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CST - BIOSCI107 > ET : N - Action Potentials > Flashcards

ET : N - Action Potentials Flashcards

1
Q

is potential inside neurons constant

A

no

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2
Q

when does potential inside neurons change

A

when ion concs or membrane permeability changes

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3
Q

hyperpolarisation occurs when it becomes more

A

negative

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4
Q

in hyperpolarisation, the potential inside the cell moves closer to the EP of

A

potassium

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5
Q

depolarisation occurs when it becomes more

A

positive

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6
Q

in depolarisation, the potential inside the cell moves closer to the EP of

A

sodium

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7
Q

what is an action potential (technically speaking)

A

a brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels, which spreads, like a wave, along an axon

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8
Q

when does an action potential occur

A

when the membrane has been depolarsied to threshold

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9
Q

information is coded in the…

A

frequency of action potentials

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10
Q

what are action potentials a key element of

A

the process of signal transmission along axons

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11
Q

look at diagram of three stages of action potentials: what is the * representing

A

a slow and graded depolarisation evoked by a stimulus

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12
Q

look at diagram of three stages of action potentials: what is the 1 representing

A

after the MP reaches threshold: fast depolarisation to approx 30mV (overshoot)

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13
Q

look at diagram of three stages of action potentials: what is the 2 representing

A

repolarisation

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14
Q

look at diagram of three stages of action potentials: what is the 3 representing (and what happens in this)

A

after-hyperpolarisation (AHP) (becomes slightly more negative than RMP before returning to RMP)

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15
Q

look at diagram of three stages of action potentials: 1+2 together represent what period

or

what period does stimulus evoking and rapid depolarisation represent

A

absolute refractory period

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16
Q

look at diagram of three stages of action potentials: what period is 3

A

relative refractory period

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17
Q

in the refractory period the neuron is not ________. if you applied a second stimulus what would happen?

A

excitable

nothing (usually)

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18
Q

absoulte refractory period means (and compare to relative)

A

even if you introduced a second stimulus which was powerful and strong, no action potential will be evoked where as in stage three (relative) it will regain its excitability but a much stronger stimulus is required to depolarise it to threshold

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19
Q

when MP reaches threshold, there is a sudden activation (opening) of…

A

voltage-gated Na+ channels

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20
Q

when MP reaches threshold and voltage gated Na+ channels are activated what is the Pk:PNa+ (used to be 40:1)

A

1:20

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21
Q

thus in overshoot the MP shifts towards…

A

E(Na)

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22
Q

do the voltage gated Na channels open for long

A

no it is short lasting as these channels inactivate quickly

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23
Q

what happens when the Na+ channels are inactivated

A

followed by transient opening of voltage-gated K+ channels, leading to repolarisation and AHP

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24
Q

when K+ channels are opened what happens to MP and Pk:PNa

A

shifts towards E(K) and 100:1

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25
Q

when the voltage threshold is reached… (sodium channels and Na+ ions)

A

sodium channels open and Na+ ions move into the cell along both the conc and electrical gradient

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26
Q

influx of Na+ slows down and stops when..

A

1) the inside potential becomes positive (moves towards E(Na)) and thus attracts Na+ ions less
2) Na+ channels inactivate

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27
Q

if the activation gate is closed, is the channel permeable

A

no

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28
Q

why is the first phase of action potential so fast

A

because when threshold is met, Na+ channels open and due to conc grad, sodium ions rush into the cell, thus rapidly bringing positive charge into the cell

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29
Q

permeability is solely determined by

A

leak channels

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30
Q

inactivation gate sense depolarisation and

A

changes conformation to block etc

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31
Q

briefly state the steps of how voltage-gated Na+ channels activate and inactivate

A

RMP = gate closed

Depolarisation = gate opens

After fraction of a second = inactivation gate blocks the channel

back to initial state when membrane repolarises (gate closed, inactivation gate no longer blocking channel)

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32
Q

each action potential is an all or none event, which contrasts…… which are …..

A

contrasts small (sub threshold) depolarisations or hyperpolarisations which are graded

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33
Q

the amplitude of action potentials is usually ___________ and does not depend on the ________ _________ (provided that this ________ is _________)

A

constant
stimulus intensity
stimulus
suprathreshold

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34
Q

what does suprathreshold mean

A

stimulus causes depolarisation which just crosses the threshold

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35
Q

what does graded mean

A

the amplitude depends on the amplitude of the stimulus while the amplitude of the whole action potential does not depend on the stimulus intensity (provided that the stimulus causes depolarisation to threshold)

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36
Q

what is one way to induce an action potential

A

applying an electrical stimuli

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37
Q

when switch is closed, one electrode becomes _____ and one electrode becomes _____, this causes a _________ ___________

A

positive
negative
potential difference

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38
Q

current follows the path of

A

least resistance

39
Q

what are the two main paths that current follows

A

1) outside from + to -

2) across membrane and inside axon

40
Q

which path can change RMP

A

the second path, across membrane and inside axon

41
Q

What happens when the current generated by an outside source flows through the cell membrane from outside to inside

A

hyperpolarisation (MP becomes more negative)

42
Q

what happens when the current generated by an outside source flows through the cell membrane from inside to outside

A

depolarisation (MP becomes less negative)

43
Q

flow goes from

A

positive to negative

44
Q

in electrolytes current is carried by

A

ions

45
Q

what is the largest component

A

the movement of cations (positively charged ions)

46
Q

what affects excitability and the potential inside

A

the smaller component of the current where current goes across cell membrane, along cytoplasm and back to cathode (negative charged electrode)

47
Q

along this path, current flows outside > inside which causes

A

accumulation of negative charge along this path

48
Q

due to accumulation of negative charge, anode expects to see

A

increase in negativity of MP, known as local hyperpolarisation

49
Q

under cathode (negative electrode) we have accumulation of

A

cations

50
Q

if postivie charge shifted to this area, this would cause

A

local depolarisation

51
Q

if the local depolarisation reaches threshold, the next event would be

A

activation of the voltage gated sodium channels and initiation of action potential

52
Q

where are AP’s first generated

A

in the axon initial segment

53
Q

why do AP’s generate in the axon initial segment

A

because it has the lowest threshold and thus serves as the ‘trigger zone’ for APs

54
Q

what is depolarisation to threshold evoked by

A

excitatory postsynaptic potentials (EPSPs) which spread mainly passively from dendrites

55
Q

what happens to APs once generated

A

APs are transmitted actively along the axon away from the cell body

56
Q

what EPSPs evoked by

A

synaptic transmission from presynaptic axons to dendrites and, to a smaller degree, cell bodies

57
Q

the current loop flows through the cell membranes in the axon initial segment region from ______ to ________

A

inside to outside

58
Q

once generated, APs also spread from axon initial segment back to the

A

cell body

59
Q

what are the two types of axons

A

unmyelinated axons and myelinated axons

60
Q

diameter difference in unmyelinated axons and myelinated axons

A 
unmyelinated = small diameter 
myelinated = larger diameter
61
Q

difference in transmission of APs in unmyelinated axons and myelinated axons

A 
unmyelinated = slow, continuous 
myelinated = fast, saltatory (in large steps)
62
Q

what are the two stages of action potential transmission in both types of axons

A

1) passive spread

2) generation of action potentials

63
Q

what are the three steps of passive spread of current

A

1) (subthreshold) depolarisation at one region of the membrane
2) passive current flow (inside and outside the axon)
3) depolarization of adjacent parts of membrane

64
Q

what is subthreshold

A

hasn’t reached threshold at which voltage gated sodium channels kick in

65
Q

when one section is depolarised it has

A

higher potential than the regions next to it

66
Q

potential difference always leads to

A

flow of current

67
Q

will there be flow of current in the extracellular space through the interstitial fluid? why?

A

yes because depolarisation is normally associated with loss of positive charges outside, there will also be a potential graident outside the axon, thus a potential difference and a flow of current in the extracellular space through the interstitial fluid

68
Q

how can current move through the membrane

A

through the leak potassium channels as current is carried by ions

69
Q

current can spread passively only over…

A

short distances

70
Q

current quickly _________ as it flows along the axon

A

dissipates

71
Q

when the circuit is enabled, you have highest potential at the _____ compared to the _______ and due to this ___________ ___________ current will flow

A

+
ground
potential difference

72
Q

what are the five specific steps of action potential transmission in unmyelinated axons

A

1) action potential
2) passive current flow
3) depolarisation of adjacent parts of membrane to threshold
4) activation of voltage-gated Na+ channels
5) new (full size) action potentials generated in adjacent parts of membrane

73
Q

if current is strong enough we expect hyperpolarisation under the ________ and depolarisation under the _______ which can cause action potential / opening of voltage gated sodium channels

A

anode

cathode

74
Q

can passive current generated by action potential depolarise other adjacent parts of membrane to threshold

A

yes

75
Q

in myelinated axons, action potential conduction velocity is

A

increased

76
Q

how does myleination increase speed of action potential conduction

A

by increasing the effciency of passive spread

77
Q

myelinated: where are APs generated

A

at nodes of ranvier

78
Q

myelinated: current flows passively between _______

A

nodes

79
Q

in myelinated axons, do AP need to be regenerated at every part of the axonal membrane

A

no

80
Q

why is speed of AP transmission in unmyelinated axons slower than in myelinated axons

A

passive current flow between two adajcent points is fast, however AP must be regenerated at every point on the membrane in unmyelinated, where as it doesn’t in myelinated. This takes time and therefore conduction is slow

81
Q

AP propogate in which axon faster

A

myelinated

82
Q

in myelinated axons, what is the myelin sheath formed by in CNS and PNS

A

oligodendrocytes in CNS and schwann cells in PNS

83
Q

what types of cells are oligodendrocytes and schwann cells

A

glia cells

84
Q

in myelinated axons, myelination is ________________; interrupted at _________ ____ _____________

A

discontinuous

nodes of ranvier

85
Q

myelination increases…

A

passive spread of current

86
Q

how does myelination increase passive spread of current

A

due to the insulating properties of myelin, there is less current dissipation as it flows across the axon

87
Q

what direction does passive conduction occur

A

in both directions

88
Q

if myelinated fibres are much more effcient, why do we still have unmyelinated fibres

A

because unmyelinated fibres have a much smaller diameter and we have limited space in CNS due to skull, so by using unmyelinated, even though slower, yu can have more of them.

89
Q

axons are able to conduct AP in both directions except…

A

under physiological conditions (ap one direction but passive conduction still goes both ways)

90
Q

why does AP conduct in only one direction under physiological conditions

A

due to the absolute refractory period - by the time the absolute refracty period is over, AP has already moved down the axon

91
Q

PNS contains axons of ___________ _______ as well as axons of _______________ and the ‘______________ ____________ ____________’

A

sensory neurons
motorneurons
Autonomic Nervous System

92
Q

explain the process of AP generation in sensory neurons

A
  • When stimulus acts on receptors in sensriy neurons, it does not immediately evoke APs
  • first it evokes a graded depolarisation, known as the receptor potential
  • the receptor potential spreads passively to more distally located trigger zone, where APs are generated
  • APs then spread along the axon (myelinated or unmyelinated) towards the CNS
93
Q

where is information about the strength of the stimulus coded (in sensory neurons?)

A

in the amplitude of the receptor potential and the frequency of APs

94
Q

go to page 231 of course guide, label the bottom diagram

A

okay

CST - BIOSCI107 (13 decks)

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