By Dr. Andy James, University of Bristol
The comedian, Jake Yapp, has been presenting on BBC Radio 4’s website a series of short comic histories of science entitled, “Everything We’ve Ever Known About …” Informative and entertaining, these are invariably funny. A recent edition concerned the heart, neatly covering the development of our understanding of heart function through the millennia, from the ancient Egyptians to the present day – where Jake concludes by commenting that if you isolate a heart muscle cell it will have its own pulse. Actually, strictly speaking, a pulse is the increase in pressure within the arteries as a result of beating of the heart. ‘Rhythm’ is a better word.
It’s a commonly held misconception, even amongst specialists in cell biology, that all heart muscle cells have their own rhythm that synchronises when the cells couple together (meaning they join and allow electricity to flow between them). Beautiful as this idea is, it simply isn’t true. The heart’s rhythm originates as electrical activity in a specialised area of the heart known as the sinus node- the heart’s physiological pacemaker. The heart muscle cells of the sinus node generate rhythmic electrical activity so that, when isolated, they contract regularly.
In the intact heart, a conduction system ensures that the electrical activity is passed to the chambers to produce a well-coordinated contraction. Many of the cells of the conduction system would also beat when isolated. But the cells from the walls of the chambers that produce the pressure to – as the English anatomist, William Harvey, would have it – propel the blood, do not. Chamber cells only need to receive the signal, but not generate it themselves.
The persuasive and attractive idea that all heart muscle cells show their own rhythm may have arisen from attempts to maintain heart muscle cells for several days in a petri dish. Heart muscle cells from adult mammalian hearts do not generally multiply under these conditions and while it is possible to keep the cells alive for a short time, after a few days, the cells die. However, cells from embryonic, or even neonatal, hearts do multiply in culture. They often show rhythmic contractions and, yes, they synchronise once they form connections with their neighbours.
So do heart cells really have their own pulse? It depends! Cells from adult hearts are programmed to behave in a fixed way- as a pacemaker cell, a conducting cell or a contractile cell from a chamber wall. Cells from immature hearts, on the other hand, retain the ability to change their properties and tend to develop pacemaker-like function in the petri dish.