Neuro: Unit #1 (HW/Exams)

You make a mutation to the Na-K ATPas such that the ionic concentrations of Na and K across the membrane have been altered (see below, Na channel is red, K channel is blue). b) What is the equilibrium potential for Na+ (ENa)?

ENa = 61/z x log ([Na]out/[Na]in) ENa = 61/1 x log [3] / [60]) ENa = -79.4 mV

You are recording from a neuron that expresses voltage-gated K channels that open at -60 mV. In your system, EK = +20 mV. Which of the following IV plots would you expect to see for your current?

Opens at -60 mV x-intercept at +20 mV

You express a gene for a channel in your cell, below, but you forgot to label the tubes so don't know if it is a K channel or a Na channel. Describe one experiment that you could do to figure out if it were a Na channel or a K channel.

Put a recording electrode into the cell to measure Vm. If Vm = x, it is a K channel. If Vm = x, it is a Na channel.

You mutate the K channel such that its conductance is reduced by about half

conduction = slope

Consider how changes in conductance affect the shape of the IV plot. Change gK from 70 to 200. Construct the IV plot from -80 mV to +80 mV in 10 mV increments.

increase in gk = increase in slope

What is the difference between a voltage-gated K+ channel and a leak K+ channel?

Leak channels are always open, whereas the open probability of voltage-gated channels changes as a function of membrane voltage.

What function do voltage-gated K+ channels and leak K+ channels typically serve in a neuron?

Leak channels set the resting membrane potential and voltage-gated K channels repolarize the AP.

Below is an image of the Na current elicited with two voltage steps, spaced at variable time intervals (from your MetaNeuron homework). a) What property of the Na channel does this graph measure?

Recovery from inactivation

How do changes in gleak affect the timing or shape of AP?

The biggest effect is that with more gleak, the time to reach AP peak becomes delayed. This is because it takes longer for gNa to exceed gleak and begin the rising phase of the AP. You also see a difference in the height of the AP. This is because lots of gleak slows the rising phase of the AP (you can see that these lines have different slopes), so under conditions where gleak is high, v-gated K channels engage before the cell has had time to get all the way to ENa.

The definition of the length constant is:

The distance a signal can travel before decaying by a specified amount

Now we will consider how changes in conductance change the shape of the IV plot. Change gNa max from 260 to 100. Draw the IV plot (from -80 mV to +80 mV in 10 mV increments).

decrease in conductance = decrease in slope (maximum channels open)

1. Below is an IV plot of a K channel. What is EK in this system? 2. Based on this EK, do you think the concentration of K is higher inside the cell or outside the cell?

1. +50 mV -x-intercept is the Ek or reversal potential 2. Outside -positive Ek = concentration of K higher on outside

Since you apparently like to live dangerously, you grab another unmarked tube and express its contents in another neuron. You find that the resting membrane potential is -11.6 mV Based on this, did the tube contain:

2 K channels and 3 Na channels

You make a mutation to the Na-K ATPas such that the ionic concentrations of Na and K across the membrane have been altered (see below, Na channel is red, K channel is blue). a) What is the equilibrium potential for K+ (EK)?

Ek = 61/z x log ([K]out/[K]in) Ek = 61/1 x log [50] / [5]) Ek = +61 mV

You have recently isolated two new compounds known to block certain types of voltage-gated K channels. You drive the neuron with a strong stimulus and see the response labeled "control". C) When you apply Compound B, the firing rate of the neuron stops in response to the same stimulus. Why do you think this is the case? D) Which channel do you think was most likely blocked by Compound B?

C) Compound B has probably blocked a K channel that helps with AP repolarization. Without this channel, the membrane cannot repolarize enough between spikes to fully de-inactivate Na channels, so the neuron goes into depolarization block, where it cannot continue firing. D) Kv3

True or false, in the relative refractory period, a neuron cannot fire, no matter how big a stimulus it gets

False

What is the cause of demyelination in Multiple Sclerosis, and which type of glial cells are affected?

MS is an autoimmune disease. The body makes Abs against its own myelin and destroys it.

Below are IV plots (real data!) of Na channels measured from cultured neurons. The blue IV plot shows data collected from neurons expressing the original Na type of channel (wildtype, WT). The green and yellow traces show IV plots of mutant variants of Na channels, "RH" in green, and "NK" in yellow. c) Does the NK mutation (yellow) affect the channels' voltage sensitivity, conductance or both? Justify your answer.

NK seems to have a big effect on voltage sensitivity. The channel needs the membrane to be much more depolarized in order to open compared to the WT channel.

You voltage clamp a neuron to study its voltage-gated Na and voltage gated K currents. You adjust the concentrations of extracellular ions such that EK = -80 mV and ENa = +40 mV. Both types of voltage-gated channels open at -50 mV. Draw the current as a function of time that you would see under the following conditions: c) Vcmd = +70 mV

Na current is outward because you are more depolarized than ENa

You voltage clamp a neuron to study its voltage-gated Na and voltage gated K currents. You adjust the concentrations of extracellular ions such that EK = -80 mV and ENa = +40 mV. Both types of voltage-gated channels open at -50 mV. Draw the current as a function of time that you would see under the following conditions: e) Vcmd = 0 mV in the presence of TEA

No K current

You voltage clamp a neuron to study its voltage-gated Na and voltage gated K currents. You adjust the concentrations of extracellular ions such that EK = -80 mV and ENa = +40 mV. Both types of voltage-gated channels open at -50 mV. Draw the current as a function of time that you would see under the following conditions: b) Vcmd = +40 mV

No Na current because you are at ENa

You voltage clamp a neuron to study its voltage-gated Na and voltage gated K currents. You adjust the concentrations of extracellular ions such that EK = -80 mV and ENa = +40 mV. Both types of voltage-gated channels open at -50 mV. Draw the current as a function of time that you would see under the following conditions: d) Vcmd = -70 mV

No current because channels are closed

You are recording from a neuron that has bistable behavior (see image below). If you hyperpolarize it, it fires in a rhythmic, bursting manner. If you keep it depolarized, it fires tonically. b) When you depolarize the neuron, why does it fire tonically, instead of in bursts?

This is because you prevent the membrane from getting hyperpolarized enough to activate Ih, and T-type Ca2+ channels are inactivated, so you don't initiate Ca2+ spikes. You are essentially masking this bursting mechanism. Instead, the neuron fires with standard Na/K spike mechanisms, because you are providing constant depolarization.

You make a mutation to the Na-K ATPas such that the ionic concentrations of Na and K across the membrane have been altered (see below, Na channel is red, K channel is blue). c) Assume the Na and K channels expressed in this neuron are not voltage gated and are always open. If gK = 200 pS and gNa = 200 pS, what is the resting membrane potential of the neuron?

Vm = [(gkEk)+(gNaENa)] / [(gk + gNa)] Vm = (200 pS * 61 mV) + (200 pS * -79.4 mV) / (200 pS + 200 pS) Vm = -9.7 mV

Function of astrocytes

[X] Support the blood brain barrier [X] Regulate blood flow [X] Regulate synaptic plasticity

Revert back to the defaults shown above. Construct an IV plot for the K+ current from -80 mV to +80 mV in 10 mV increments.

standard K channel IV plot

Use the program to measure the Na current when voltage is set from -80 mV to +80 mV, in 10 mV increments. Change command voltage by varying "amplitude" under Stimulus 1. Draw the IV plot.

standard Na channel IV plot

Below are IV plots (real data!) of Na channels measured from cultured neurons. The blue IV plot shows data collected from neurons expressing the original Na type of channel (wildtype, WT). The green and yellow traces show IV plots of mutant variants of Na channels, "RH" in green, and "NK" in yellow. d) In what domain(s) of the channel (S1-S6) is the NK mutation most likely to be found and why?

the S4 voltage sensor

Now let's consider how changes in equilibrium potential change the shape of the IV plot. Change Na+ equilib potential to 0 mV. Draw the IV plot (from -80 mV to +80 mV in 10 mV increments).

x-intercept is now at 0 mV, so it shifted to the left

What type of glial cells give rise to myelin in the peripheral nervous system?

Schwann cells

Below is an image of the Na current elicited with two voltage steps, spaced at variable time intervals (from your MetaNeuron homework). b) If this particular neuron needs 90% of its Na channels available to fire an action potential, what is the absolute refractory period of this cell?

7 ms

You have recently isolated two new compounds known to block certain types of voltage-gated K channels. You drive the neuron with a strong stimulus and see the response labeled "control". A) When you apply Compound A, the firing rate of the neuron is much faster in response to the same stimulus. Why do you think this is the case? B) Which channel do you think was most likely blocked by Compound A?

A) Compound A has probably blocked a K channel that prolongs the relative refractory period. Such a channel would be open between action potentials, creating more K current for the cell to overcome in order to get to spike threshold. With this current blocked, the neuron can get to spike threshold faster, hence, speeding up firing. B) BK channel

Below are IV plots (real data!) of Na channels measured from cultured neurons. The blue IV plot shows data collected from neurons expressing the original Na type of channel (wildtype, WT). The green and yellow traces show IV plots of mutant variants of Na channels, "RH" in green, and "NK" in yellow. e) In a different set of experiments, you study the effects of these Na channel mutations on the firing properties of neurons. You find that neurons expressing the NK (yellow) mutation need a much stronger stimulus in order to fire than either the RH or wildtype channels. Why do you think this is?

Because the NK mutation makes the channels open only at higher voltages. This would mean that Vm would have to become more depolarized before an all-or-nothing action potential would be assured of happening, thus making it harder for neurons to get to spike threshold.

Below are IV plots (real data!) of Na channels measured from cultured neurons. The blue IV plot shows data collected from neurons expressing the original Na type of channel (wildtype, WT). The green and yellow traces show IV plots of mutant variants of Na channels, "RH" in green, and "NK" in yellow. b) In what domain(s) of the channel (S1-S6) is the RH mutation most likely to be found and why?

Because the main effect of RH is on conductance, this would be related to mutations in the S5-6 (pore forming) domains.

What is the cause of demyelination in Charcot Marie Tooth Disease, and which type of glial cells are affected?

CMT is cause by a genetic mutation in the gap junctions (Cx36) that allow nutrients to pass through the extent of the tightly wrapped Schwann cell. With these gap junctions, the Schwann cells die from lack of nutrition.

How do changes in conductance through v-gated Na channels affect the timing or shape of the AP?

Changes in gNa affect the rising phase of the AP but not the falling phase. The AP reaches its peak faster with more gNa. Also, you can see that it gets up to ENa when gNa is high, but doesn't quite reach ENa when gNa is low. This I because with low gNA, the v-gated K channels have time to engage before cell gets all the way to ENa (because rising phase is slower).

How do changes in conductance through v-gated K+ channels affect the timing or shape of action potentials?

Decreasing K+ broadens the action potential. This is because v-gated K+ channels are important for the repolarization phase. They have a very small effect on AP height that you really only see when gK is very low. This is because their kinetics are so slow compared to v-gated Na channels that the cell gets almost all the way to ENa before K+ channels have activated enough to repolarize the cell. When K+ channels are nearly completely absent (g = 1 mS/cm2), you can see the AP height is a little taller. This tells you that at other values of gK, you are getting repolarization a little before the cell has gotten all the way to ENa, but again, this effect is small.

3. If the conductance of the membrane (above) was doubled for K, draw approximately what the new curve would look like on the axis below.

Double slope, same x-intercept

You decrease the extracellular K+ concentration

EK = 61 log [k]out/[k]in]) So, decrease in [k]out = decrease Ek

You are recording from a neuron that has bistable behavior (see image below). If you hyperpolarize it, it fires in a rhythmic, bursting manner. If you keep it depolarized, it fires tonically. a) Describe the mechanism (sequence of ion channel activity) that causes the neuron to burst upon hyperpolarization.

Hyperpolarization causes activation of Ih channels. This drives a Ca2+ spike due to activation of T-type Ca2+ channels, which then gets you depolarized enough to activate L-type Ca2+ channels. The Ca2+ spike is terminated once SK channels have activated enough to bring Vm back down to a very hyperpolarized potential, which triggers opening of Ih, and the cycle starts again. While Vm is depolarized during the Ca2+ spike, Na/K mediated action potentials can take place.

You apply a drug that weakens the Na-K ATPase

Na-K ATPase keeps K concentrations inside the cell high

What type of glial cells give rise to myelin in the central nervous system?

Oligodendrocytes

Below are IV plots (real data!) of Na channels measured from cultured neurons. The blue IV plot shows data collected from neurons expressing the original Na type of channel (wildtype, WT). The green and yellow traces show IV plots of mutant variants of Na channels, "RH" in green, and "NK" in yellow. a) Does the RH mutation (green) affect the channels' voltage sensitivity, conductance or both?

RH seems to reduce the channel's conductance, because there is less current at each holding potential, compared to the WT channel.

Which term best describes depolarization block (choose one):

When a neuron can no longer sustain firing because its Na channels are inactivated