Midterm 1 sample open ended questions

Question 1

A toxin prevents the Na+ /K+ ATPase from working. You apply it to a cell with a resting membrane potential of –60 mV, and you wait several hours. This will have which effect on the cell’s membrane potential?

depolarize
hyperpolarize
no effect

Answer 1

depolarize

This pump sets up the concentration gradients of sodium and potassium which underlie the cell’s membrane potential. Without it, concentrations run down until equal concentrations of both sodium and potassium are outside/inside. This causes the cell’s membrane potential to go to 0mV. Since 0mV is more positive than -60mV, this change in potential is a depolarization

Question 2

A cell has leak channels for Na+, K+ and Cl-. The cell has Na+/K+ ATP pumps,
but no chloride pump, transporter or exchanger. The resting membrane potential of this cell is -60 mV.
You can assume that the equilibrium potentials for Na+ and K+ are those of common neurons. Make no
assumptions about the equilibrium potential of Cl- and instead use the information given in this problem
to answer the questions.

What is the equilibrium potential for chloride in this cell? Explain your answer.

Answer 2

The equilibrium potential for Cl- will be: -60mV.

If there are no pumps, transporters or exchangers, the cell cannot regulate the concentrations of
chloride inside and outside the cell. Therefore, regardless of what the initial concentrations of chloride
are inside/outside the cell, the ions will move in/out in such a way that they will establish a
concentration gradient that would correspond to an equilibrium potential equal to the resting potential of
the cell.

Question 3

If the equilibrium potential for chloride is the same as the resting potential of the neuron, opening chloride channels will cause depolarization or hyperpolarization?

Answer 3

Neither - Net of 0

Question 4

Is a cell’s membrane potential at the peak of an action potential equal to the sodium equilibrium
potential?

Answer 4

No.
The peak of an action potential approaches the sodium equilibrium potential because at that time the
permeability of the membrane to sodium is huge. However, the membrane is still permeable to
potassium, due to open potassium leak channels and a few voltage gated potassium channels that
have already opened, so the peak would be slightly lower than the equilibrium potential of Na+

Question 5

After a voltage gated sodium channel becomes inactivated, how can it be put into a state where it is
ready to open again?

Answer 5

The channels must be in hyperpolarized environments in order to become de-innactivated. The removal
of the inactivation will take a couple of miliseconds from the moment the cell becomes hyperpolarized
below -50mV.

Question 6

Why is having a long time constant advantageous for dendrites?

Answer 6

It allows the summation of multiple inputs across a larger temporal window, allowing for larger overall
local responses which will have a higher chance of reaching the cell body before decaying. (temporal
summation)

Question 7

You inject current into an axon halfway between the axon hillock and the axon terminal. This current
causes the axon to depolarize above action potential firing threshold.

Which direction will the action potential travel along the axon?

Answer 7

Both directions - The axon does not have any intrinsic polarity making the action potential travel one way or the
other. Since the action potential is being initiated in the middle of the axon, there will not be any
area of the axon that already has inactivated voltage gated sodium channels, so the action
potential can move both ways.

Question 8

A neuron with a resting membrane potential of –58 mV is at room temperature. It has leak chloride channels that are open at rest. The cell does not have any chloride pumps,
transporters or exchangers. You measure the extracellular chloride concentration and discover that it is
10 mM. What is the intracellular chloride concentration? (be sure to show your work and explain your
answer!)

Answer 8

1nM

Because the cell does not regulate chloride concentrations, chloride ions will reach equilibrium at
concentrations where the chloride equilibrium potential is equal to the resting membrane potential.

so it you set Ecl = rmp and plug in everything to solve for internal chloride concentration, you’ll get 1nm