2005-10-25T03:21:00.000-07:00

November 15, 1998

Dear Family and Friends

I can't believe I'm already well into my second year of medical school. And more unbelievable than that, I'm really enjoying it.

In our Function of the Human Body curriculum, we're doing a sequence on Electrophysiology of the Heart. I was a biophysicis major in college. I took some courses in electronics, and magnetism. So electrophysiology isn't unfamiliar to me. My professor, an electrophysiologist, says I show an aptitude for interpreting EKGs. So after class, I talked to him about seeing some "real action." He said I could go to the electrophysiology lab next week and see what really happens!

In the meantime, I'm just amazed by the human heart. Given an adequate supply of oxygen and nutrients (via the coronary circulation), the heart will continue to beat—even if the nerves to the central nervous system have been cut. This is why transplanted hearts resume beating once the coronary circulation is restored in the implanted heart. However, the central nervous system does exert some control over the heart, primarily to regulate the frequency of the heartbeat to meet blood supply needs to other parts of the body.

Transplanted hearts generally beat at a constant rate established within the heart itself and don’t, for example, beat faster during periods of exertion.

Like skeletal muscle, contraction of the myocardium is stimulated by electrical activity within the cells—from a polarized (“positive” charge) state, during which the myocardium is relaxed, to a depolarized (“negative” charge) state, during which the myocardium contracts. The change from a polarized state to a depolarized state and back again occurs through the exchange of electrically charged calcium, sodium, and potassium ions across myocardial cell membranes. The chemical exchange resulting in electrical polarity is complex, and we won’t discuss it further in this module.
The important point to understand is that depolarization (and contraction) travels down the heart in a “wave” from the atria to the ventricles. As a result, the atria contract first, followed closely by the ventricles. The myocardium then repolarizes (and relaxes) in preparation for the next “wave” of depolarization and contraction. The ability of the heart to depolarize, contract, and repolarize is intrinsic (inherent in itself) and autoregulatory in nature. Certain myocardial cells are designed to depolarize and repolarize in a regular, controlled fashion, and these autorhythmic cardiac cells trigger the coordinated wave of depolarization that travels down the heart, from myocardial cell to myocardial cell, causing the heart to beat as a coordinated unit.[Marieb:696-7]
Autorhythmic cardiac cells are locat

The Making of A Cardiologist