LEC18: Membrane Excitability Flashcards Preview

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Flashcards in LEC18: Membrane Excitability Deck (32):

what kind of channels are in membrane that exhibits passive membrane behavior? 

what does this mean?

current flow across membrane of only voltage-independent "leak" channels

resulting voltage change: as predicted by Ohm's law, w/ delay b/c of capacitance 

Vm returns directly to original value when current ends


what kind of channels are in membranes that show active properties? 

membranes including voltage-dependent channels - conductances respond to changes in Vm, which is reflected in current value



how do excitable membranes show active behavior? what do they respond to?

respond to thrshold depolarization with an action potential: all or none, stereotyped sequence of Vm changes 


what is automaticity? what cells exhibit this?

cells that spontaneously & rhythmically generate action potentials

excitable cells that're specialized to act as pacemakers do this


point on an action potential when: 

- threshold for initiatino

- reversal of membrane polarity at the peak

- afterhyperpolairzaiton before turn to Vm resting value 


A image thumb

what would the effect on depolarization of a membrane be if inward rectifiers close? why?

depolarization would be larger than expected b/c membraen conductance for K+ is reduced


what controls resting potential of Vm in a neuron?

leak K+ channel


what channels cause large spike in depolarization in a neuron?

opening of voltage-gated Na+ channels


at hte peak of an action potential in a neuron, which channels are open? 

voltage-gated K+ channels begin to activate 

Na+ channels inactivate


which channels repolarize the membrane? which phase?

voltage-gated K+ channels and leak K+ channels 

during afterhyperpolarization


what happens to the membrane to cause an action potential?

a strong enough stimulus depolairzes the membrane to threshold for an action potential initiation 

result: rapid, lare depolarization 

membrane then beings to repolarize


which channels dominate at rest of action potential?

leak K+


once Vm is depolarized to the threshold, which channels are in action?

voltage-gated Na+ channels open, drive Vm toward VNa 

these channels begin to inactivate soon aftr peak of depolairzation 

voltage-gated K+ channels also activate, repolarize the membrane


which channels repolarize the membrane of an action potential

voltage-gated K+ channels


what does Vm approach during afterhyperpolarization? why?

approaches Vk 

because until voltage-gated K+ channels deactivate, K+ conductance is larger than it was in resting membrane, making Vm get close to Vk 


what kind of event is an action potential? what does this mean?

all or none - a stronger suprathreshold stimulus would cause Vm to reach threshold value more quickly, but wouldn't change the shape of the subsequent action potential


what and when is the absolute refractory period?

right after action potential, it's impossible to initiate another one 

some time is required for recently inactivated Na+ channels to de-inactivate before they can generate another action potential

period ends when full complement of Na+ channels has been de-inactivated


what is state of channels during relative refractory pd? how might this show?

some Na channels have de-inactivated, but most have not 

during this pd, may see small-amplitude, shallow slope spikes of Na+ channels as they de-inactivate 


when does the refractory period end?

when full complement of Na+ channels has been de-inactivated

then can launch full new action potential


how does action potential travel down a neuron's axon?

like a "wave" - action potential propagates down the axon 

axon contains: leak K+ channels, voltage-gated and inactivating K+ and Na+ channels 

1) voltage-gated Na+ channels open, Na+ enters cytoplasm, results in depolarization 

2) depolarization spreads along membrane due to redistribution of charges per entry of uncompensated positive ions 

3) per Na+ influx, K+ channels open, K+ exits cell through delayed rectifiers

membrane is repoalirzed, action potential terminated in that region


A image thumb

what do all excitable heart muscle cells have in common?

1) long phase of depolarization compared to neurons 

2) prominent use of Ca2+ as the depolarizing charge carrier 


are heart muscle cells usually depolarized or polarized? why? 

usually depolarized 

b/c mechanical pumping action of heart relies on contractions that last hundreds of milliseconds


describe action potential of the ventricles


Q image thumb

At rest, cells are polarized at about -90 mV; Kir channels dominant conductance 

1) Phase 0: membrane depolarizes to threshold for action potential initiation; voltage-gated Na+ channels activated; rapid depolarization & upstroke

2) Phase 1: early/fast repolarization; Na+ channels inactivate

3) Phase 2: plateau; delayed opening of voltage-gated Ca2+ channels; voltage-gated K+ channels slowly activate

4) Phase 3: repolarization; K+ current builds via various voltage-gated K+ channels, and Ca2+ current diminishes as voltage-gated Ca2+ channels inactivate

5) Phase 4: diastole: begins after membrane returns to well-polarized resting potential



what causes ventricular caridac muscle cell to return to its resting potential?

combination of voltage-gated Ca2+ channels inactivaing & voltage-gated K+ channels activating


what are the different heart muscle cells, what is each responsible for?

pacemakers: impulse generation

His-purkinje: impulse propagation 

"working" muscle: contraction


what is normal sequence of events in cardiac cycle? 

1) RA pacemaker cells spontaneously fire in the SA node, initiate an action potential 

2) depolarization spreads through the atria, results in atrial contraction & stimulation of the AV node and His-Purkinje fibers that innervate ventricles

3) initiation, spread of action potentials in the ventricles, produces contraction that pumps blood to rest of the body


what does an electrocardiogram (ECG) show?

what are the different parts of the line?

electrical activity of the various excitable cells & localization in the heart 

1) 1st deflection: P-wave, depolarization of the atria, corresponds to Phase 0 of Atria

2) QRS complex, corresponds to Phase 0 of Ventricle

3) T-wave, corresponds to Phase 3 repolarization of ventricles 


what is the Q-T interval?

how long is it, normally? 

the duration of the ventricular action potential and contraction 

<450 ms normally


what happens in a patient with long QT syndrome? 

delayed, slow repolarization 

can give rise to premature action potentials during Phase 3, without waiting for normal trigger from atria 

can lead to dangerous arrhythmia, called torsade de pointes - ventricles contract in uncoordinated manner, independent of normal atrial rhythm; reduces pumping efficiency of heart


what kind of pathology is LQTS?

"channelopathy," pathology caused by mutation in a channel 


what cardiac channel mutations have been linked to LQTS?

1) voltage-gated Na+ channel; mutations impair inactivation, result in relative large inward current during plateau phase & deplayed repolarization 

2) 2 of the voltage-gated K+ channels of Phase 3; causes delays and slows repolarization 


what happens in LQTS w/ overexpression of hERG? 

hERG is a K+ channel; mutated G628S site in pore of the channel, reduces its conductance

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