Cardiac muscle

Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscles, with the other two being skeletal and smooth muscles. It is an involuntary, striated muscle that constitutes the main tissue of the walls of the heart. The myocardium forms a thick middle layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium), with blood supplied via the coronary circulation. It is composed of individual heart muscle cells (cardiomyocytes) joined together by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix. Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscles, with the other two being skeletal and smooth muscles. It is an involuntary, striated muscle that constitutes the main tissue of the walls of the heart. The myocardium forms a thick middle layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium), with blood supplied via the coronary circulation. It is composed of individual heart muscle cells (cardiomyocytes) joined together by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix. Cardiac muscle contracts in a similar manner to skeletal muscle, although with some important differences. An electrical stimulation in the form of an action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation contraction coupling. Diseases of heart muscle are of major importance. These include conditions caused by a restricted blood supply to the muscle including angina pectoris and myocardial infarction, and other heart muscle disease known as cardiomyopathies. Cardiac muscle tissue or myocardium forms the bulk of the heart. The heart wall is a three layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. On the outer aspect of the myocardium is the epicardium which forms part of the pericardium, the sack that surrounds, protects, and lubricates the heart. Within the myocardium there are several sheets of cardiac muscle cells or cardiomyocytes. The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner they allow the ventricle to squeeze in several direction simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion (similar to wringing out a damp cloth) to squeeze the maximum amount of blood out of the heart with each heartbeat. Contracting heart muscle uses a lot of energy, and therefore requires a constant flow of blood to provide oxygen and nutrients. Blood is brought to the myocardium by the coronary arteries. These originate from the aortic root and lie on the outer or epicardial surface of the heart. Blood is then drained away by the coronary veins into the right atrium. When looked at microscopically, cardiac muscle can be likened to the wall of a house. Most of the wall is taken up by bricks, which in cardiac muscle are individual cardiac muscle cells or cardiomyocytes. The mortar which surrounds the bricks is known as the extracellular matrix, produced by supporting cells known as fibroblasts. In the same way that the walls of a house contain electrical wires and plumbing, cardiac muscle also contains specialised cells for conducting electrical signals rapidly (the cardiac conduction system), and blood vessels to bring nutrients to the muscle cells and take away waste products (the coronary arteries, veins and capillary network). Cardiac muscle cells or cardiomyocytes are the contracting cells which allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells - known as a functional syncytium - working to efficiently pump blood from the heart, and if this coordination breaks down then – despite individual cells contracting – the heart may not pump at all, such as may occur during abnormal heart rhythms such as ventricular fibrillation. Viewed through a microscope, cardiac muscle cells are roughly rectangular, measuring 100–150μm by 30–40μm. Individual cardiac muscle cells are joined together at their ends by intercalated disks to form long fibres. Each cell contains myofibrils, specialised protein fibres that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when looked at through a microscope, similar to skeletal muscle. These striations are caused by lighter I bands composed mainly of a protein called actin, and darker A bands composed mainly of myosin. Cardiomyocytes contain T-tubules, pouches of membrane that run from the surface to the cell's interior which help to which improve the efficiency of contraction. The majority of these cells contain only one nucleus (although they may have as many as four), unlike skeletal muscle cells which typically contain many nuclei. Cardiac muscle cells contain many mitochondria which provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue.