**2. Cardiomyocyte: structure and function**

Cells are the fundamental unit of life, and the large number of them reflects the diversity of their function. The myocardium is composed principally of specialized muscle cells called cardiomyocytes. The major components of cardiac myocytes, cell membrane (sarcolemma) and T-tubules, necessary in excitationcontraction coupling, thereby facilitate a fast and synchronous contraction across the entire cell volume, sarcoplasmic reticulum, nucleus, and contractile elements. Cardiomyocytes contain many more mitochondria than skeletal muscle cells reflecting the dependence of cardiac muscle cell on aerobic metabolism. The functional intracellular contractile unit of cardiac muscle is the sarcomere that contains the contractile proteins myosin and actin. Sarcomeres also contain the regulatory proteins troponin and tropomyosin. Functional integration of myocytes is mediated by intercalated disks, which join individual cells and within which specialized intercellular junctions permit mechanical and electrical coupling. Intercalated disk contains various junctional complexes including the zonula adherens, desmosomes, and gap junctions. One of the most important types of adhesive interactions required for the formation and maintenance of tissues is that mediated by the cadherin family. The expression and distribution of many of these junctional components are often perturbed in cardiovascular disease [1–4]. One of the components of intercalated disks is gap junctions which facilitate synchronous myocyte contraction by providing electrical coupling. Deregulation of gap junction in cardiovascular disease may contribute to electromechanical dysfunction and arrhythmias. In addition, the cardiac conduction system consists of specialized myocardial fibers that conduct electrical impulses more readily than surrounding myocardial fibers and regulate the rate and the rhythm of the heart.
