The musculoskeletal system has four major functional components: the bones and ligaments, the synovial and fibrocartilaginous joints, the muscles and tendons, and the nerves innervating the muscles. Tendons anchor muscle to bone while ligaments anchor bone to bone. The overall shape and contour of the body is attributable to its bony structure and overlying muscles.
Bones provide mechanical support for the body and protection for the viscera within the body cavities, vertebral column and skull. Mature bone forms by mineralization of osteoid laid down by osteoblasts on a cartilaginous matrix in the epiphyses of the long bones, endplates of the vertebrae and cartilaginous structures (endochondral bones of the skull and face). Cortical bone forms a thick cortex surrounding a central hollow, the marrow space. Trabecular bone forms an intricate lattice laid down along the lines of stress within the marrow cavity. Bone reabsorption by osteoclasts and new bone formation by osteoblasts is continuous. Bone must maintain strength sufficient to resist compression and tension applied by mechanical loading forces and muscle traction. Bone strength depends upon normal architecture, collagen, and mineralization; abnormality of any component results in susceptibility to fracture. The shape of mature bone is influenced by the pull of the muscles at their anatomic origins and insertions during skeletal maturation. Muscles that insert on a small portion of bone and exert large forces deform the bone into prominences. Adjacent bones are connected by ligaments, collagenous bands continuous with the collagen of the bone itself. Ligaments stabilize the bones relative to one another and the intervening joints.
Joints separate articulated bones. Fibrocartilage separates bones at joints where motion is minimal, for example, the intervertebral discs. Synovial (diarthrodial) joints separate bones where motion is extensive. The bone surfaces of diarthrodial joints are covered with hyaline cartilage architecturally designed to resist repeated axial loading with minimal deformation while providing a smooth, virtually friction-free surface for motion. The avascular cartilage derives its nutrition from the synovial fluid by diffusion. The diarthrodial joints are enclosed within a synovial envelope of collagen lined with a single synovial cell layer that secretes the proteoglycans necessary for joint lubrication. Joint integrity is maintained by ligaments anchoring bone to bone and the forces exerted by muscles whose tendons cross the joint.
Muscles, Tendons, and Bursae
See also Chapter 14, The Neurologic Examination. Striated skeletal muscles exert contractile force proximally on one bone at its origin and distally via a variably elongated tendon onto its insertion on another bone. Therefore, muscular contraction serves to change the relative position of the bones. Most muscles originate diffusely directly from the periosteum, while some attach by tendons at both their origin and insertion, for example, the long head of the biceps. Tendons are elongated, relatively avascular collagen structures that are continuous at their origin with the interstitial collagen of the muscle and ...