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Structure and Function of Bones
- Bones provide support and protection for the body’s tissues and organs and are important sources of minerals and blood cells.
- Bone formation begins with the production of an inorganic matrix by bone cells. Bone minerals crystallize in and around collagen fibers in the matrix, fiving bone its characteristic hardness and strength.
- Bone tissue is continuously being resorbed and synthesized by basic multicellular units of osteoclasts and osteoblasts.
- Bones in the body are made up of compact bone tissue and spongy bone tissue. Compact bone is highly organized into haversian systems that consist of concentric layers of crystallized matrix surrounding a central canal that contains blood vessels and nerves. Dispersed throughout the concentric layers of crystallized matrix are small spaces containing osteocytes. Smaller canals, called canaliculi, interconnect the osteocyte-containing spaces. The crystallized matrix in spongy bone is arranged in bars or plates. Spaces containing osteocytes are dispersed between the bars or plates and interconnected by canaliculi.
- There are 206 bones in the body divided into the axial skeleton and the appendicular skeleton. Bones are classified by shape as long, short, flat, or irregular. Long bones have a broad end (epiphysis), broad neck (metaphysis), and narrow midportion (diaphysis) that contains the medullary cavity.
- Bone injuries are repaired in stages. Hematoma formation provides the fibrin framework for formation and organization of granulation tissue. The granulation tissue provides a cartilage model for the formation and crystallization of bone matrix. Remodeling restores the original shape and size to the injured bone.
Structure and Function of Joints
- A joint is the site where two or more bones attach. Joints provide stability and mobility to the skeleton.
- Joints are classified as synarthrosis, amphiarthrosis, or diarthrosis, depending on the degree of movement they allow. Joints are also classified by the type of connecting tissue holding them together. Fibrous joints are connected by dense fibrous tissue, ligaments, or membranes. Cartilaginous joints are connected by fibrocartilage or hyaline cartilage. Synovial joints are connected by a fibrous joint capsule. Within the capsule is a small fluid-filled space. The fluid in the space nourishes the articular cartilage that covers the ends of the bones meeting in the synovial joint.
- Articular cartilage is a highly organized system of collagen fibers and proteoglycans. The fibers firmly anchor the cartilage to the bone, and the proteoglycans control the loss of fluid from the cartilage.
- Joints help move bones and muscle.
Structure and Function of Skeletal Muscles
- Skeletal muscle is made up of millions of individual fibers.
- Whole muscles vary in size (2 cm to 60 cm) and shape (fusiform, pennate). They are encased in a three-part connective tissue framework. The fundamental concept of muscle function is the motor unit, defined as those muscle fibers innervated by a single motor nerve, its axon, and anterior horn cell.
- Satellite cells are dormant myoblasts; however, they can regenerate muscle when activated.
- Muscle fibers contain bundles of myofibrils arranged in parallel along the longitudinal axis and include the muscle membrane, myofibrils, sarcotubular system, aqueous sarcoplasm, and mitochondria. There two types of muscle fibers, type I and type II, determined by motor nerve innervation.
- Myofibrils and myofilaments contain the major muscle proteins, actin and myosin, which interact to form cross-bridges during muscle contraction. The nonprotein muscle constituents provide an energy source for contraction and regulate protein synthesis, enzyme systems, and membrane stabilization.
- Muscle contraction includes excitation, coupling, contraction, and relaxation.
- Muscle strength is graded by the all-or-nothing phenomenon and recruitment. Speed of contraction is affected by several factors: muscle fiber type, temperature, stretch, and weight of the load.
- There are two types of muscle contraction: isometric and isotonic. Muscle shortening occurs during contraction but can be seen also during pathologic and physiologic contracture.
- Skeletal muscle requires a constant supply of adenosine triphosphate (ATP) and phosphocreatine to fuel muscle contraction and for growth and repair. ATP and phosphocreatine can be generated aerobically or anaerobically.
Aging & the Musculoskeletal System
- Sarcopenia, or age-related loss in skeletal muscle, is a direct cause of decrease in muscle strength. A slow decline in dynamic and isometric strength is evident after age 70 years.
- The regenerative function of muscle tissue remains normal in elderly persons.
- On average, people lose about one third of a pound of muscle every year after age 40 years and gain at least as much body fat.
- Reduced basal metabolic rate and decreased lean body mass are also noted in the elderly population.
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