Depolarization Phases of cardiac cell and electrolytes influx and eflux - USMLE Forums
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Old 02-16-2011
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Heart Depolarization Phases of cardiac cell and electrolytes influx and eflux

In uworld it is written that "In SA node, automaticity is the result of slow phase 4 depolarization that occurs due to a slow inward calcium current".
But in kaplan it is said that slow calcium current occurs during phase 0..
and in my understanding slow sodium influx or funny current not calcium influx occurs during phase 4 ..
can anyone explain plz?
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Old 02-16-2011
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I think Ca isn't a main factor for automaticity of SA.

the resting potential of cardiac cells is -90 mV, meanwhile in Pacemakers (SA) this value is changed during a Dyastole. In SA node occurs process called: "Slow Spontan Diastolic Depolarisation" and it is the result of special character of SA node cell membranes. They(membranes) increase Na flow into cell during Diastole and resting potential is rased to -60mV. after that Normal cycle begins.
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Old 02-16-2011
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As far as I know, transient calcium channel open around ~-50mV.

Since phase 4 starts in below -60mV and then depolarizes slowly till action potential, before ~-50mV the most important current is what is called "funny current".

However, after that, there is a calcium current, that is important to the end of phase 4.

Once you've reached the threshold, Slow calcium channels open and calcium influx causes action potential.

Note there are two different calcium channels, and they play different roles in different phases of node potential.

I hope I helped you!

Last edited by GGossling; 02-16-2011 at 08:10 AM. Reason: Adding information.
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