Wednesday, April 10, 2013

Why are there differences in cardiac action potential in different parts of the heart?

Cells in different regions of the heart do not have the same action potential.

The difference is major between conducting system and the myocytes.
And there is a minor variation in the action potential in distinct parts of the ventricle (endocardial and epicardial)

What is epicardium and endocardium?
Epicardium is the outer side of the cardiac muscles and endocardium is the inner side (towards the cavity of the ventricle)

What causes the difference in action potential?
This is due to the nature of repolarization.
Unlike depolarization, the repolarization is not a propagating phenomenon.

When a cell depolarizes, another cell close to it then depolarizes and produces an electric field which triggers the depolarization phenomenon.
In this way, the depolarization proceeds as a propagating wave within cardiac tissue.

Repolarization in a cell occurs because the action pulse has only a certain duration; thus the cell repolarizes at a certain instant of time after its depolarization, not because of the repolarization of an adjoining cell.

If the action pulses of all cells are of equal duration, the repolarization would of course accurately follow the same sequence as depolarization.
In reality, however, this is not the case in ventricular muscle. The action pulses of the epicardial cells (on the outer surface) are of shorter duration than those of the endocardial cells (on the inner surface).

Coming back to our original statement.. What is the minor variation in the action potential of endocardial and epicardial fibres?

The epicardial action potential exhibits a prominent notch between phase 1 and phase 2 that results in a spike and dome configuration. The notch is smaller in midmyocardial cells and absent in endocardial cells.
Subendocardial fibres lack phase 1.
The action potential notch is due to the presence of a transient outward current (Ito), which diminishes in amplitude from the epicardial to endocardial surfaces.

This current is due to potassium efflux (potassium going out of the cell)
You see, if the sodium channels simply closed at phase 0 the action potential would stay up there. It is the closure of potassium channels which cause the repolarization and the phase 1.

It simply means epicardium has more outward current than endocardium.
Outward potassium current is little in endocardial cells.
So very little potassium is going out.. And that too is balanced by calcium coming in. That's why no phase 1. Hope that made sense ^_^"

In real heart muscle, since the action potential duration at the epicardium is actually shorter than at the endocardium, the recovery phase appears to move from epicardium to endocardium, that is, just the opposite to activation.

Why is there a difference in action potential?
According to me, the differences in action potentials cause a difference in conducting velocities and difference in refractory period (changing their respective response to stimulation rate)
It is these differences that help the heart contract in an organized fashion and prevent haywire spread of cardiac excitation.
I mean, we don't want the impulse transmitted to the epicardium from the endocardium go back into the endocardium now, do we?
So we let the epicardium repolarize first (shorter action potential) and make sure that the endocaridum is refractory till then.
Otherwise we'd have an impulse going back and forth from the epicardium to the endocardium!

That is why, during depolarization, the impulse is carried from endocardium to epicardium, and during repolarization, the impulse moves from epicardium to endocardium.

What helps one of my best friend's remember this is "What depolarizes last, repolarizes first"
So to sum it all up, it's the variation in potassium (potassium conductance) which plays a major role in impulses going here and there in the heart. That's why potassium injections are lethal if not in range.
Thank you so much for being there in my life (:

Cool fact: 
Purkinje fibres have a prominent phase 1.
The heart cells in the subendocardial region lack phase 1 and the conducting cells (Purkinje fibres) in the same region have a prominent phase 1.
It is breathtakingly beautiful how our body is designed :)

Another cool fact: The changes in action potential also change the response of various drugs on different parts of the heart. This I came to know during my extensive Google search :P
So all of these minor changes might have relevance in your future practice as a doctor =D

That's all!

If you search hard enough for something, you will find it.. Yes, even the answers to your silly questions! ^_^

Read behind the scenes, which questions lead me to which answers & making of this post by clicking here.

-IkaN

References: 
http://www.bem.fi/book/06/06.htm
http://cpt.sagepub.com/content/2/1/61.short
http://en.wikipedia.org/wiki/Endocardium
http://physrev.physiology.org/content/85/4/1205.full

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