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The wall of the heart consists of three layers: the epicardium (external layer), the myocardium (middle layer) and the endocardium (inner layer). The epicardium is the thin, transparent outer layer of the wall and is composed of delicate connective tissue. The myocardium, comprised of cardiac muscle tissue, makes up the majority of the cardiac wall and is responsible for its pumping action. The thickness of the myocardium mirrors the load to which each specific region of the heart is subjected. The endocardium is a thin layer of endothelium overlying a thin layer of connective tissue. It provides a smooth lining for the chambers of the heart and covers the valves. The endocardium is continuous with the endothelial lining of the large blood vessels attached to the heart.

The wall of the heart consists of three layers: the epicardium (external layer), the myocardium (middle layer) and the endocardium (inner layer). The epicardium is the thin, transparent outer layer of the wall and is composed of delicate connective tissue. The myocardium, comprised of cardiac muscle tissue, makes up the majority of the cardiac wall and is responsible for its pumping action. The thickness of the myocardium mirrors the load to which each specific region of the heart is subjected. The endocardium is a thin layer of endothelium overlying a thin layer of connective tissue. It provides a smooth lining for the chambers of the heart and covers the valves. The endocardium is continuous with the endothelial lining of the large blood vessels attached to the heart.

===Structure of Cardiac Muscle===

====Structure of Cardiac Muscle====

Cardiac muscle fibres are shorter in length and larger in diameter than skeletal muscle fibres. They also exhibit branching, which gives an individual fibre a Y-shaped appearance. A typical cardiac muscle fibre is 50-100μm long and has a diameter of about 14μm. Normally, there is only one centrally located nucleus, although occasionally a cell may have two nuclei. The sarcoplasm of cardiac muscle is more abundant than that of skeletal muscle and the mitochondria are larger and more numerous. Cardiac muscle fibres have actin and myosin filaments arranged in the same way as skeletal muscle fibres and possess a well-developed T-tubule system. In contrast to skeletal muscle, cardiac muscle does not fatigue, cannot be repaired when damaged and is regulated by the autonomic nervous system.

Cardiac muscle fibres are shorter in length and larger in diameter than skeletal muscle fibres. They also exhibit branching, which gives an individual fibre a Y-shaped appearance. A typical cardiac muscle fibre is 50-100μm long and has a diameter of about 14μm. Normally, there is only one centrally located nucleus, although occasionally a cell may have two nuclei. The sarcoplasm of cardiac muscle is more abundant than that of skeletal muscle and the mitochondria are larger and more numerous. Cardiac muscle fibres have actin and myosin filaments arranged in the same way as skeletal muscle fibres and possess a well-developed T-tubule system. In contrast to skeletal muscle, cardiac muscle does not fatigue, cannot be repaired when damaged and is regulated by the autonomic nervous system.

Although cardiac muscle fibres branch and interconnect with each other, they form two separate functional syncytia, one for the atria and another for the ventricles. The ends of each fibre in a network connect to its neighbours by irregular transverse thickenings of the sarcolemma called intercalated discs. The discs contain desmosomes, which hold the fibres together, and gap junctions, which allow ions to travel between cells and permit the rapid propagation of action potentials. Consequently, excitement of a single fibre of either network results in stimulation of all the other fibres in the network. As a result, each network contracts as a functional unit.

Although cardiac muscle fibres branch and interconnect with each other, they form two separate functional syncytia, one for the atria and another for the ventricles. The ends of each fibre in a network connect to its neighbours by irregular transverse thickenings of the sarcolemma called intercalated discs. The discs contain desmosomes, which hold the fibres together, and gap junctions, which allow ions to travel between cells and permit the rapid propagation of action potentials. Consequently, excitement of a single fibre of either network results in stimulation of all the other fibres in the network. As a result, each network contracts as a functional unit.