Cardiac Muscle: The Unique Tissue of the Heart
Cardiac muscle is a type of muscle tissue that is found only in the heart and is responsible for pumping blood throughout the body. Unlike skeletal muscle, which is under voluntary control, cardiac muscle is involuntary and contracts rhythmically without external stimulation. Cardiac muscle is also different from smooth muscle, which is found in the walls of blood vessels and other organs, in that it has striations or bands of alternating light and dark regions that are visible under a microscope.
Cardiac muscle cells, also called cardiomyocytes or myocytes, are typically unicellular and have a single nucleus. They are connected to each other by specialized cell junctions called intercalated discs, which allow electrical impulses and mechanical forces to be transmitted from cell to cell. Intercalated discs also contain gap junctions, which are small pores that allow ions and small molecules to pass between adjacent cells. This enables cardiac muscle cells to synchronize their contractions and function as a single unit.
Cardiac muscle cells have a complex structure that allows them to generate and propagate electrical signals, contract forcefully and efficiently, and resist fatigue. They contain many mitochondria, which are organelles that produce energy from oxygen and glucose. They also have a well-developed sarcoplasmic reticulum, which is a network of membrane-bound sacs that store and release calcium ions, which are essential for muscle contraction. Cardiac muscle cells also have a system of transverse tubules, which are invaginations of the plasma membrane that carry electrical signals deep into the cell.
The main contractile proteins of cardiac muscle cells are actin and myosin, which form thin and thick filaments respectively. These filaments are arranged into repeating units called sarcomeres, which are the basic functional units of muscle contraction. Sarcomeres have a characteristic striped pattern due to the overlapping of thin and thick filaments. When a cardiac muscle cell receives an electrical signal from the sinoatrial node, which is the natural pacemaker of the heart, calcium ions are released from the sarcoplasmic reticulum and bind to troponin, which is a regulatory protein on the thin filaments. This causes tropomyosin, another protein on the thin filaments, to move away from the binding sites for myosin on actin. Myosin then uses energy from ATP to pull actin towards the center of the sarcomere, causing the sarcomere to shorten and the muscle cell to contract.
Cardiac muscle is organized into layers of myocardial tissue that wrap around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, resulting in a decrease in the heart chamber size and an ejection of blood into the pulmonary and systemic vessels. The amount of blood pumped by the heart per minute, or cardiac output, depends on the contractile force and frequency of cardiac muscle cells, as well as on the resistance and pressure in the blood vessels. Cardiac output varies to meet the metabolic needs of different tissues and organs in the body.
Cardiac muscle is essential for life, as it maintains blood circulation and oxygen delivery to all cells. Cardiac muscle can adapt to changes in workload and demand by altering its size, shape, and function. However, cardiac muscle can also be affected by various diseases and disorders that impair its structure or function, such as coronary artery disease, myocardial infarction, cardiomyopathy, arrhythmia, heart failure, and congenital heart defects. These conditions can cause symptoms such as chest pain, shortness of breath, palpitations, fatigue, edema, and reduced exercise tolerance. Treatment options may include medication, surgery, implantable devices, or transplantation.