Potential
-> we can’t mess around with magnitude, so we measure with frequency, ie. how many APs are there in a given period

• if we have a 10 msec subthreshold stimulus, no action potential will be generated as a subthreshold stimulus does not cause a great enough mV change
• if we have a 10 msec threshold stimulus, one action potential may be generated
• however, if we have a 20 msec threshold stimulus, of greater stimulus intensity, two action potentials may be generated within closer range of each other, ie greater frequency

Question 17

what are refractory periods and the two types ?

within the types, can another AP be configured?

Answer 17

• After we generate an AP and inactivate (ball blocks the exit) the Na+ channels, we have a period in which all or some Na+ channels are inactivated
• Na+ channels remain inactivated until membrane potential drops below ‘threshold’, then channels reconfigure to their original state (ball is removed from end) and membrane becomes excitable again
• Absolute RP: none of channels are reconfigured ie. all Na+ voltage channels are inactivated - another AP cannot be created– MP above the threshold
• Relative RP: some but not all of the Na+ channels are reconfigured to their original state (not blocked by the ball and chain) (generally 2-5 ms duration for the protein channels to reconfigure since they all do at different paces) – other AP can be created here since the MP will be below the threshold, but will require a suprathreshold since there are less voltage gated sodium channels to work with and the MP is below resting state (more stimulus to reach -55mv)

Question 18

what do you need to do to completely block the membrane from producing an AP

Answer 18

depolarization blocks:

• Keep the membrane depolarized! (remember that in order to generate an AP, we need to repolarize the membrane to below threshold level to reconfigure the Na+ channels to its original state)
• in depolarization, Na channels are open but then quickly inactivate and can only open again when the membrane is repolarized
• If you permanently depolarize the membrane at the inactive state, ie. keep it at 20 mV (above threshold), the Na+ channels will be permanently inactivated, and you will not be able to generate another AP

Question 19

what do we need to increase in the extracellular space to keep the membrane depolarized (creating a depolarization block)

Answer 19

• You need to destroy the concentration gradient for K+ to create a depolarization block (remember that K+ current is responsible for keeping the MP polarized to -70 mV as K+ ions travel out of the cell, making it more negative)
• this can be done by introducing more K+ in the extracellular space so the concentration gradient is disturbed and depolarized (more positive as K+ cannot leave- above threshold) (excess [K+] — e.g. with KCl injection)
• This will result in permanent Na+ inactivation and the membrane will remain in absolute refractory state where it is in-excitable

Question 20

what is after hyperpolarization ? And what is the major contributing factor for this ?

Answer 20

• K+ leakage channels make the membrane more negative (polarize) as positive charges are taken out
• K+ voltage gated channels are extra K+ channels that reopen when the membrane is depolarized
• if we have both of these K+ channels removing K+ ions, we will have a greater outflow K+ current
• thus will result in the MP to be even more negative than the resting state of -70 mV (around -80 mV)
• thus, the membrane will overshoot polarization or hyperpolarize before it returns to its resting MP
• thus the presence of K+ voltage gated channels results in the hyperpolarization of the membrane after the AP