Membrane depolarization and the action potential

Question 1

What are the factors that determine whether a presynaptic release will result in propagation of an action potential by postsynaptic cell?

Answer 1

Stimulus number
Stimulus magnitude
Excitatory/Inhibitory stimulus
Stimulus location

Question 2

The release of a neurotransmitter causes a local membrane permeability and allows the flow of ion current and a small, localized depolarization. Will this always result in an action potential?

Answer 2

No, action potential will occur if there are voltage-gated channels in the area and only if the depolarization is large enough

Question 3

The greater the flow of current the greater change in the what of depolarization?

Answer 3

Magnitude of depolarization

Question 4

How is the flow of current related to the depolarization area?

Answer 4

Large flow of current = larger area of depolarization

Question 5

If 2 cells receive the same stimulus, which cell are have a larger ara of depolarization?

Answer 5

The cell with the larger length constant

Question 6

What is the spatial summation of non-propagating depolarization?

Answer 6

Overlapping zones of depolarization constructively reinforce one another resulting in greater depolarization

Question 7

For an action potential to be propagated, what must be present in the zone of depolarization?

Answer 7

Axon hillock

Question 8

What is decremental conduction of depolarization?

Answer 8

Postsynpatic membrane depolarizes in response to the presynpatic release of excitatory transmitter but propagation will occur depending on length constant of the cell

Question 9

If a zone of depolarization overlaps with a high Cl- conductance, what is the result?

Answer 9

Reduced propagation of depolarization

Question 10

What is temporal summation?

Answer 10

Post synaptic effects that occur close in time will build on each other to cause depolarization

Question 11

What is the difference between spatial and temporal summation?

Answer 11

Spatial summation is 1 action potential occurring in 2 overlapping sections to create a depolarization

Temporal summation is 2 action potentials occurring in one section to cause depolarization

Question 12

Signal propagation depends on the density of sensitivity of what channel? Where are these channels most dense?

Answer 12

Nav

Axon hillock

Question 13

During an action potential, membrane potential increases initially due to what?

Answer 13

Electrotonic conduction

Question 14

What is electrotonic conduction?

Answer 14

A depolarization zone is caused by a nearby depolarization without a change in membrane conductance

Question 15

Why do Na+ channels open before K+ channels?

Answer 15

Na+ channels open at more negative potentials than K+ channels

Question 16

At what point in the action potential is the K+ conductance increasing most rapidly?

Answer 16

Peak of action potential

Question 17

What causes afterhyperpolarization?

Answer 17

K+ conductance is larger than during resting conditions

Question 18

Why is hyperpolarization important?

Answer 18

Nav channels deinactivate

Question 19

When K+ channels deinactivate?

Answer 19

At resting membrane potential

Question 20

Does electrotonic depolarization rely on voltage-gated channels?

Answer 20

No

Question 21

The magnitude of depolarization depends on what 2 factors?

Answer 21

Magnitude of depolarization at the source (current density) and distance from source (length constant)

Question 22

Are voltage-gated Ca2+ channels located at the axon hillock?

Answer 22

Yes

Question 23

Are Ca2+-activated K+ channels located at the axon hillock?

Answer 23

Yes

Question 24

Describe the steps of depolarization at the axon hillock with short relative refractory period

Answer 24

Electrotonic depolarization opens Nav channels

Nav channels induce action potential

K+ channels open at peak of action potential
Ca2+ voltage-gated channels open (little effect)

Kv & Ca2+ channels inactivate

Exchangers & ATPase return resting membrane potential

Question 25

How is calcium pumped out of the cell?

Answer 25

Na-Ca exchanger

Ca2+-ATPase

Question 26

Describe the steps of depolarization at the axon hillock with long relative refractory period

Answer 26

Electrotonic depolarization opens Nav channels

Nav channels induce action potential

Ca2+ voltage-gated channels open
Ca2+-sensitive K+ channels open (hyperpolarization)

Nav channels deinactivate during hyperpolarization

Kca2+ channels have prolonged open probability as long as cytosol Ca2+ levels are elevated

Kca2+ channels causes prolonged hyperpolarization

Exchangers and ATPases return resting membrane potential

Question 27

Do action potentials lose amplitude during spread from soma to terminal? What is this called?

Answer 27

No

Undiminished (reinforced) signal conduction

Question 28

If an action potential occurs at the axon hillock, will it result in an excitatory response?

Answer 28

Yes

Question 29

What are absolute refractory periods?

Answer 29

Duration during which an action potential cannot occur

Question 30

What are relative refractory periods?

Answer 30

Duration during which it takes a stronger than normal stimulus to make an action potential

Question 31

When does the absolute refractory occur?

Answer 31

During activation & inactivation of Nav channels

Question 32

When does the relative refractory period occur?

Answer 32

During membrane hyperpolarization

Question 33

Can action potentials move retrograde? Why or why not?

Answer 33

No because those ion channels are inactivated

Question 34

If a nerve is stimulated between the soma and the terminus which way does the action potential propagate?

Answer 34

Both ways

Question 35

What determines the length constant?

Answer 35

Ratio of membrane resistance to cytoplasm resistance

Question 36

What 2 ways can the length constant be increased (better propagation)?

Answer 36

Decrease cytosolic resistance (increase diameter)

Increase membrane resistance (myelination)