The Beat Goes On
♥ The Cardiomyocyte: Your heart beats about 72 times per minute, or 100,000 times a day, clocking an average of 2.5 billion times in a lifetime and working harder than any other muscle in your body. After all, it has to pump some 2000 gallons of blood around 60 miles of blood vessels each day. No matter how hard you train, the skeletal muscles of your arms or legs could never keep this up. So spare some love for the cardiac muscle cell, or cardiomyocyte.
♥ The Pacemaker: The heart marches to it’s own tune, unlike the skeletal muscle which gets direct input from a motor nerve. Indeed, the heart would be an unwieldy mess if each individual fiber needed a motor nerve connection. Instead, every beat of the heart starts within the sinoatrial node (SAN) containing <10,000 pacemaker cells equipped with a built-in clockwork mechanism to fire rhythmically. These electrical impulses spread through the muscle fibers by direct communication from cell to cell via special channels called gap junctions, that synchronize the contraction. If the pacemaker fails, the ~5 billion working cardiomyocytes don’t get their marching orders and the heart slows down or becomes arrhythmic. Promising new research aims to convert ordinary cardiomyocytes to pacemaker cells by expressing a master regulator gene, Tbx18 to replace those lost by disease or defects.
♥ Sparks and Stripes: Each cardiomyocyte is packed with ordered arrays of thin and thick filaments that slide past each other to make the muscle contract. The thin filaments are made of actin seen as red stripes in the image. The thick filaments are an assembly line of myosin motors that use a rowing motion to pull on the actin filaments. In the absence of an electrical signal, the muscle is relaxed, with the filaments kept apart by a guardian protein called troponin C. The magical molecule that sets the contraction in motion is calcium, seen in the gif as sparks and waves. Each electrical impulse releases a packet of calcium that binds to troponin C, and moves it out of the way to trigger contraction. But the calcium is quickly captured (by calcium pumps and exchangers) and moved back into stores, so the muscle relaxes..before it all begins again.
Another installment in the series on cell types.
▶ Adipocyte: http://goo.gl/S4fQFS
▶ Erythrocyte: http://goo.gl/R5R6Y0
▶ Astroycte: http://goo.gl/SMpXMV
REF: Direct conversion of quiescent cardiomyocytes to pacemaker cells by expression of Tbx18. Kapoor et al., 2013 http://www.nature.com/nbt/journal/v31/n1/full/nbt.2465.html