I must say that I’m very glad I took the Health and Medicine course first before I started the personal training course. The anatomy portion is a lot easier in comparison, and feels more like a review.

It’s still kind of crazy for me to think about how our heart just knows what to do without conscious control. The heart cells are so meticulously designed and well-fitted together, and they can withstand a ton of pressure when the heart contracts (with desmosomes).

Did you know that the pacemaker is actually a special kind of heart cells? They literally set the rhythm of the heart! And there are three levels of fallback systems, from sinoatrial node to atrioventricular node to Bundle of His. Should all three fail, then we need a manmade pacemaker device.

How does a heart know when to contract? The cardiac muscles are involuntary. The SA node’s pacemaker cells spontaneously depolarize on their own. Although the autonomic nervous system doesn’t initiate this, it modulates its rate: sympathetic nerves release norepinephrine to speed it up, parasympathetic nerves release acetylcholine to slow it down.

When depolarization happens, it reaches T-tubule and triggers voltage-gated Ca2+ channels to let in extracellular Ca2+. That trigers sarcoplasmic reticulum (SR) to dump out more Ca2+ (aka calcium-induced calcium release CICR). The Ca2+ binds with troponin and moves tropomyosin out of the way so that the myosin head can bind to the actin. The myosin head already hydrolyzed ATP to ADP and Pi, the myosin head binds to actin, releases the phosphate, triggers the power stroke and then releases ADP.

All this is how a heart muscle contracts.

Isn’t this amazing?? Makes me want to go out and run a 5K - just doing my part to keep the heart healthy.