Chapter 4 - The action potential

Question 1

What are other names for the action potential?

Answer 1

Spike, nerve impulse, and discharge.

Question 2

What are the parts of an action potential?

Answer 2

1. Rising phase
2. Overshoot
3. Falling phase
4. Undershoot
5. Gradual restoration of the resting potential

Question 3

How long does an action potential last?

Answer 3

About 2 milliseconds (msec)

Question 4

How does the action potential begin in the thumbtack example?

Answer 4

1. The thumbtack enters the skin
2. The membrane of the nerve fibers in the skin is stretched
3. Na+ permeable channels open
   - Because of the large concentration gradient and the negative charge of the inside of the membrane, Na+ crosses the membrane through these channels and depolarizes the membrane; the cytoplasmic surface of the membrane becomes less negative.

Question 5

What causes action potentials?

Answer 5

Depolarization of the membrane beyond the threshold.

Question 6

What affects the rate of action potential generation?

Answer 6

The magnitude of the continuous depolarizing current.

Question 7

What is the maximum firing frequency of action potentials?

Answer 7

About 1000 Hz. Once an action potential is initiated, initiating another one is impossible for about 1 msec. This is the ABSOLUTE REFRACTORY PERIOD.

In addition, it can be relatively difficult to initiate another action potential for several milliseconds after the end of the absolute refractory period. This is the RELATIVE REFRACTORY PERIOD. The amount of current required for depolarization is elevated above normal during this period.

Question 8

What is OPTOGENETICS?

Answer 8

Controlling neural activity with light. Introducing into neurons foreign genes that express membrane ion channels that open in response to light.

Question 9

What is the relationship between the ionic driving force, ionic conductance, and the amount of ionic current that will flow, for K+?

Answer 9

I_K = g_K (V_m - E_K), or more generally

I_ion = g_ion (V_m - E_ion)

Question 10

Explain the ins and outs of an action potential as in the example on page 90.

Answer 10

1. Membrane permeable only to K+, V_m = E_K = -80 mV
2. Membrane not permeable to Na+, so there is a large driving force on Na+ ([V_m - E_Na] = [- 80mV - 62 mV] = -142 mV
3. When the ionic permeability of the membrane is changed, there is a large driving force pushing on Na+. Thus, we can generate a large sodium current, I_Na, across the membrane.
4. Assuming the membrane permeability is now far greater to sodium than to potassium, the influx of Na+ depolarize the neuron until V_m approaches E_Na, 62 mV.
5. The membrane potential could rapidly be reversed by switching the dominant membrane permeability from K+ to Na+.
6. The falling phase occurs when sodium channels quickly close and potassium channels remain open: K+ flows out of the cell until the membrane potential again equals E_K.

Question 11

What is the VOLTAGE-GATED SODIUM CHANNEL?

Answer 11

The protein forms a pore in the membrane that is highly selective to Na+, and the pore is opened and closed by changes in membrane voltage.

It is created from a single long polypeptide, and has four distinct domains.

Question 12

What is the VOLTAGE-GATED POTASSIUM CHANNEL?

Answer 12

Gates that allow potassium to flow through, and open in response to depolarization of the membrane. They open about 1msec after depolarization, unlike potassium gates. This is why this conductance is called the delayed rectifier.

There are many different types of voltage-gated potassium channels.

Question 13

Explain the stage of action potential: THRESHOLD

Answer 13

The membrane potential at which enough voltage-gated sodium channels open so that the relative ionic permeability of the membrane favors sodium over potassium.

Question 14

Explain the stage of action potential: RISING PHASE

Answer 14

When the inside of the membrane has a negative electrical potential, there is a large driving force on Na+. Therefore, Na+ rushes into the cell through the open sodium channels, causing the membrane to rapidly depolarize.

Question 15

Explain the stage of action potential: OVERSHOOT

Answer 15

Because the relative permeability of the membrane greatly favors sodium, the membrane potential goes to a value close to E_Na, which is over 0 mV.

Question 16

Explain the stage of action potential: FALLING PHASE

Answer 16

The behavior of two types of channels contributes to the falling phase.

1. Voltage-gated sodium channels inactivate
2. Voltage-gated potassium channels finally open
3. K+ rushes out of the cell, causing membrane to become negative again.

Question 17

Explain the stage of action potential: UNDERSHOOT

Answer 17

The open voltage-gated potassium channels add to the resting potassium membrane permeability. Because there is very little sodium permeability, the membrane potential goes toward E_K, causing a hyperpolarization relative to the resting membrane potential until the voltage-gated potassium channels close again.

Question 18

Explain the stage of action potential: ABSOLUTE REFRACTORY PERIOD

Answer 18

Sodium channels inactivate when the membrane becomes strongly depolarized. Cannot be activated again, until the membrane potential becomes sufficiently negative to deinactivate the channels.

Question 19

Explain the stage of action potential: RELATIVE REFRACTORY PERIOD

Answer 19

The membrane potential stays hyperpolarized until the voltage-gated potassium channels close. Therefore, more depolarizing current is required to bring the membrane potential to threshold.

Question 20

What is the force that maintains the ionic concentration gradients that drive Na+ and K+ through their channels during the action potential?

Answer 20

The sodium-potassium pump

Question 21

What is ORTHODROMIC CONDUCTION?

Answer 21

Action potentials in the direction from the soma to the axon terminal.

Question 22

What is ANTIDROMIC CONDUCTION?

Answer 22

Backward propagation, from the axon terminal to the soma.

Question 23

What is the typical velocity of an action potential?

Answer 23

10m / sec. The length of the membrane engaged in it can be calculated since we know that from start to finish it takes about 2 msec.

Question 24

Does the current flow down the inside of the axon or across the axonal membrane?

Answer 24

It depends on whether the axon is wide and if there are many open membrane pores. Wide axon = flows inside axon. Many open membrane pores = most will flow down the membrane.

Action potential conduction velocity increases with increasing axonal diameter.

Question 25

How does myelin affect action potential conduction?

Answer 25

The myelin sheath consists of many layers of membrane. It facilitates current flow down inside the axon, thereby increases the action potential conduction velocity.

Question 26

What is SALTATORY CONDUCTION?

Answer 26

The skipping of action potentials from node to node (of Ranvier), resulting in quick propagation.

Question 27

What is the SPIKE-INITIATION ZONE?

Answer 27

The AXON HILLOCK, which contains voltage-gated sodium channels.

Question 28

Define membrane potential (V_m) and sodium equilibrium potential (E_Na). Which of these, if either, changes during the course of an action potential?

Answer 28

Membrane potential: The voltage across the neuronal membrane at any moment. Changes during an action potential.
Sodium equilibrium potential: The steady equilibrium potential that would occur if the membrane were permeable only to that ion.

Question 29

What ions carry the early inward and late outward currents during the action potential?

Answer 29

Sodium and potassium ions.

Question 30

Why is the action potential referred to as “all-or-none”?

Answer 30

Because it either occurs completely if the threshold is reached, or does not occur at all.

Question 31

Some voltage-gated K+ channels are known as delayed rectifiers because of the timing of their opening during an action potential. What would happen if these channels took much longer than normal to open?

Answer 31

The membrane potential would take much longer to reset to normal.

Question 32

Imagine we have labeled tetrodotoxin (TTX) so that it can be seen using a microscope. If we wash this TTX onto a neuron, what parts of the cell would you expect to be labeled? What would be the consequence of applying TTX to this neuron?

Answer 32

TTX clogs the Na+ permeable pores, and blocks all sodium-dependent action potentials.

Question 33

How does action potential conduction velocity vary with axonal diameter? Why?

Answer 33

The action potential conduction velocity increases with axonal diameter, since the potential travels quicker in the axon than in the membrane.