Lecture 2 - Chapter 2/4: Action Potentials are Generated by Ion Currents

Question 1

What is electric current?

Answer 1

Flow of electrical charge, i.e. flow of potassium and sodium (ampere = coulomb/second)

Question 2

What is membrane resistance?

Answer 2

A measure of how much the membrane opposes the passage of electrical charge (through ion channels)

Question 3

What is membrane conductance?

Answer 3

The permeability of the cell membrane to those ions

Question 4

What is voltage?

Answer 4

Difference in charge between two points (i.e. membrane gradient). It creates energy that pushes charge to move.

Question 5

What is the membrane potential?

Answer 5

The voltage difference between the inner and outer surface of the cell membrane.

Question 6

What is meant by the fact that membranes are capacitors?

Answer 6

The membrane, composed of a lipid bilayer, is impermeable to ions. It can therefore store charges.

Question 7

What technique is able to control the membrane potential and simultaneously measure their underlying permeability changes? And what animal was used to study electrical signals?

Answer 7

A voltage clamp, first used in squids. Squids have enormous axons of 1 mm in diameter compared to 1-2 um in mammals.

Question 8

How is voltage clamp measurement achieved?

Answer 8

The device is able to control membrane potentials at any level desired.

1. Here, the membrane potential is measured by an electrode placed inside the cell, e.g. on an axon. This electrode is connected to the voltage clamp amplifier.
2. The amplifier compares this membrane potential with the desired (command) potential.
3. When Vm/membrane potential is different from the command potential, the clamp amplifier injects current into the axon through a second electrode. This way the membrane potential becomes the same as the command potential.
4. The current flowing back into the axon, and thus across its membrane, can be measured here.

Question 9

What is a capacitive current?

Answer 9

The capacitive current determines how fast the cell membrane potential responds to the flow of ion channel currents. It is the flow in membrane current due to a changing potential of the electrode which charges or discharges the membrane (i.e. the capacitor) → thus only flows when the membrane potential is changing.

Question 10

What happens when the membrane potential is hyperpolarized from the resting level (-65 mV) to -130 mV?

Answer 10

There’s redistribution of charge across the axonal membrane. The capacitive current is a nearly instanteneous. Besides this, very little current flows when the membrane is hyperpolarized.

Question 11

What happens when the membrane potential is depolarized from -65 mV to 0 mV?

Answer 11

First, there is a outward capacitive current. After this, a transient inward current is rapidly produced, where positive charges enters the cell. This results in a more slowly rising delayed outward current.

Question 12

What ions are (probably) responisble for the transient inward and delayed outward current?

Answer 12

• Transient inward current → sodium
• Delayed outward current → potassium

Question 13

Name three ways how it can be tested what ions are involved e.g. depolarization or hyperpolarization.

Answer 13

1. By determining at what membrane potential the current reverses and comparing it to the electrochemical equilibrium potential of ions in solution
2. By changing the external concentrations of certain ions and testing if the current is altered or reversed.
3. By blocking certain channels and testing if this also block the current.

Question 14

So it’s most likely sodium is responsible for the transient inward current during depolarization. Already mentioned is that this can be tested by blocking sodium channels. How is this achieved?

Answer 14

By blocking the voltage-gated Na+ channels with tetrodotoxine (TTX) (but this wasn’t possible in the earlier days so they used something else)

Question 15

So it’s most likely sodium is responsible for the transient inward current during depolarization. Already mentioned is that this can be tested by changing the external/extracellular concentration of sodium ions. What happens to the inward current when sodium is removed from the extracellular space?

Answer 15

The inward current changes to an outward current. When sodium is recovered extracellularly, the inward current is also recovered. This shows that for this inward current, extracellular sodium is needed.

(This can also be tested for potassium, where potassium is removed intracellularly, which removes the delayed outward current).

Question 16

Another method that is commonly used to research membrane potentials is the Patch Clamp method. What’s different in this method compared to the Voltage Clamp method?

Answer 16

• The Voltage Clamp method used giant squid axons to control the membrane potential and simultaneously measure their underlying permeability changes.
• The Patch Clamp method is used to study the function of single ion channels in cells.

Question 17

Explain how the Patch Clamp method works.

Answer 17

1. Patch-clamping setup → a micropipette (diameter 1 µm) is placed against a cell.
2. Membrane patch isolation → gentle suction is applied to form a tight seal between the pipette and the plasma membrane. Typically only one or few channels will be in the membrane within the pipette.
3. The flow of ions is recorded while the membrane is subjected to a depolarizing step in voltage, yielding traces of individual Na+ currents during channel opening.

Question 18

What can be recorded when multiple ion channels are sealed between the pipette and the plasma membrane?

Answer 18

The macroscopic sodium flow

Question 19

Where is the ionic current/flow proportionate to?

Answer 19

Ionic current = conductance x (membrane potential - equilibrium potential)

Note: conductance is permeability

Question 20

How does ion conductivity increase?

Answer 20

By the opening/closing of channels.

So what you see in this picture, is that first the ion conductance of sodium increases rapidly (which means that sodium channels are opening), which is followed by a slight increase in potassium conductance (i.e. opening of potassium channels).

Question 21

What happens during a resting membrane potential?

Answer 21

During the resting membrane potential, there’s a higher potassium conductance than for sodium (g_K > g_Na). Furthermore, there’s more Na+ outside the cell then inside and there’s more K+ inside the cell then outside.

Question 22

Explain the steps that occur during an action potential.

Answer 22

1. Na+ channels open → g_Na increases
2. Na+ enters the cell
3. Membrane potential gets depolarized further
4. Electrochemical driving force for Na+ diminishes (when the membrane potential becomes more positive, more Na+ channels close again)
5. K+ channels open and g_K increases and the Na+ current is inactivated
6. K+ exits the cell (and the membrane potential gets less positive → repolarization)
7. K+ channels also close

Note: as the picture also states → any depolarizing force will bring the membrane potential closer to threshold. This will cause Na+ channels to open.

Question 23

What is meant by the fact that action potentials are self-supported and are thus termed as a ‘all- or nothing event’ and why is this important?

Answer 23

That an action potential is always a full response. Either a stimuli stimulates the membrane to reach the threshold or the stimuli isn’t strong enough to stimulate the membrane to reach the threshold. So, there’s no such thing as a strong or weak action potential.

This minimizes the possibility that information will be lost along the way.

Question 24

What’s the refractory period?

Answer 24

The last step of an action potential is the closing of the potassium channels. But since these channels are a little slow with closing, K+ ions are still capable to exit the cell. This makes the membrane potential go even more negative than the resting membrane potential, which is called hyperpolarization. During this event, the cell is not capable of creating a new action potential. This is called the refratory period.

Question 25

How does the cell recover during the refractory period (i.e. how does the cell go back to its resting membrane potential)?

Answer 25

There's a sodium potassium **pump**, which is responsible for putting sodium and potassium back in place. So it ensures that more sodium is moved out of the cell than K+ exits the cell via its K+ channel. This restores the membrane potential to its resting membrane potential.

Question 26

What is meant by the fact that action potentials can self propagate?

Answer 26

It means that one action potential automatically triggers the neighboring membrane areas into producing an action potential via local currents. Local currents induce depolarisation of the adjacent axonal membrane and where this reaches a threshold, further action potentials are generated. Thus once threshold is reached action potentials always propagate down the axon to the synaptic region of the axon.

Question 27

Action potentials do **not** propagate backwards. What is meant by this and how is this possible?

Answer 27

An action potential is propagated when the local current that is induced by the **first action potential** triggers the adjacent (Na+) channels to initiate the next action potential, so that the action potential can be moved down the axon to the synaptic region of the axon. Action potentials can not propagate backwards means that action potentials can not be activated at the same place where the previous action potential has occured. This is due to the fact that the sodium channels that have been opened by the previous action potential are still inactivated. Therefore, at this place an action potential can not propagate.

Question 28

What process speeds up action potential propagation?

Answer 28

Myelination → myelin is produced by glial cells and causes the action potential to propagate uicker since myelin is a very good isolator. This allows neurotransmission to be very fast.

Question 29

Just study this picture. If you understand this picture, you also probably understand everything that is discussed before.

Answer 29

Ok