Action Potential

Question 1

What are the phases of the action potential?

Answer 1

1- Rising (depolarizing) phase
2- Overshoot phase
3- Falling (repolarizing) phase
4- Undershoot phase

Question 2

What phase incorporates the refractory period?

Answer 2

undershoot (hyperpolarization) phase

Question 3

Which ion is essential for the generation of an action potential?

Answer 3

Sodium (Na+ channels contribute to the depolarizing phase)

Question 4

What results if external sodium levels are decreased?

Answer 4

Smaller and slower action potentials

Question 5

What are the three different voltage sensitive mechanisms that generate action potentials?

Answer 5

1- activation of Na+ conductance
2- delayed activation of K+ conductance
3- inactivation of Na+ conductance

Question 6

What does the release of K+ from the cell cause?

Answer 6

repolarization and hyperpolarization

Question 7

Can a cell “fire” action potentials in response to stimulation during the refractory period?

Answer 7

When in the refractory period the cell does not”fire” action potentials in response to stimulation.

Question 8

Why does hyper polarization occur?

Answer 8

The K+ channels stay open longer than the Na+ channels.

Question 9

Explain how Hodgkin and Huxley demonstrated how volt gated channels work?

Answer 9

They varied the membrane potential in squid giant axon to measure the changes in membrane conductance through volt gated Na+ and K+ channels.

Question 10

What two types of voltage-dependent ionic currents were revealed in Volt-Clamp technique?

Answer 10

Early inward current Na+ and Late outward K+

Question 11

Which drug blocks sodium channels and not potassium channels?

Answer 11

Tetrodotoxin (TTX)

Early current is blocked (because no Na+)

Question 12

Which drug blocks potassium channels and not sodium channels?

Answer 12

Tetraethylammonium (TEA)

Late current is blocked (because no K+)

Question 13

What is active current flow?

Answer 13

the gating of the voltage-gated channels and associated influx of Na+ current

Question 14

what is passive current flow?

Answer 14

The depolarization wave that precedes the action potential.

Question 15

Is there passive current flow though the membrane?

Answer 15

No, only through channels

Question 16

What is the purpose of passive current flow?

Answer 16

To discharge the membrane capacitance and lead to sodium channel activation

Question 17

What occurs during the absolute refractory period?

Answer 17

Na+ channel inactivation (which limits the frequency of firing and allows the signal to be propagated in a single direction)

Question 18

What is the relative refractory period?

Answer 18

Brief hyperpolarized membrane potential due to open resting and voltage gated K+ channels that ends when the voltage gated K+ channels close

Question 19

What are the four different colt-sensitive mechanisms of action potentials?

Answer 19

1- activation of Na+ conductance
2- activation of K+ conductance
3- inactivation of Na+ conductance
4- closing of voltage gated K+ channels

Question 20

Which type of cells form myeline sheaths?

Answer 20

Glial cells (oligodendrocytes in the CNS and Schwann cells in the PNS)

Question 21

What is the purpose of myelination?

Answer 21

1- increases insulation which leads to reduced leak of passive flow

2- decreased capacitance (which is irreversible to the thickness of the membrane)

Question 22

What are nodes of ranvier?

Answer 22

gaps in myelin sheath that contain full compliment of Na+ and K+ channels, where action potentials can be generated

Question 23

What is the conduction velocity of unmylinated nerve as compared to the myelinated nerve?

Answer 23

unmyelinated= 0.5 to 10 m/s

myelinated= about 150 m/s

Question 24

What is saltatory conduction?

Answer 24

Action potential that jumps from node to node