The musculoskeletal system aids in movement and posture and provides support to the body. It consists of muscles, bones, and connective tissue such as cartilage and ligaments. This system is divided into two parts: the skeletal system and the muscular system. The skeletal system consists of bones of different types that are joined together by ligaments in the body. This system provides support to the body and allows movement. It also creates a structure that protects soft tissue organs such as the brain, the heart, and the lungs. In addition, the skeletal system is the storage site for fat and minerals, and some components produce blood cells.The second part of the musculoskeletal system is the muscular system. This system consists of three types of muscle tissue: smooth, cardiac, and skeletal muscle. The skeletal muscles are connected to bone. This muscle tissue functions to aid in movement with the skeletal system, provide joint stabilization, protect organs, and maintain posture. Skeletal muscles work with bone to control voluntary movement such as running and walking. Cardiac muscles form the structure of the heart and allow the heart to contract to move blood. Smooth muscle lines vessels of the circulatory system and organs of the digestive system. These muscles contract in a twisting motion that pushes blood through arteries or food through the small intestine. Joints are places where bones interact with each other. Some joints are involved in growth, some provide flexibility, and others promote movement. There are different types of joints with different functions. Sutures are joints between the bones of the skull and are not movable joints. The joints between the vertebrae of the spine have limited motion. Some joints, such as those in the hips and shoulders, are ball and socket joints that can rotate in multiple planes. Hinge joints, such as those in the knee and elbow, can bend in one plane but not rotate. Pivot joints, such as those found in the neck, allow rotation in a limited plane. Tendons and ligaments are dense regular connective tissues. Ligaments attach bone to bone. Ligaments function to hold structures together and maintain stability. Tendons attach muscle to bone and allow the movement of the bone by contraction of the attached muscle. Cartilage is another type of connective tissue that cushions many joints.
Skeletal muscles move bones by contracting and shortening. Muscle fibers contain thick filaments of myosin alternating with thin filaments of actin along the length of the fiber. A nerve signal to the muscle causes calcium ions, Ca2+, to be released. The ions bind to a protein associated with the actin filaments, exposing the active site of the actin filament. Part of the myosin structure can interact with the active site of actin and pull the actin filament, causing it to slide past the myosin filament and thereby shorten the muscle. As the muscle shortens, it pulls on the tendon, which pulls on the bone. This process occurs repeatedly until the muscle has shortened enough to the move the bone as required and the nervous signal stops. Muscles can only pull on bone; they cannot push. In order for the bone to move back the other way, a different muscle must contract while the original muscle relaxes.