!

Questions

Question 1!

In a laboratory, scientists often study neurons in isolation, outside of a living creature, in a dish. In this setting, one can have a good deal of control over the local ionic environment in which the cell exists, because we can control the makeup of the solution – the 'medium' – in the dish. This situation is a bit like replacing the extracellular fluid that the cell would normally be bathed in with some other fluid.!

There are a variety of different kinds of media that scientists use for different purposes. Let’s consider a few standard ones, including Dulbecco’s Modified Eagle’s Medium (DMEM)* and "depolarization" medium.!

The table below lists the ionic concentrations for these media. The "intracellular" column lists the concentrations inside a particular neuron. All concentrations below are in mM (millimolar).!

!

ion Intracellular DMEM "depolarization medium"

K+ 140 5.3 55

Na+ 10 154 105

Cl- 12 119 119

Ca2+ 0.0001 1.8 1.8

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!!!Based on this chart, what is the Nernst potential for K+ for the cell, bathed in DMEM at 37C?!

Check the best option!

• 87 mV!

• -87 mV!

• -25 mV!

• 25 mV!

• -60 mV!

!What is the Nernst potential for Cl- in DMEM at 37C?!

Check the best option!

• 80 mV!

• -80 mV!

• 61 mV!

• -61 mV!

!What is the Nernst potential for Na+ in DMEM at 37C?!

Check the best option!

• 73 mV!

• -73 mV!

• 63 mV!

• -63 mV!

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If we assume relative ion permeabilities of 100, 4, 10 for K+, Na+, and Cl-, respectively, what would the resting potential of the cell be when the cell is bathed in DMEM at 37C? (assume Ca2+ permeability is neglible)!

Check the best option!

• 90 mV!

• -90 mV!

• 66 mV!

• -66 mV!

!What is Nernst potential for K+ in the "depolarization" medium at 37C?!

Check the best option!

• -87 mV!

• 63 mV!

• -25 mV!

• 10 mV!

!What would the resting potential of the cell be when it is bathed in depolarization medium at 37C?!

Check the best option!

• 0 mV!

• -25 mV!

• 25 mV!

• 50 mV!

!!!

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When the neurons are bathed in DMEM at 37C, which ion experiences the smallest driving force if we apply a voltage of -100mV across the membrane?!

Check the best option!

• K+!

• Na+!

• Cl-!

• Ca++!

!Which direction would the net flux be in for that ion?!

Check the best option!

• Extracellular to intracellular!

• Intracellular to extracellular!

• At the Nernst potential, so no net movement!

!Which ion experiences the largest driving force if a voltage of -100mV is applied?!

Check the best option!

• K+!

• Na+!

• Cl-!

• Ca++!

!!!!!!

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Which ion experiences the largest driving force if a voltage of -50 mV is applied?!

Check the best option!

• K+!

• Na+!

• Cl-!

• Ca++!

!Question 2!

Hypokalemia and hyponatremia are the medical terms referring to low levels of potassium and sodium in the extracellular fluid, respectively. The former occurs in various conditions, including overmedication with certain antibiotics, while the latter has many causes including diseases of the kidney. Let's explore what effect, if any, each of these conditions would have on the membrane potential of a neuron.!

What would happen to the resting potential if there were a lower than normal extracellular K+ concentration?!

Check the best option!

• no change!

• depolarization!

• hyperpolarization!

!What would happen to the resting potential if there were a lower than normal extracellular Na+ concentration?!

Check the best option!

• no change!

• depolarization!

• hyperpolarization!

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Which would have a greater impact on the resting potential: lowering the extracellular Na+ concentration by 2 mM, or lowering extracellular K+ concentration by 2 mM?!

Check the best option!

• Na+!

• K+!

• the effect would be equal!

!Question 3!

Suppose that we discover a new kind of channel that we can activate chemically, to turn on or off permeability to one particular ion. Let's try to determine which ion this new channel passes.!

When we activate the channel, the membrane potential does not change very much, depolarizing by a small amount, even though we are sure that we are activating many channels and that the permeability to the unknown ion should be high when the channels are active.!

Assume the follow ionic parameters for the cell:!

Which of the following ions is the most likely ion for the channel to pass?!

Check the best option!

• Na+!

• K+!

• Cl-!

• Ca+!

ion Intracellular (mM) Extracellular (mM) Resting Permeability

K+ 140 4.5 100

Na+ 10 135 4

Cl- 12 125 10

Ca2+ 0.0001 1.8 0.01

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If we open up this channel how do think it will affect action potentials?!

Check all options that apply:!

• Make action potentials reverse polarity!

• Make it more difficult to trigger one!

• Make it easier to trigger one!

• Nothing will change!

What would happen if we reversed the concentration gradient of this ion, by swapping the intracellular and extracellular concentrations of the ion it is selective for? What would happen if we opened the channel then?!

Check the best option!

• It would make it more difficult to trigger an action potential!

• Still nothing would change!

• It would trigger an action potential!

!Question 4!

Myelin speeds action potential conduction by...!

Check all options that apply:!

• decreasing membrane capacitance!

• decreasing axial resistance!

• increasing membrane resistance!

• increasing the membrane potential!

!!!!!!

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Question 5!

In each picture, we have an input signal and a measurement of the membrane potential from an adjacent patch of membrane. Which membrane has a higher capacitance, assuming that all other properties of the membrane are the same?!

" !

capacitance plots!

Check the best option!

• A!

• B!

Question 6!

If we increase membrane resistance (Rm)...!

Check all options that apply:!

• the length constant increases!

• the length constant decreases!

• the time constant increases!

• the time constant decreases!

!!!!!

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Question 7!

If we decrease membrane capacitance (Cm)...!

Check all options that apply:!

• the length constant increases!

• the length constant decreases!

• the time constant increases!

• the time constant decreases!

!Question 8!

What would happen if we made an axon have a larger diameter, while keeping the density of channels per unit area of the membrane constant?!

Check all options that apply:!

• Raxial decreases!

• Raxial increases!

• Rm decreases!

• Rm increases!

!Question 9!

Consider an ion for which there are only passive leakage channels. If the membrane potential is farther from the ion’s Nernst potential, the magnitude of the current associated with that ion will be...!

Check the best option!

• greater!

• smaller!

• the same!

• we can’t say!

!

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Question 10!

The relative refractory period of the action potential is caused by...!

Check all options that apply:!

• open delayed rectifier K+ channels!

• voltage-gated Na+ channel inactivation!

• sodium/potassium pump activity!

• an increase in membrane capacitance!

!Question 11!

The absolute refractory period of the action potential is caused by...!

Check all options that apply:!

• open delayed rectifier K+ channels!

• voltage-gated Na+ channel inactivation!

• sodium/potassium pump activity!

• an increase in membrane capacitance!

!!!!!!!!!!!

Page � of �10 25

Question 12!

Fill in the blanks:!

Label the phases of the action potential!

" !

action potential diagram!

A: [ resting phase | stimulation/initiation | saltatory phase | rising phase | refractory period ]!

B: [ resting phase | stimulation/initiation | saltatory phase | rising phase | refractory period ]!

C: [ resting phase | stimulation/initiation | saltatory phase | rising phase | refractory period ]!

!!!!!!

Page � of �11 25

Question 13!

Fill in the blanks:!

Complete this paragraph by filling in the best word:!

The action potential is a propagated change in the [ membrane | Nernst | equilibrium ] potential of a neuron that begins with a [ depolarization | hyperpolarization ] of the membrane. If enough voltage gated [ sodium | chloride | potassium | calcium ] channels open, then we enter the [ rising | refractory | falling ] phase of the action potential. The rapid change is due to the [ sodium | chloride | potassium | calcium ] current from the rapidly opening channels. The next phase of the action potential, the [ rising | refractory | falling ] phase, sees the membrane potential [ hyperpolarize | depolarize ] due to the opening of voltage gated [ sodium | chloride | potassium | calcium ] channels.!

!!!!!!!!!!!!!!!!

Page � of �12 25

Question 14!

See the circuit diagram below.!

" !

circuit diagram!

If we reduce by half the value of R, then we will...!

Check all options that apply:!

• double I!

• halve I!

• double V!

• halve V!

!Question 15!

If you stimulate an axon in the middle, what will happen?!

Check all options that apply:!

• action potentials will propagate in both directions!

• an action potential will only propagate in the direction away from the cell body!

• an action potential will only propagate in the direction towards the cell body!

Page � of �13 25

!Question 16!

Cells in the electric eel's electric organ have a larger-than-normal membrane potential, sitting at roughly -150 mV. How might a cell achieve a more negative resting potential?!

Check all options that apply:!

• Higher extracellular K+ concentration!

• Lower extracellular K+ concentration!

• Higher intracellular K+ concentration!

• Lower intracellular K+ concentration!

What other modifications would this larger membrane potential require?!

Check all options that apply:!

• different Na/K pump directionality!

• higher Cm!

• lower Cm!

• more Na/K pump activity!

• lower Raxial!

• higher Raxial!

!!!!!!!!!

Page � of �14 25

Question 17!

Ca2+ plays an important role in many biological processes. It is present in the following concentrations:!

This means that:!

Check all options that apply:!

• Ca2+ will readily flow into the cell if a channel selective for Ca++ opens!

• Ca2+ will readily flow out of the cell if a channel selective for Ca++ opens!

• Ca2+ can only flow into the cell when the cell is depolarized (e.g. at the peak of the action potential).!

• Ca2+ can only flow out of the cell, but only when the cell is hyperpolarized!

!Question 18!

True or false?: The GHK equation includes a term describing the permeability of the Na+/K+ pump.!

True or False?!

• True!• False!

!Question 19!

True or false?: The Na+/K+ pump requires energy to work.!

True or False?!

• True!• False!

!!

ion Intracellular (mM) Extracellular (mM)

Ca2+ 0.0001 1.8

Page � of �15 25

Question 20!

True or false?: Energy is required to maintain an equilibrium.!

True or False?!

• True!• False!

!Question 21!

True of false?: A system that is at equilibrium would also be considered to be at a steady state.!

True or False?!

• True!• False!

!Question 22!

Glia, the supporting cells of the nervous system, are primarily only permeability to a single ion species, K+.!

Given this information, which of the following equations could be used to estimate the resting potential of a glia cell? Check all options that apply.!

Check all options that apply:!

• Ohm's law!

• The Nernst equation!

• The GHK equation!

• The Hodgkin Huxley equations!

!!!!

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Question 23!

The selective potassium leakage channel keeps out sodium on the basis of its...!

Check all options that apply:!

• difference in charge!

• difference in driving force!

• difference in ionic radius!

!Question 24!

To produce a -70 mV resting potential, roughly how many K+ ions must translocate across the membrane?!

Check the best option!

• on the order of 1 in 100,000!

• on the order of 1 in 100!

• on the order of 1 in 10!

!Question 25!

True or false?: the concentration of Na+ outside the cell varies by up to 20% during an action potential.!

True or False?!

• True!• False!

!!!!!!

Page � of �17 25

Question 26!

True or false?: the concentration of K+ inside the cell varies by up to 20% during an action potential!

True or False?!

• True!• False!

!Question 27!

True or false?: most invertebrate neurons have giant axons.!

True or False?!

• True!• False!

!Question 28!

True or false?: giant axons are common in vertebrates.!

True or False?!

• True!• False!

!Question 29!

True or false?: long axons in vertebrates animals are typically myelinated.!

True or False?!

• True!• False!

!!!

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Question 30!

True or false?: if a large enough voltage is applied, charged particles will move across the lipid bilayer membrane, even without ion channels.!

True or False?!

• True!• False!

!Question 31!

A myelin sheath primarily works because...!

Check all options that apply:!

• it is thicker than the normal membrane!

• its dielectric constant is higher the normal membrane!

• it lowers the axial resistance of the cell!

• it prevents ions from flowing through the membrane!

!!

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Question 32!

Fill in the blanks:!

Label the parts of this axon:!

" !

axon diagram!

A: [ node of Ranvier | glia (supporting cell) | myelin sheath | betz cell | axon hillock | axonal bouton ]!

B: [ node of Ranvier | glia (supporting cell) | myelin sheath | betz cell | axon hillock | axonal bouton ]!

C: [ node of Ranvier | glia (supporting cell) | myelin sheath | betz cell | axon hillock | axonal bouton ]!

!!!!

Page � of �20 25

Question 33!

True or false?: a voltage-gated Na+ channel would be open 100% of the time at -40mV membrane potential.!

True or False?!

• True!• False!

!Question 34!

True or false?: a voltage-gated Na+ channel would be closed most of the time at -80mV membrane potential.!

True or False?!

• True!• False!

!Question 35!

True or false?: a patch of membrane can always be stimulated to produce an action potential -- it’s just a matter of how much current is needed to make it happen.!

True or False?!

• True!• False!

!Question 36!

True or false?: the limitations of action potential propagation keep axons from being more than a few mm long.!

True or False?!

• True!• False!

!Page � of �21 25

Question 37!

True or false?: Larger neurons need to have more negative resting potentials in order to function.!

True or False?!

• True!• False!

!Question 38!

True or false?: Larger neurons must expend more energy at rest.!

True or False?!

• True!• False!

!Question 39!

If we artificially made the resting potential more negative by 10mV, then, all other things being equal, we’d need:!

Check the best option!

• a larger injected current to stimulate an action potential!

• a smaller injected current to stimulate an action potential!

• no change in injected current to stimulate an action potential!

• it would no longer be possible to stimulate an action potential!

!!!!!!

Page � of �22 25

Question 40!

The propagation velocity of an action potential traveling in an unmyelinated axon depends on...!

Check all options that apply:!

• the length constant of the membrane!

• the time constant of the membrane!

• the number of voltage-gated channels in the membrane!

• the axial resistance of the axoplasm!

!Question 41!

If we had twice as many voltage-gated sodium channels in the membrane, then!

Check all options that apply:!

• the action potential amplitude would double!

• the action potential would be wider in time!

• it would be easier to initiate an action potential!

• it would be harder to initiate an action potential!

!!!!!!!!!!

Page � of �23 25

Question 42!

A newly discovered species of sea snail is found to produce a potent venom containing several neurotoxic substances. Three of these substances – Substances X, Y, and Z – are isolated, and applied one by one to a neuron in a dish to discover their function.!

When Substance X is applied to a neuron in a dish, action potentials initiate normally, but the membrane potential takes many times longer to return to baseline.!

What part of the neuron might the poison be affecting/attacking?!

Check all options that apply:!

• voltage-gated sodium channels!

• delayed rectifier voltage-gated potassium channels!

• the Na+/K+ pump!

• K+ leakage channels!

• Na+ leakage channels!

• myelin!

When Substance Y is applied the cell, the cell functions normally at first, but as time goes on, action potential amplitude gradually decreases, and the resting potential gradually becomes less negative. If the cell is driven to spike more, both of these effects occur more rapidly.!

What part of the neuron might the poison be affecting/attacking?!

Check all options that apply:!

• voltage-gated sodium channels!

• delayed rectifier voltage-gated potassium channels!

• the Na+/K+ pump!

• K+ leakage channels!

• Na+ leakage channels!

• Myelin!

!Page � of �24 25

When Substance Z is applied the cell, within a few seconds, axon potentials are completely abolished – no amount of stimulation is able to trigger one.!

What part of the neuron might the poison be affecting/attacking?!

Check all options that apply:!

• voltage-gated sodium channels!

• delayed rectifier voltage-gated potassium channels!

• the Na+/K+ pump!

• K+ leakage channels!

• Na+ leakage channels!

• myelin!

!

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