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The musculoskeletal system provides mechanical support for the body, protects internal organs and permits movement. It is composed of bones, cartilage, skeletal muscles, joints, and connective tissues such as tendons and ligaments. Bones also serve as the body’s main mineral reservoir, they store calcium and phosphate and release them according to the body’s needs. Red bone marrow is the body’s production center for blood cells.
The central nervous system controls body movements by stimulating skeletal muscles to contract. Contraction of skeletal muscles moves bones, which act as levers. Bones articulate with each other through joints. Cartilage provides padding for the ends of bones within joints. Muscles are connected to bones by tendons, while bones are held together by ligaments.
Bones are classified according to their shapes and corresponding functions: long bones are responsible for most body movements; short bones provide some limited motion; while flat bones and irregular bones are mainly protective and supportive.
The bone tissue, or osseous tissue, is composed of bone cells and a characteristic extracellular matrix. Bone matrix is made of an organic component, mainly collagen, and an inorganic component of minerals, mainly calcium. Collagen gives bones flexibility while calcium provides stiffness. Without calcium, bones would be soft and bend easily. On the other hand, without collagen, bones would be brittle like chalk.
Bones renew and remodel throughout a person’s life in a process known as bone remodeling, which constantly removes old bone tissue and adds new bone tissue. Bone remodeling serves to re-shape bones to adjust to changing mechanical needs and to repair everyday micro-damages as well as fractures following injuries. This process also underlies the mechanism by which the constant levels of plasma calcium and phosphate are maintained. Bone remodeling is performed by 2 types of cells: osteoclasts, which dissolve bone matrix, and osteoblasts, which deposit new matrix around themselves to form new bone tissue. Bone remodeling is under control of complex signaling pathways. Major regulators include parathyroid hormone, vitamin D, growth hormones, glucocorticoids, thyroid hormones, estrogen and testosterone.
The most common bone disease is osteoporosis, or porous bone, in which bones lose mass and weaken, increasing risks of fractures. Osteoporosis is commonly due to old age and some other unavoidable factors, but can also develop from, or worsen by hormone imbalances, deficiencies in calcium, vitamin D or proteins, and sedentary lifestyles.
The most common and also most movable type of joint is synovial joint. The bones of a synovial joint are separated by a cavity containing synovial fluid, which serves as lubricant. Together, the fluid and the cartilage that lines the bone surfaces make the movements at synovial joints almost friction-free. There are also small fibrous sacs containing synovial fluid, called bursae, located between muscles, or between a tendon and a bone. Bursae cushion muscle movements and help tendons slide smoothly over the joints.
The most common disease of joints is arthritis. There are 2 main types of arthritis:
– Osteoarthritis, also called degenerative joint disease, is the “wear and tear” condition of the joint, commonly due to old age. Osteoarthritis is characterized by loss of cartilage, bone spurring and no major inflammation.
– Rheumatoid arthritis is a result of joint inflammation, with immune cells and inflammatory chemicals causing damage to the joint. It’s not clear how rheumatoid arthritis starts but genetic predisposition together with infection of the joint are likely to be among the causes.
Muscular tissue consists of specialized elongated muscle cells, called muscle fibers, which are bundled into fascicles. Muscle fibers, fascicles and whole muscles are wrapped in layers of connective tissue, which provides support and protection. These connective tissue coverings are continuous with the tendon that connects to a nearby bone. Fascicle arrangement determines the strength of a muscle and the direction it pulls. Most common muscle disorders are caused by injury or overuse, and include sprains, strains, cramps, and tendinitis.
