mani
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in specialized cardiac cells, stage 4 depolarization is caused by:
1. slow inward Na current
2. inward Ca current
3. decreased K efflux
4. decreased Cl influx
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| mash
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decreased K efflux
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| mani
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there is controversy in BRS and kaplan about this, kaplan says 3 while BRS says 1.
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| mash
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mani, this has been discussed earlier too on this forum.
actually phase 4 depolarization is due to 3 factors n im copying a post by suvi
The Sa nodal diastolic depolarization (spontaneous depol) is due to three factors: 1) Decreased K conductance( This also contributes to their less negative resting membrane potential, i.e. because relative conductance of K to total ionic conductance is less, so it doesn't reach the nerst potential of K as other cells do). 2) Na conductance; this is a slow Na conduction through special channels called the "funny conduction channels", as they seem to be non-specific in their conduction. These are NOT the fast conducting Na channels found in ventricular muscle. 3) Ca conduction to some extent.
but as far as i know dec K conductance is the major factor coz SA nodal cells do not ve enough Na channels...
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| mani
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thnx for reply mash. i was confused that what to choose when th equestion appears in exam. thats why i intentionally reposted the question. can u give me reference for the last sentence of ur posting?
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| medical1
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spontaneous depolarization relating to phase 4 ie progressive decrease in k conductivity; not efflux. so slow inward Na currant is my ans.
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| mani
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decrease in conductivity would decrease efflux, isnt it!!??
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| medical1
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yes but there is no way that u declare option c incorrect. probably NA H DOMINENT ROLE .
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| Idiopathic
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NA is dominant, and this is how class I antiarrythmics slow nodal conduction, through decreased current through these channels.
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| rubyduby
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medical and idiopathic u guys are rt too and mani is rt too. becuz when evr i took kaplan prep courses they srongly implicated on potassium.. and as far as i herd dat in specialised cells depolarisation occur by calcium in phase 0,, becuz they says they lacks Na chanels, but senor medical u gota point too dat then why do we give class 1 antiarythmic.. so its contorvesial yet. :?:
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| rubyduby
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i was confusing it wid pahse 3 and 0.. its phase 4,, stable phase
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| sudipto1973
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I have encountered this dilemma in a number of questions. However for specialized cells like SA node the answer has to be slow inward Na influx through slow Na channels (SA cells lack fast voltage gated Na channels). Decreased K conductivity is a contributing factor. SO the answer is 1 period. Remember all the electrical events happening in heart is due interplay of number of ions and USMLE requires us to know the predominant ion involved.
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| mjl1717
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Im goinig with Proffessor Mash's explaination.
{This is a fairly important concept}
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| mash
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"mjl1717" wrote:
Im goinig with Proffessor Mash's explaination.
{This is a fairly important concept}
hey wat does dat mean?
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| mjl1717
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It means you teach a lot of people on this forum whether you know it or not.
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| mash
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look mj, i dont teach people out here. I just give my point. Everybody is preparing fr the same exam and putting forward ur pt doesnt mean teaching someone.
Anyways, if someone has problems with dat , i cant help it.
and one thing more, in the process of teaching(as u call it), i ve learnt alot.
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| mjl1717
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OK buddy I get your drift, just breaking the monotony-but humour is one of the mature defense mechanisms
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