Lecture 3 - Electrical Signals of Nerve Cells (The Resting Membrane Potential)

Question 1

What is the name of this equation?

Answer 1

Nernst Equation

Question 2

True or False?:

Capacitance is a measure of permeability to ionic flux.

Answer 2

False

Conductance is a measure of permeability to ionic flux.

Question 3

True or False?:

Ion channels are protein structures.

Answer 3

True

Question 4

True or False?:

Under normal conditions, the V_m is set solely by E_Na.

Answer 4

False

Under normal conditions, the V_m is set by some balance of the E_K, E_Na, and E_Cl.

Question 5

Why can’t ions cross the membrane without channel proteins? What do ion channels do to selectively allow them to cross the membrane?

Answer 5

Ions in solution carry a hydration shell which can not pass through the hydrophobic interior of the lipid bilayer membrane. Ion channels selectively strip ions of this hydration shell so that they can cross.

Question 6

True or False?:

Just like at rest, changing [Na]_O during an action potential has a very little impact on membrane voltage.

Answer 6

False

Unlike at rest, changing [Na]_O during an action potential has a large impact on membrane voltage.

Question 7

True or False?:

Receptor & synaptic potentials are graded. Action potentials are “all-or-nothing” and generative.

Answer 7

True

Question 8

True or False?:

One way to think about the resting membrane potential is that its a way for the cell to store energy in the form of ion gradients.

Answer 8

True

Question 9

Explain why there is flux of K⁺ from 1 to 2 on the left, no flux of K⁺ in the middle, and flux of K⁺ from 2 to 1 on the right (membrane is permeable to K⁺).

Answer 9

On the left, there is no electromotive force to move ions so the only factor is the concentration gradient, which moves K⁺ from where there is a high concentration (compartment 1) to a low concentration (compartment 2). In the middle, the voltage is set at the equilibrium voltage (given by the Nernst equation), so the movement from compartment 1 to compartment 2 due to the concentration gradient is equal to the movement from compartment 2 to compartment 1 due to the charge imbalance. On the right, the charge imbalance is increased, so there is more movement from compartment 2 to compartment 1 due to that than from compartment 1 to compartment 2 due to the concentration gradient.

Question 10

What does the Nernst equation do?

Answer 10

The Nernst equation restates the concentration gradient in electrical terms (membrane voltage at which there is no net movement of ions). It finds the most energetically favoured membrane voltage for a given concentration gradient across the membrane.

Question 11

If you inject negative current into a cell, does it depolarize or hyperpolarize the cell?

Answer 11

Hyperpolarize

Question 12

Explain what is happening in the following diagram.

Answer 12

A membrane permeable only to K⁺ (gold spheres) separates compartments 1 and 2, which contain the indicated concentrations of KCl. As K⁺ ions diffuse and encounter the membrane channels, more will pass from the side where [K⁺] is higher, simply by chance. Consequently, there is a net movement of K⁺ ions from the high concentration (compartment 1) to the low concentration (compartment 2) sides. However, this now separates the positive charge of the K⁺ ions (compartment 2) from the negative charge of the Cl^- ions (compartment 1), creating an electrical potential difference between the two sides. This potential difference results in an electrostatic force that drives K⁺ ions back from compartment 2 to the negative compartment 1 (because opposites attract). At equilibrium, the number of K⁺ ions diffusing down their concentration gradient into compartment 2 equals the number of K⁺ ions drawn back into compartment 1 due to the charge inbalance.

Question 13

Who discovered the squid giant axon’s function?

Answer 13

John Z. Young

Question 14

The GHK equation calculates a weighted “average” of Nernst potentials for multiple ions, using relative permeabilities as the weighing factor.

Answer 14

True

Question 15

What ions contribute to the resting membrane potential? Do they all contribute equally?

Answer 15

Sodium, potassium, and chloride contribute to the resting membrane potential. Though, they don’t all contribute equally.

Question 16

How does the negative resting potential help the neuron?

Answer 16

The normal negative resting membrane potential helps the neuron because it is a way to store the energy used for rapid signaling.

Question 17

True or False?:

Applying a voltage across the membran will modify its ionic flux.

Answer 17

True

Question 18

What is the diameter of the squid giant axon? What is the diameter of a mammalian axon?

Answer 18

The squid giant axon has a diameter of approximately 800 µm while a typically mammalian axon is approximately 2 µm in diameter.

Question 19

Do ion transporters or ion channels create ion concentration gradients?

Answer 19

Ion Transporters

Question 20

What is the advantage to having a hyperpolarized resting membrane potential?

Answer 20

Cells have a hyperpolarized resting membrane potential so that they can send signals very rapidly without having to consume ATP or do some sort of slow metabolic reaction. They use ATP earlier through the transporters to establish a gradient that gives them access to “instant energy” to move charge across the membrane and propagate signals.

Question 21

How does varying the membrane permeabilities for Na⁺ and K⁺ cause an action potential?

Answer 21

At rest, the membrane is more permeable to potassium than sodium. Since potassium’s equilibrium potential is very low (-84 mV), it results in a negative, hyperpolarized membrane potential. During an action potential, the membrane permeability of sodium increases and becomes greater than that of potassium. Since sodium’s equilibrium potential is very high (+57 mV), it has a very large driving force that causes sodium to rush into the cell to depolarize it to a positive potential closer to that of sodium’s equilibrium potential. After the peak of the action potential, the permeability of sodium decreases back to its resting permeability and the cell goes back to its resting potential.

Question 22

True or False?:

Chemical diffusion is balanced by electrostatic forces at equilibrium.

Answer 22

True

Question 23

Glutamate receptors pass both K and Na upon binding glu. Why does this cause depolarization?

Answer 23

This causes the membrane to depolarize towards the average of E_K and E_Na which is close to 0 mV. This is the basis of the so-called excitatory postsynaptic potential (EPSP).

Question 24

True or False?:

The GHK equation describes in electrical terms (mV) the force produced by each ion’s gradient. The Nernst equation predicts the resting membrane potential (V_m) at which all 3 ions will be at equilibrium.

Answer 24

False

The Nernst equation describes in electrical terms (mV) the force produced by each ion’s gradient. The GHK equation predicts the resting membrane potential (V_m) at which all 3 ions will be at equilibrium.

Question 25

Who is responsible for the third law of thermodynamics, the [his name] lamp, the Neo-Bechstein-Flügel electric piano, chemical warfare, and the [his name] equation?

Answer 25

Walther Nernst

Question 26

What happens if you suddenly greatly increase the membrane permeability to Na⁺ ions?

• Na⁺ ions will rush out of the cell with K⁺ ions because they share the same charge.
• Na⁺ ions will rush into the cell because of their concentration gradient ([out] > [in]) which will rapidly equalize [Na⁺] inside and outside the cell.
• Na⁺ ions will rush into the cell causing the extracellular Na⁺ level to drop dramatically.
• Na⁺ ions will rush into the cell which will rapidly cause the membrane potential to find a new stable level nearer to Na⁺’s equilibrium voltage.
• Cell membrane voltage will go even lower as Na⁺ exits the cell making it more negative.

Answer 26

Na⁺ ions will rush into the cell which will rapidly cause the membrane potential to find a new stable level nearer to Na⁺’s equilibrium voltage.

Question 27

Do ion transporters or ion channels allow ions to diffuse down their concentration gradient?

Answer 27

Ion Channels

Question 28

True or False?:

The driving force on an ion is lower the further V_m gets from that ion’s equilibrium potential.

Answer 28

False

The driving force on an ion is greater the further Vm gets from that ion’s equilibrium potential.

Question 29

True or False?:

Neurons are able to actively transport enough ions to significantly change the extracellular space.

Answer 29

False

The extracellular space is infinite as far as the cell is concerned, so one cell is not able to pump enough ions to significantly change the extracellular space. Though, it may have a small local impact.

Question 30

What is the name of the group of passive membrane channels that contribute to the permeability of the membrane at rest?

Answer 30

Leak Channels

Question 31

What do ion transporters typically consume to transport ions?

Answer 31

ATP

Question 32

In which directions will potassium and sodium ions want to flow across the membrane (through ion channels)? Why don’t they actually exit or enter the cell in the directions their gradients would dictate?

Answer 32

Sodium ions will want to flow into the cell and potassium ions will want to flow out of the cell. They don’t actually do this because the channels are highly-selective and the ions have an electrostatic force.

Question 33

Do ion transporters or ion channels actively move ions against their concentration gradient?

Answer 33

Ion Transporters

Question 34

True or False?:

The intracellular space typically has a high concentration of sodium and chloride and a low concentration of potassium while the extracellular space typically has a high concentration of potassium and a low concentration of sodium.

Answer 34

False

The extracellular space typically has a high concentration of sodium and chloride and a low concentration of potassium while the intracellular space typically has a high concentration of potassium and a low concentration of sodium.

Question 35

What does the Nernst equation look like?

Answer 35

Question 36

What does the Nernst equation look like at 25 °C when the natural logarithm has been converted into a base 10 logarithm.

Answer 36

E = (58 mV / z) * log([ion_out]/[ion_in])

Question 37

What are the relative permeabilities of potassium, sodium, and chloride in a resting neuron?

Answer 37

P_K : P_Na : P_Cl = 1.0 : 0.04 : 0.45

Question 38

What is driving force?

Answer 38

Driving force is the force that drives ions into or out of a cell. When a channel for an ion with a high driving force (like sodium) opens, its going to cause a big change in membrane voltage, whereas the opening of a channel for an ion with a low driving force (like chloride), its going to produce a relatively small change in membrane voltage. The driving force for ion flux is equal to V_m - E_x where V_m is membrane potential and E_x is the equilibrium potential (reversal potential) for an ion.

Question 39

What does the Goldman-Hodgkin-Katz (GHK) equation look like?

Answer 39

Question 40

What is RT/F equal to at 25 °C?

Answer 40

0.0257 J/C = 0.0257 V = 25.7 mV

Question 41

GABA can activate either GABA-A or GABA-B receptors. GABA-A receptors conduct Cl^- in response to GABA which transiently forces V_m down towards E_Cl and GABA-B indirectly causes more K⁺ channels to open, forcing V_m down towards E_K. If the resting V_m is higher than E_Cl or E_K then GABA will result in an IPSP. But sometimes inhibition still is effective when, for example, E_Cl and V_m are the same. Why?

Answer 41

Although the driving force for Cl^- is small at rest (since E_Cl = -67 mV), as the cell gets depolarized by EPSPs, the driving force for chloride will get proportionally larger. Now, in addition to Na⁺ and K⁺ trying to depolarize the membrane to their equilibrium potential, you have to factor in Cl^- which would be able to hyperpolarize the cell (since its driving force is increased from the depolarization). In essence, the more you depolarize the cell, the better the GABA receptor will be at pulling it back down. This “clamping” of the voltage is known as shunting inhibiton.