Confused about ventricular systole

[DwayneEMTP]

Yeah, thanks Doc, I guess I understood the difference, but with them being so closely related (normaly) I tended to think of them together.

But I still have the issue with this sentence:

"So ventricular contraction (systole) spans depolarization and repolarization of the ventricles."

Wouldn't contraction (systole) cease to be present upon repolarization? Or maybe repolarization is normaly considered part of the systolic cycle? Though I couldn't find any reference to systole that referenced anything but depolarization.

I'm, so far, comfortable with what the EKG is saying happens durine each phase of the cardiac cycle, at least the way the book explains each step. It just seems that the book is telling me that during the ST segment and T wave that the ventricles are repolarizing (I can see that) yet the above sentence says it's still part of systole...and I can't seem to reconcile that except as a contradiction.

I'm trusting that you all understand that I'm in no way arguing with you!! I know I'm wrong headed on this...I just can't make it work..

Thanks for your help Doc...

Dwayne

Edit: Oh hell...is it possible that what you are saying is that though systole/diastole are happening physically, they are not exactly in concert with the electrical impulses shown on the EKG so that perhaps is why the terms don't match up? Though the ventricles are in systole (physical) the wave of depolarization has completed (electrical) so though the EKG shows the end of depolarization, contraction is still occuring? Is there a lag between impulse-->EKG-->physical response maybe? Crap...I'm just making myself look dumber aren't I....?

[AZCEP]

But I still have the issue with this sentence:

"So ventricular contraction (systole) spans depolarization and repolarization of the ventricles."

The contraction won't necessarily last from the beginning of depolarization to the end of repolarization. Depolarization (QRS complex) initiates the contraction, but it has to happen prior to the start of systole. Repolarization is started when the electrolyte balance determines that they need to move back to the starting point. As the amount of sodium and potassium reach their diffusion thresholds, they will move to balance. The calcium and magnesium that follow, cause the contraction to end and relaxation(diastole) to begin.

[DwayneEMTP]

So, is contraction and repolarization happening in the same myocytes at the same time? Maybe that's where I'm going wrong, I believed them to be mutually exclusice acts.

Depolarization occurs, triggering contraction, depolarization ends, contraction continues, repolarization begins, contraction continues, repolarization and contraction end...Some thing like that?

Dwayne

[AZCEP]

For a brief moment, it is possible for the muscle to be contracting and the myocyte to be repolarizing.

The cell uses the calcium ion to initiate the contraction. Because it moves after the sodium, it will be a bit later to leave the cell allowing relaxation.

Keep in mind that most biologic processes are occurring at the same time. It is very difficult to say that one process ends and another begins in black and white. It would make things simpler, but look for the gray.

[DwayneEMTP]

Thanks AZ, I knew I was wrong headed somewhere. I makes much more sense if you think of them as overlapping processes instead of linear.

Does it make sense now where I was getting stuck on the terminology?

I was so sure I was clear on the physiology of each concept at the molecular level. But it seems every time I start feeling over confident I find I've been waving at the crowd with my zipper down.

Thanks for your help everyone! That was like a misquito buzzing around my head...It just had me in vapor lock.

Life, as I know it, can now continue on a normal basis....

Have a great day!

Dwayne