L16 Action Potentials Initiation and Conduction Flashcards

1
Q

what kind of AP conduction is seen in unmyelinated axons?

A

continuous conduction

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2
Q

what kind of AP conduction is seen in myelinated axons?

A

saltatory conduction

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3
Q

what drives the membrane potential towards Ena?

A

an increased influx of Na+ ions into the cell down its electrochemical gradient

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4
Q

describe the influence of the influx of Na+ ions on the rising phase of the AP?

A

at threshold, Na+ causes the rising phase of the impulse

at threshold, the influx of Na+ is greater than the efflux of K+ = more Na+ channels open= more depolarization

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5
Q

describe the influence of the efflux of K+ ions on the repolarizing phase of the AP

A

depolarization causes the slow opening of K+ channels = delayed efflux of K+ down their electrochemical gradient = repolarization

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6
Q

recognize the different time courses of change in sodium and potassium conductance

A

fast time course for Na+ channels opening

slower time course of the opening of K+ channels

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7
Q

explain how the structural features of voltage-gated Na+ channels are related to the influence of depolarization on the closed and open states of this channel

A

prolonged depolarization inactivated voltage gated Na+ channels

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8
Q

what are the characteristics of the Na+, K+ and Ca2+ voltage gated channels?

A
  • four identical subunits
  • 6 transmembrane spanning segments
  • segment 4 = charged components = voltage sensing element for channel protein
  • pore loop that endows the channel pore with its particular ion selectivity
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9
Q

what is refractory period?

A

when the sodium channel reverts from inactivated state to the normal closed state

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10
Q

what is the absolute refractory period?

A

when all Na+ channels are inactivated

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11
Q

what is the relative refractory period?

A

following absolute refractory period, when an impulse can be evoked by a LARGE stimulus

*some sodium channels are in the normal closed state and can be opened
the large number of K+ channels remain open and tend to hold the potential close to Ek = opposes excitiation of the axon

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12
Q

where are voltage gated Na+ and K+ channels in unmyelinated axons?

A

distributed uniformly

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13
Q

where are voltage gated Na+ and K+ channels in myelinated axons?

A

Na channels = axon initial segment, presynaptic terminales and node of Ranvier

K channels = paranodal and juxtaparanodal regions

*impulses only occur at nodes of rangier
repolarized at paranodal regions

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14
Q

what does conduction velocity depend on?

A

fiber diameter

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15
Q

why are unmyelinated axons slower than myelinated axons?

A
  • small diameter

- continuous conduction require that all of the axonal membrane must be driven to theshhold sequentially

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16
Q

conduction velocity is _____x the fiber diameter in unmyelinated axons

17
Q

conduction velocity is ____x the fiber diameter in myelinated axons?

18
Q

both unmyelinated and myelinated axons propagate AP without decrement =?

A

all of none!

19
Q

what are the advantages of myelin?

A
  • enhanced velocity of conduction because impulses are generated only at nodes of Ranvier
  • large velocity is achieved in fibers with relatively small diameters - space saving without fall in velocity
  • confining impulses to nodes means that Na+ loading during intense impulse traffic is minimized in myelinated axons giving a huge energy saving!
20
Q

what can impair impulse conduction?

A
  • TTX and saxitoxin
  • local anesthetics
  • demyelination
21
Q

what does TTX do?

A

blocks Na+ channels at extracellular site

pufferfish = paralysis, convulsions, mental impairment, cardiac arrhythmia, numbness of lips and tongue, complete paralysis then death within 4-6 hours

22
Q

what does saxitoxin do?

A

blocks Na+ channels at extracellular site to!

you get similar symptoms of TTX but it results in flaccid and respiratory paralysis

23
Q

what do local anesthetics do

A
  • block Na+ channel at intracellular site
  • they block small myelinated axons first, then unmyelinated axons then large myelinated axons
    pain perception is blocked!
24
Q

what does demyelination do?

A

agents destroy myelin and leave axons bare
causes redistribution of Na+ and K+ channels along the axons

*frequency related block
total conduction block
“crosstalk” between axons

25
what is frequency related block?
failure of some impulses in a train to propagate through the demyelinated region
26
what is total conduction block?
failure to excite the node
27
what is "crosstalk" between axons?
during impulse conduction some current circulating may deplarize an adjacent demyelinated hyperexcitable axons