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Flashcards in L4: action potentials - Levy Deck (36)
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1
Q

the farther above threshold the stimulus the greater the frequency of action potential firing. This relationship holds until…

A

until the fiber reaches its maximum frequency of firing

2
Q

what limits the frequency of action potential firing?

A

refractory period

3
Q

what approximately is resting potential in vertebrates?

A

-70 mV

4
Q

describe the “all-or-none” property of an action potential

A

once threshold is reached, an amplitude will always attain a fixed amplitude that is independent of stimulus strength

5
Q

describe the “time-limited” property of an action potential

A

action potential duration does not vary with duration of stimulus – action potential repetitions will change according to duration of stimulus and frequency will change according to strength of stimulus until maximum frequency is reached (due to refractory period)

6
Q

describe the “non-decremental” property of an action potential

A

unlike local potentials, an action potential propagates along the cell membrane without decrement, which is valuable for conveying info over long distances

7
Q

describe the “voltage dependence” of an action potential

A

when a critical voltage is reached across the membrane, voltage-dependent cahnnels open and allow a series of passive ion movements to produce the action potential

8
Q

when in the course of an action potential is the absolute refractory period

A

from the beginning of the action potential to the point when it has repolarized to a value near threshold (before undershoot)

9
Q

when in the course of an action potential is the relative refractory period

A

from repolarization to threshold through undershoot to depolarization back to resting potential

10
Q

what happens to the threshold voltage during the relative refractory period

A

threshold is raised

11
Q

what is the orthodromic direction

A

from receptive region to presynaptic region of neuron

12
Q

what is the antidromic direction

A

from presynaptic region to receptive region of neuron

13
Q

T/F concentration gradients change significantly during the course of an action potential

A

false - concentration gradients hardly change during the course of an action potential

14
Q

how is conductance related to current and driving force?

A

g = I / (Vm - E)

15
Q

why is the slope of depolarization steep?

A

g Na+ is increasing because voltage gated Na+ channels are opening

16
Q

why is the slope of repolarization steep?

A

g K+ is increasing because voltage gated K+ channels are opening

17
Q

this technique used a constant Vm and allowed the measurement of ion currents and calculation of voltage dependence of ion conductances

A

voltage clamp method

18
Q

the puffer nerve poison tetrodotoxin (TTX) blocks ___

A

blocks voltage gated Na+ channels and therefore Na+ current due to with no effect on K+ current

19
Q

tetraethylammonium ions (TEA) block ___

A

the delayed voltage gated K+ channels and therefore the delayed K+ current but have no effect on Na+ current

20
Q

this substance blocks Na+ current with no effect on K+ current

A

TTX puffer nerve poison tetrodotoxin

21
Q

this substance blocks the delayed K+ current but have no effect on Na+ current

A

TEA tetraethylammonium ions

22
Q

describe the entire sequence of events that gives rise to an action potential

A
  • local depolarization activates voltage-gated Na+ channels, allowing influx of Na+ and depolarization
  • local depolarization also increases K+ driving force (Vm - EK+), influx K+ and repolarization
  • threshold is reached when sufficient voltage Na+ channels are opened and overcome outward K+ current. Na+ influx depolarizes further and opens more Na+ voltage channels in positive feedback
  • Na+ channels are inactivated and I Na+ turned off
  • (with some overlap to previous step) delayed K+ voltage channels open and efflux, repolarize, hyperpolarize, and then close to depolarize back to normal Vm resting
  • absolute refractory is about threshold to threshold when Na+ channels are either open or inactivated
  • relative refractory is from re-threshold to Vresting when K+ channels are still open and Na+ are changing from inactivated to closed
23
Q

what are the voltage sensors that open voltage gates?

A

groups of charged amino acid residues on transmembrane a-helices

24
Q

what confers channel selectivity for either Na+ or K+ ions?

A

particular geometric arrangement of negative amino acids at narrowest part of pores

25
Q

what are 4 conformations of Na+ voltage gated channel over course of action potential

A
  • rest (closed)
  • activated (open)
  • inactivated
  • inactivated and closed
  • back to rest (closed)
26
Q

this period of an action potential is due to inactivation of Na+ voltage gates

A

absolute refractory

27
Q

explain the absolute refractory period

A

so few Na+ channels have recovered that even if all of them were opened, inward current would not exceed outward K+ current that keeps cell polarized

28
Q

explain the relative refractory period

A

fewer than normal Na+ channels are available, so a larger fraction of them must be opened to carry sufficient inward current to offset considerable outward K+ current. hence, a larger depolarizing stimulus (threshold) is necessary

29
Q

T/F like the Na+ channel, the K+ channel has an inactivation gate

A

false - contrary to the Na+ channel, the K+ channel has no activation gate

30
Q

when do the K+ channels open and close?

A
  • delayed open after action potential depolarization

- begin to close during hyperpolarization and finish closing when back to resting membrane potential

31
Q

when do the Na+ channels open and close?

A
  • open - depolarization
  • inactivate - near top of depolarization peak
  • back to rest (closed) ~ re-threshold before hyperpolarization
32
Q

why doesn’t an action potential re-invade a region it has immediately passed through?

A

region becomes refractory and cannot generate an action potential for a certain time

33
Q

myelin consists of

A

fused membranes of satellite cells wrapped around the axon

34
Q

nodes of Ranvier

A

regularly spaced gaps in myelin

35
Q

what are 3 functions of myelin?

A
  • decrease leakage current from axoplasm during conduction
  • faster conduction through saltatory conduction
  • require less metabolic energy to maintain Na+ K+ concentration gradients (less room for leak channels, less need for Na K pump)
36
Q

saltatory conduction is

A

faster conduction jumping from nodes of Ranvier between myelin