Action Potential

Question 2

What is an action potential?

Answer 2

A rapid, regenerative electrical signal caused by depolarization of the membrane that conveys information along the axon.

Question 3

What does it mean that the resting membrane is ‘polarized’?

Answer 3

The inside of the neuron is negatively charged relative to the outside.

Question 4

What is depolarization?

Answer 4

A shift in membrane potential toward more positive values.

Question 5

What is hyperpolarization?

Answer 5

A shift in membrane potential toward more negative values.

Question 6

What is the threshold of an action potential?

Answer 6

The membrane potential that must be reached to trigger opening of voltage-gated Na+ channels and initiate a spike.

Question 7

What happens during the rising phase of the action potential?

Answer 7

Na+ channels open rapidly, allowing massive Na+ influx that drives the membrane potential upward.

Question 8

What happens during the falling phase of the action potential?

Answer 8

Na+ channels inactivate and voltage-gated K+ channels open, causing K+ efflux that repolarizes the membrane.

Question 9

What is the undershoot (after-hyperpolarization)?

Answer 9

A period where membrane potential becomes more negative than resting due to prolonged K+ channel opening.

Question 10

What is the absolute refractory period?

Answer 10

The period when no new action potential can be generated because Na+ channels are inactivated.

Question 11

What is the relative refractory period?

Answer 11

The period after the absolute refractory period when a stronger-than-normal stimulus is required to trigger a spike.

Question 12

What feature of the action potential conveys information?

Answer 12

The frequency and timing of action potentials, not their amplitude.

Question 13

Why are action potentials ‘all-or-none’?

Answer 13

Once threshold is reached, the full spike occurs; subthreshold events do not trigger partial spikes.

Question 14

What causes the rapid depolarization during the rising phase?

Answer 14

Rapid opening of voltage-gated Na+ channels and inward Na+ current.

Question 15

What causes rapid repolarization during the falling phase?

Answer 15

Closing of Na+ channels and opening of voltage-gated K+ channels causing outward K+ current.

Question 16

What did Hodgkin and Huxley demonstrate using the squid axon?

Answer 16

They identified voltage-gated Na+ and K+ currents as the basis of the rising and falling phases of the action potential.

Question 17

What does the voltage-clamp technique do?

Answer 17

Holds the membrane voltage constant to measure ionic currents needed to maintain that voltage.

Question 18

What did voltage-clamp experiments reveal about Na+ channels?

Answer 18

They open rapidly after depolarization but inactivate within milliseconds.

Question 19

What did voltage-clamp experiments reveal about K+ channels?

Answer 19

They open more slowly and stay open as long as the membrane remains depolarized.

Question 20

What happens when Na+ channels inactivate?

Answer 20

Na+ influx stops, contributing to the end of the rising phase and preventing immediate re-firing.

Question 21

What happens when K+ channels open fully?

Answer 21

The membrane potential is pulled toward the K+ equilibrium potential, causing the falling phase and undershoot.

Question 22

What is deinactivation of Na+ channels?

Answer 22

The process by which Na+ channels reset from inactivated to closed state after repolarization.

Question 23

Why does the action potential travel in one direction?

Answer 23

The absolute refractory period prevents Na+ channels from reopening behind the spike.

Question 24

Where is an action potential normally initiated?

Answer 24

At the axon initial segment (hillock), where voltage-gated Na+ channel density is highest.

Question 25

What does tetrodotoxin (TTX) do?

Answer 25

Blocks voltage-gated Na+ channels, preventing action potentials.

Question 26

How can Na+ channel mutations cause febrile seizures?

Answer 26

Temperature-dependent increases in excitability can lead to excessive firing when channel gating is altered.

Question 27

What determines the activation threshold for an action potential?

Answer 27

The balance of Na+ inward current versus K+ leak currents and membrane depolarization.

Question 28

What are the steps of an action potential?

Answer 28

1. Initial depolarization → 2. Threshold reached → 3. Na+ channels open → 4. Rising phase → 5. Na+ inactivation + K+ opening → 6. Falling phase → 7. Undershoot → 8. Return to rest.

Question 29

What ensures the regenerative nature of the action potential?

Answer 29

Local depolarization triggers opening of adjacent voltage-gated Na+ channels.

Question 30

What challenge do long axons face during signal propagation?

Answer 30

Signal attenuation and the need to regenerate the spike repeatedly over long distances.

Question 31

What role does myelin play in action potential propagation?

Answer 31

Myelin increases membrane resistance and decreases capacitance, speeding conduction.

Question 32

What is saltatory conduction?

Answer 32

Action potentials 'jump' from node to node (Nodes of Ranvier), increasing speed and efficiency.

Question 33

What happens at the Nodes of Ranvier?

Answer 33

Voltage-gated Na+ channels regenerate the action potential where myelin is absent.

Question 34

What happens if myelin is lost, as in MS?

Answer 34

Action potentials slow or fail because current leaks out and cannot reach threshold at distant nodes.

Question 35

Why do K+ channels contribute to the undershoot?

Answer 35

They remain open briefly after repolarization, allowing continued K+ efflux.

Question 36

Why is the amplitude of the action potential always the same?

Answer 36

Ion gradients and channel properties are fixed, making spikes uniform regardless of stimulus strength.

Question 37

Why does a stronger stimulus increase firing rate rather than spike amplitude?

Answer 37

Greater depolarization triggers more frequent threshold crossings, not larger spikes.

Question 38

How do patch-clamp recordings work?

Answer 38

A tiny electrode seals onto a small membrane patch to measure currents through single ion channels.

Question 39

What did patch-clamp reveal about Na+ channels?

Answer 39

They open in brief, stochastic bursts but collectively produce predictable macroscopic currents.

Question 40

What determines the speed of action potential propagation?

Answer 40

Axon diameter, myelination, and the density/distribution of voltage-gated channels.

Question 41

Why does increased axon diameter increase conduction velocity?

Answer 41

It reduces internal resistance to ion flow along the axon.

Question 42

What is the effect of temperature on action potentials?

Answer 42

Higher temperatures speed channel kinetics; very high temperatures can destabilize firing patterns.

Question 43

What is delayed rectification?

Answer 43

The slow opening of K+ channels in response to depolarization, allowing repolarization.

Question 44

Why is Na+ influx self-reinforcing?

Answer 44

Depolarization opens more Na+ channels → more Na+ enters → further depolarization (positive feedback).

Question 45

Why is K+ efflux stabilizing?

Answer 45

It counteracts depolarization and restores resting membrane potential (negative feedback).

Question 46

What prevents infinite positive feedback during the rising phase?

Answer 46

Na+ channel inactivation and delayed K+ channel opening.

Question 47

What is the ionic basis of the refractory periods?

Answer 47

Absolute: Na+ channels inactivate. Relative: K+ channels still open, making neurons harder to depolarize.

Question 48

Why is the initial segment the spike trigger zone?

Answer 48

It has the highest density of voltage-gated Na+ channels, making it the easiest site to reach threshold.