Be still my beating heart!

I’m not in particularly great shape at the moment – this was brought very dramatically to my attention today as, when I got home after a run, my heart felt like it was going to explode out of my chest, alien-style, as it was beating so hard.

But, in a terribly geeky way, it did make my marvel at the wonder that is the human heart. It is a phenomenal organ. Sitting behind the protective ribcage, it sits regulating itself, fiercely contracting and sending blood surging around the body. You can easily see why Aristotle, back in the 4th Century B.C., identified the heart as the most important organ in the body.

The heart can beat independently of any input from the body. This, I believe, it the most incredible thing about it. Sitting in isolation in a dish in a lab, heart cells, known as myocytes, will happily beat away; waves of contraction pulsing across sheets of cells.

If you remember back to school days and biology classes you might remember that there is a small patch of cells, called the sinoatrial node, which act as the heart’s pacemaker.

In these pacemaker cells, like other muscle and nerve cells, a charge exists across their cell membranes, resulting from tiny charged atoms, ions, being in different concentrations on either side of the membrane. However, unlike other cells, a unique combination of channels and pumps in the myocyte walls allow specific ions across the walls at certain times, generating electric pulses. It’s remarkable how, because of these ion channels opening and closing on different timescales, the cells are able to regulate themselves.

Because of ions seeping across the myocyte walls through one set of ion channels, the cells are constantly depolarizing. This means that the charge across the myocyte wall becomes more positive. When the charge becomes positive enough, it reaches a threshold level and the change in charge results in a second set of ion channels to open. These channels allow calcium ions to flood into the cells, which drastically depolarises them. This is the myocyte ‘firing’ an electrical signal.

However, these calcium ion channels rapidly close again, and a third set of channels open. These channels allow potassium ions to move out of the myocytes, returning the charge across the myocyte membranes to negative levels. Having returned to this negative charge the first set of ion channels once again allow ions to leak into the myocytes, beginning to depolarize them again. And so the cycle continues.

Fig. 2 from Korzick (2011): Characteristic cardiac sinoatrial node action potential. The intrinsic rhythmicity of the sinoatrial node is thought to be controlled by three time- and voltage-dependent membrane currents: inward L-type Ca2+ current (ICa), outward K+ current (IK), and nonspecific cation “funny” current (If).

Isn’t it amazing that evolution has created such a sophisticated structure where so many molecules work in partnership with such ease to circulate life-supporting blood around your body?

Here is a great clip, not of the sinoatrial node, but a regular cardiac cell contracting. Just imagine thousands of these cells working in unison to result in a powerful muscle contraction which forces blood through the lungs and pumps it around the body.

Figure source: Korzick, D (2011) From syncitium to regulated pump: a cardiac muscle cellular update. Adv Physiol Educ 35(1): 22-27

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