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Membranes and Receptors > Session 4 - The action potential > Flashcards

Session 4 - The action potential Flashcards

1
Q

During an action potential, there is increased permeability to which ion?

A

-Na

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2
Q

What is meant by action potentials being all of nothing?

A

-Action potentials are only propagated if the cell is depolarised to threshold, otherwise there is no response, ie there is never a partial action potential

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3
Q

What is meant by ‘action potential propagation is without loss of amplitude’?

A

-As the distance from the site of initiation increases, the AP size/frequency/amplitude is not lost

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4
Q

Are all action potentials the same? Give an axample

A
  • No they vary between tissues

- In cardiac ventricles they are longer

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5
Q

What is conductance?

A

-The capacity of a membrane to conduct a current of a particular ion across itself

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6
Q

What determines the level of conductance?

A

-The permeability of a membrane to that particular ion, ie how many ion channels are open and activated

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7
Q

What happens to membrane potential if the conductance of a single ion species is increased?

A

-The membrane potential will move closer to the Ep of that ion

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8
Q

What is the consequence of increasing a cells conductance to K+?

A
  • The RMP will move towards that of Ek and the cell will become hyperpolarised as K leaves the cell down its concentration gradient
  • This makes the cell less easily excitable
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9
Q

What is the consequence of increasing conductance to Na?

A

-The membrane potential will move towards that of Na, ie depolarisation will occur and the cell will become more easily excitable

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10
Q

What is the result of a decreased extracellular Na on an action potential?

A
  • Depletion of the gradient between the intracellular and extracellular compartment
  • Na influx would decrease when Na channels open, decreasing the upstroke of the action potential
  • Cell may not depolarise to threshold
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11
Q

What is the consequence on action potentials if the extracellular Na is increased?

A
  • Steeper gradient between the extracellular and intracellular compartments
  • Greater influx of Na when Na channels open
  • Upstoke of AP becomes steeper as the cell moves quicker towards ENa
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12
Q

Why does the peak of an AP never truely reach ENa?

A

-There are other ions involved in the AP and the cell is not perfectly permeable

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13
Q

What is capacitance?

A

-The ability of a lipid bilayer to store charge

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14
Q

What effect does high capacitance have on action potential propagation?

A

-Cells with high capacitance require a higher current to depolarise the membrane

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15
Q

What is voltage clamping? How does it work?

A
  • An experiment to measure the effects of different membrane potentials on the conductance of Na and K
  • The membrane potential is controlled using a voltage clamp and the currents flowing through the membrane are measured through a microelectrode
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16
Q

What happens to voltage-gated channels upon depolarisation?

A

-They open

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17
Q

Describe the ionic changes during an action potential and state the effects on membrane potential

A
  • There is rapid opening of Na+ channels, influx of Na causes depolaristion
  • Membrane potential moves towards towards ENa
  • Na channel inactivation
  • K channels opening is delayed and occurs slowly, allowing gradual efflux of K+
  • Membrane potential moves back towards Ek
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18
Q

What happens to the Na channels shortly they have opened?

A

-They undergo inactivation

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19
Q

Why does hyperpolarisation occur after depolarisation?

A
  • K channels remain open allowing efflux of K+

- When this is coupled with ceased Na influx hyperpolarisation occurs

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20
Q

How does the membrane potential return to resting after hyperpolarisation following an acion potential?

A

-As K channels close and the conductance of K returns to normal, the resting membrane potential returns

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21
Q

Where is an action potential generated within an axon?

A

-Axon hillock

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22
Q

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

A

-Depolarisation has to be reached during synaptic potentials between dendrites and other neurones

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23
Q

What is summation?

A

-Multiple synaptic potentials occur together and reach threshold to generate an action potential

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24
Q

What is the positive feedback loop of an action potential? How is it the basis of the all or nothing law?

A
  • Na channels open, Na enters cell,Membrane depolarisation causes Na channels to open -> Cycle is entered
  • If depolarisation is below threshold then not enough Na channels are opened to initiate loop and no AP is generated
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25
Q

What is repolarisation?

A

-After depolarisation, K channels opening and returning the membrane potential to resting is repolarisation

26
Q

Is the NaKATPase involved in repolarisation?

A

-No it just maintains the gradients over time

27
Q

What is the absolute refractory period?

A
  • The period after depolarisation where nearly all Na channels are in the inactive state and thus do not pass current so there is 0 excitability
  • No AP can be generated no matter how much stimulation
28
Q

What is the relative refractory period?

A

-Na channels are recovering from inactivation and excitability is returning towards normal as the number of Na channels in the inactive state decreases

29
Q

What is accomodation?

A

-Accommodation is when depolarisation occurs to threshold, however depolarisation occurs so slowly that some of the open Na channels become inactivated before there are enough channes open to initiate the feedback loop so no AP is generated

30
Q

What type of channels are Na and K channels?

A

-Voltage gated

31
Q

Describe the molecular properties of a Na channel

A
  • 4 repeating subunits which make 1 polypeptide functional channel
  • Each subunit has 6 membrane spanning domains
  • The membrane spanning domain S4 is a voltage sensor
  • Has an inactivation particle located at the N-terminus
32
Q

How does the voltage sensor of a Na channel work?

A
  • Detects change in voltage field

- Induces a conformational change which opens the channel

33
Q

How does the inactivation particle of Na channels work?

A
  • Acts as a ball on a chain
  • Once depolarisation has occured it swings into the channel and blocks the pore, causing the channel to become inactivated
34
Q

Describe the key molecular properties of a K channel

A
  • 4 separate subunits form a functional channel
  • Each subunit has 6 membrane spanning domains
  • S4 is the voltage sensor
  • No inactivation particle
  • Strictly impermeable to Na
35
Q

What is the function of a local anaesthetic?

A

-To stop the action of local pain-sensing fibres

36
Q

How does the hydrophobic pathway of local anaesthetics stop AP production?

A
  • Anaesthetic crosses the membrane and bind to and blocks the Na pores
  • There is no use-dependance
37
Q

How does the hydrophillic pathway of local anaesthetics stop AP generation?

A
  • hydrophillic molecules cannot cross pm
  • Has to use the pore to pass through
  • Upon doing so the molecules becomes protonated and remains in the pore, active as an open-channel blocker
  • Requires use dependence
38
Q

In what order to local anaethetics block axons?

A
  • Small myelinated axons
  • Unmyelinated axons
  • Large myelinated axons
39
Q

What is diphasic recording of an AP?

A
  • AP is recorded at two electrodes A and B
  • Electrode A is the stimulating electrode
  • Electrode B is the recording electrode
40
Q

What is monophasic recording of AP?

A

-One area of a membrane under an elecrtode is deliberately damaged so only one value is recorded

41
Q

How can extracellular recording be used to measure conduction velocity?

A
  • Measure the distance between the stimulating electrode and the recording electrode
  • Measure the time gap between the stimulus and the action potential being registered by the recoding electrode
  • CV=distance/time
42
Q

What is local current spread?

A
  • The influx of Na+ repels the intracellular positive charges and make them spread along the inside of the axon
  • This resultant charge on the adjacent membrane causes an immediate local change in the membrane potential - depolarisation as the mp is less negtive
43
Q

How is an action potential propagated?

A

-When local current spread causes depolarisation of the adjacent membrane to threshold, and action potential can then be initiated and this continues down the axon

44
Q

What ensures that the axon is propagated in the correct forward direction?

A

-The AP will not go backwards as that part of the axon will be in an absolute refractory period and thus an AP can not be initiated

45
Q

What determines whether local spread is enough to raise depolarisation to threshold?

A

-The amount of Na entering/size of depolarisation of the initial AP has to be sufficient to cause high local current spread to cause depolarisation to threshold

46
Q

What is the length constant of a membrane potential?

A

-The distance it takes for the potential to fall to 37% of its original value

47
Q

What is the relationship between the distance from the initial depolarisation and the local change in membrane potential?

A

-As distance from the initial depolarisation is increased, the local change in membrane potential decreases

48
Q

What determines how far the local current spreads?

A
  • The capacitance of the membrane

- The resistance of the membrane

49
Q

What is membrane resistance?

A

-The number of ion channels open ie the more ion channels open the lower the resistance

50
Q

What is the effect on the length constant if membrane resistance is high?

A

-The length constant is increased as there is decreased loss of local current spread

51
Q

What is the effect of high capacitance on depolarisation and local current spread?

A
  • Requires a higher current to depolarise to threshold

- Decreases local current spread

52
Q

Why is the length constant behind an AP shorter than infront?

A

-Local change in membrane potential is shorter because K channels are still open, meaning membrane resistance is lower and there is more loss of the local current across the membrane

53
Q

How is local current spread linked to conduction velocity?

A

-The further the local current spreads down the axon, the faster the conduction velocity as an AP can be generated further away from the initial AP, ie it travels faster

54
Q

What properties of an axon lead to a high conduction velocity?

A
  • High membrane resistance
  • Low membrane capacitance
  • Large axon diameter (low cytoplasmic resistance)
55
Q

How is conduction velocity linked to fibre diameter in:

i) myelinated nerves
ii) unmyelinated nerves

A

i) velocity is proportional to fibre diameter with max 120m/s
ii) velocity is proportional to (square root)diameter with max 20m/s

56
Q

Which axons are mostly myelinated?

A

-Ones with large diameters such as motor neurones

57
Q

What effect does myelination have on capacitance and resistance?

A
  • Reduces capacitance x100

- Increases resistance x100 (few channels under myelin)

58
Q

What cells myelinate axons in CNS and PNS?

A
  • CNS-> oligodendrocytes myelinate many axons

- PNS -> schwann cell myelinates one axon

59
Q

How does myelination result in saltatory propagation?

A

-The actual AP spread over the axonal surface is slower than the local current spread so conduction occurs in a saltatory manner

60
Q

Why is local current spread so quick in myelinated axons?

A

-Reduced capacitance in internodal regions causes the local currrent to spread further down the axon and depolarise the next node of ranvier without firing an AP within the internodal region, thus the AP jumps from node to node

61
Q

Describe the consequence of demyelination in multiple scerosis

A
  • Decreases conduction velocity
  • The loss of myelin lowers the resistance and increases the capacitance of the axons, this means the local spread does not raise the membrane potential in the next node to threshold and thus saltatory conduction does not occur, resulting in alot of AP failing
62
Q

What is an action potential?

A

-A rapid change in voltage across a cell membrane associated with the passage of an impulse along a membrane

Membranes and Receptors (13 decks)

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