The Musculoskeletal System: Bones

• The musculoskeletal system provides form, support, stability, and movement to the body.
• It is composed of bones, muscles, tendons, ligaments, cartilage, and other connective tissues.

Bone

• Bone is a dynamic tissue composed of cells (osteocytes, osteoblasts, osteoclasts), fibers, and ground substance.
• It provides support, protects organs, anchors muscles, and stores minerals.
• Bones are classified by shape: long, short, flat, irregular, and sesamoid.
• Long bones include a diaphysis (shaft) and epiphyses (ends).
• The outer surface of bone is covered by the periosteum, a fibrous membrane.
• Bone marrow, found within the medullary cavity of long bones and spaces in other bones, is responsible for hematopoiesis (blood cell formation).
• Compact bone is dense and strong, while spongy bone contains trabeculae and is lighter.
• Bone formation (ossification) occurs through intramembranous and endochondral ossification.
• Bone remodeling is a continuous process involving bone resorption by osteoclasts and bone formation by osteoblasts.

Joints

• Joints (articulations) are points where two or more bones meet.
• They are classified structurally (fibrous, cartilaginous, synovial) and functionally (synarthrosis, amphiarthrosis, diarthrosis).
• Fibrous joints are connected by dense connective tissue and allow little to no movement (e.g., sutures of the skull).
• Cartilaginous joints are connected by cartilage and allow slight movement (e.g., intervertebral discs).
• Synovial joints have a joint cavity filled with synovial fluid, allowing free movement.
• Synovial joints include a capsule, synovial membrane, articular cartilage, and often ligaments.
• Types of synovial joints include plane, hinge, pivot, condylar, saddle, and ball-and-socket joints.

Skeletal Muscle

• Skeletal muscle is responsible for voluntary movement.
• It is composed of muscle fibers (cells), connective tissue, blood vessels, and nerves.
• Muscle fibers contain myofibrils, which are composed of sarcomeres.
• The sarcomere is the basic contractile unit of muscle, containing actin and myosin filaments.
• Muscle contraction occurs via the sliding filament mechanism, where actin and myosin filaments slide past each other, shortening the sarcomere.
• Muscle contraction is initiated by a nerve impulse, which triggers the release of calcium ions from the sarcoplasmic reticulum.
• Calcium ions bind to troponin, exposing binding sites on actin for myosin.
• ATP provides the energy for muscle contraction and relaxation.
• Skeletal muscles are arranged in antagonistic pairs, where one muscle (the agonist) causes a movement and the other (the antagonist) opposes it.
• Muscle actions include flexion, extension, abduction, adduction, rotation, and circumduction.

Tendons and Ligaments

• Tendons connect muscles to bones.
• They are composed of dense regular connective tissue.
• Ligaments connect bones to bones at joints.
• They provide stability to joints and prevent excessive movement.
• Ligaments are also composed of dense regular connective tissue, but with more elastic fibers than tendons.

Cartilage

• Cartilage is a type of connective tissue composed of cells (chondrocytes) and a matrix of fibers and ground substance.
• Hyaline cartilage is the most common type, providing smooth surfaces for joint movement.
• Elastic cartilage is more flexible and is found in the ear and epiglottis.
• Fibrocartilage is tough and resilient and is found in intervertebral discs and the menisci of the knee.
• Cartilage is avascular, receiving nutrients through diffusion.