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Physiology and Pharmacology > Action potentials > Flashcards

Action potentials Flashcards

1
Q

What is an action potential?

A

Rapid change in membrane potential across the membrane of a cell

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2
Q

What is the resting membrane potential of an axon?

A

-70mV

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3
Q

Where does an action potential start in an axon?

A

Axon hillock

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4
Q

What is required for an action potential to start at the axon hillock?

A

Must be depolarised to threshold value

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5
Q

What is meant by action potentials being all or nothing?

A

They only occur when the membrane is depolarised to threshold
otherwise they don’t occur at all

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6
Q

How is the axon hillock depolarised?

A

Inputs from pre-synaptic neurones to dendrite of neurone

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7
Q

What is the shape of an axon action potential?

A

Straight line at -70mV
upstroke to 40mV
then downstroke to below -70mV
then slow upstroke to -70mV

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8
Q

What is responsible for the upstroke of an axon action potential?

A

Membrane depolarisation to threshold causes voltage-gated sodium ion channels to open
get influx of sodium ions giving further depolarisation

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9
Q

What is responsible for the downstroke of an axon action potential?

A

Voltage-gated sodium ion channels inactivate and voltage-gated potassium ion channels are finally open
efflux of potassium ions gives repolarisation and then hyperpolarisation

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10
Q

What is meant by voltage-gated sodium ion channels being inactivated?

A

Not closed

but do not allow sodium ions to pass through them

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11
Q

How do voltage-gated sodium ion channels recover from inactivation?

A

By hyperpolarisation of the membrane

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12
Q

When do voltage-gated potassium ion channels close?

A

When the membrane has been repolarised to -70mV

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13
Q

Can action potentials occur immediately one after another? Why?

A

No

because of the refractory period

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15
Q

What are the two stages of the refractory period?

A

Absolute refractory period

Relative refractory period

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15
Q

What is responsible for the refractory period?

A

Voltage-gated sodium ion channels are inactivated

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16
Q

What is happening to voltage-sodium ion channels in the absolute refractory period?

A

All voltage-gated sodium ion channels are inactivated

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17
Q

What is happening to voltage-gated sodium ion channels in the relative refractory period?

A

Voltage-gated sodium ion channels are recovering from inactivation

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18
Q

How fast do voltage-gated potassium ion channels close? What is the importance of this?

A

Slowly

There is still some potassium ion efflux even when membrane potential has repolarised, giving hyperpolarisation

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19
Q

How is an action potential propogated along an axon?

A

Local current theory

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20
Q

What is the local current theory?

A

Influx of sodium ions repels other positively charged ions which spread out to nearby regions in the axon
this depolarises nearby regions of the axon to threshold

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21
Q

How are the local currents affected with increasing distance? Why?

A

As the local currents move further along the axon

they become dissipated as the positively charged ions have become more spread out

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22
Q

What is capacitance?

A

The ability of the membrane to store a charge

in other words, to prevent it from leaking out

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23
Q

What is resistance?

A

How impermeable the membrane is to a particular ion

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24
Q

How can resistance to an ion be increased? And decreased?

A

Increased - by decreasing the number of open ion chanels for that ion

Decreased - by increasing the number of open ion channels for that ion

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25
Q

Is it advantageous to an axon membrane to have high or low capacitance? Why?

A

Low capacitance - so more charge contributes to electrical excitability

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26
Q

Is it advantageous to axon membranes to have high resistance or low resistance? Why?

A

High resistance - to prevent ions leaking out of the axon and dissipating the current

27
Q

How long is an axon action potential?

A

5ms

28
Q

How is the action potential affected as it is propogated along the axon membrane?

A

Stays exactly the same

29
Q

What type of feedback occurs in the upstroke of the action potential? How?

A

Positive feedback

Influx of sodium ions gives depolarisation which results in opening of more voltage-gated sodium ion channels which gives more sodium ion influx

30
Q

What is the basic structure of a voltage-gated sodium ion channel?

A

Single polypeptide chain, called alpha subunit
Four parts
Each part is made up of six transmembrane spanning regions

31
Q

What is the function of the fourth transmembrane spanning region in voltage-gated sodium ion channels?

A

Voltage sensor

32
Q

What is the significance of the fifth and sixth transmembrane spanning region in voltage-gated sodium ion channels?

A

Between them is pore region

33
Q

What is the significance of the third and fourth parts of the transmembrane spanning region in voltage-gated sodium ion channels?

A

Between them is the inactivation particle

34
Q

What is the basic structure of a voltage-gated potassium ion channel?

A

Four polypeptide chains, called alpha subunits

Each one is made up of six transmembrane spanning regions

35
Q

What is the function of the fourth transmembrane spanning region in voltage-gated potassium ion channels?

A

Voltage sensor

36
Q

What is the significance of the fifth and sixth transmembrane spanning regions in voltage-gated potassium ion channels?

A

Between them is pore region

37
Q

How do local anasthetics e.g. procaine affect voltage-gated sodium ion channels?

A

Block them

38
Q

What are the two mechanisms by which local anasthetics block voltage-gated sodium ion channels?

A

Use-dependent

Use-independent

39
Q

How do use-dependent local anasthetics work?

A

Block the ion channels only when they’re open

remain there when ion channel is inactivated or closed

40
Q

How do use-independent local anasthetics work?

A

Block the ion channel even when they’re inactivated or closed because can diffuse across plasma membrane
remain there

41
Q

What is conduction velocity?

A

Speed of action potential propogated along an axon

42
Q

What is the exterior surface of some axons covered with?

A

Myelin sheath, has gaps in it

43
Q

What is the myelin sheath?

A

Tightly packed membrane

44
Q

What is responsible for forming the myelin sheath?

A

CNS - oligodendrocytes

PNS - Schwann cells

45
Q

How do Schwann cells form the myelin sheath on axons in the PNS?

A

Continuously wrap around the axon

46
Q

What are the gaps in the myelin sheath called?

A

Nodes of Ranvier

47
Q

How are ion channels distributed along the axon in myelinated and unmyelinated axons?

A

Myelinated - lots of ion channels at nodes of Ranvier

Unmyelinated - ion channels distributed evenly along the axon

48
Q

What is the function of the myelin sheath?

A

To act as in insulator

prevent the leakage of ions

49
Q

How is an action potential propogated along a myelinated axon?

A

Saltatory conduction

50
Q

What is saltatory conduction?

A

Refer to how action potentials seemingly jump from one node of Ranvier to the next

51
Q

How does saltatory conduction work?

A

Action potential occurs at node of Ranvier

by local current theory, next node of Ranvier becomes depolarised to threshold

52
Q

How does myelination affect the conduction velocity of an axon? Why?

A

Increases it

Less leakage of ions so more of them spread to next node of Ranvier
raising it to threshold quicker

53
Q

How does myelination affect resistance? Why?

A

Increases it

Fewer ion channels for ions, since only occur at nodes of Ranvier although in large numbers

54
Q

What is the most common demyelinating disease?

A

Multiple sclerosis

55
Q

How does demyelination occur in multiple sclerosis?

A

Autoantibodies against proteins in myelin sheath

destroy myelin sheath

56
Q

How does demyelination affect the propogation of an action potential along the axon?

A

Leakage of ions, current dissipates
next node of Ranvier may not be depolarised to threshold
action potential not generated

57
Q

What is the solubility of use-dependent local anasthetics? And use-independent local anasthetics?

A

Use-dependent - hydrophilic, insoluble in lipids

Use-independent - hydrophobic, lipid-soluble

58
Q

In how many directions is an action potential propogated along an axon? Why?

A

One direction only

Because region of axon behind action potential is in refractory period
so action potential can only be propogated forward

59
Q

Which ion channels are located in the axon membrane?

A

Voltage-gated sodium ion channels

Voltage-gated potassium ion channels

60
Q

Will a strong stimulus generate a stronger action potential than a normal-level stimulus?

A

No

action potentials are all the same size

61
Q

How is the strength of a stimulus transmitted through action potentials?

A

By changing the frequency of action potentials

stronger stimulus means increased frequency of action potentials

62
Q

How fast do voltage-gated potassium ion channels open? What is the significance of this?

A

Slowly

By the time they’ve opened, the voltage-gated sodium ion channels have already opened, given sodium ion influx and inacivated
so potassium ion efflux occurs after this
i.e. downstroke after upstroke

63
Q

Do axons with a larger diameter of smaller diameter have a greater conduction velocity of action potentials? Why?

A

Greater diameter axons

Because they offer less resistance to ions spreading out through them
so ions can move through them faster

Physiology and Pharmacology (20 decks)

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