Musculoskeletal System Overview

The musculoskeletal system provides structural support, enables movement, protects organs, stores minerals, and houses haematopoietic tissue. It comprises bones, skeletal muscles, tendons, ligaments, cartilage, and joints.

Bone is a dynamic connective tissue composed of an organic matrix (~35% — primarily type I collagen providing tensile strength) mineralised with hydroxyapatite crystals (Ca10(PO4)6(OH)2, ~65% — providing compressive strength). Long bones have a compact cortical shell and a trabecular (cancellous) spongy interior housing red bone marrow. Flat bones (e.g., skull, sternum) have two cortical plates with diploe between them. Bone is continuously remodelled by osteoclasts (resorption, multinucleate, from monocyte lineage, activated by RANKL/OPG ratio and PTH) and osteoblasts (formation, from mesenchymal stem cells, regulated by Wnt signalling). Bone mass peaks at ~30 years. Osteoporosis reflects excessive resorption relative to formation; bisphosphonates inhibit osteoclast function.

Three muscle types exist. Skeletal muscle is striated, voluntary, and multinucleate. Smooth muscle is non-striated, involuntary, and mononucleate; found in hollow organs including blood vessels, GI tract, and airways. Cardiac muscle is striated, involuntary, and mononucleate with intercalated discs containing gap junctions for electrical coupling.

Skeletal muscle contraction follows the sliding filament theory. An action potential travels down a motor neuron and reaches the neuromuscular junction (NMJ). Acetylcholine is released from motor nerve terminals, binds nicotinic receptors on the motor end plate, and depolarises the muscle fibre. The action potential spreads along the T-tubule system, triggering Ca2+ release from the sarcoplasmic reticulum (SR) via ryanodine receptors. Ca2+ binds troponin C, causing a conformational change in the troponin-tropomyosin complex that exposes myosin-binding sites on actin. Myosin heads (powered by ATP hydrolysis) bind actin and undergo the power stroke, sliding thin filaments toward the centre of the sarcomere and shortening the muscle.

Joints are classified by structure and movement. Fibrous joints (e.g., sutures of the skull) permit minimal movement. Cartilaginous joints include synchondroses (e.g., growth plates — hyaline cartilage) and symphyses (e.g., pubic symphysis — fibrocartilage). Synovial joints are the most common and mobile; a joint capsule encloses a fluid-filled cavity lined by synovium. Types include hinge (elbow, knee — one plane), ball-and-socket (hip, shoulder — multiaxial), and pivot (atlantoaxial — rotation).

Tendons connect muscle to bone and transmit mechanical force. Ligaments connect bone to bone and stabilise joints. Both are composed primarily of type I collagen. Cartilage lacks blood vessels and nerves; it relies on diffusion of nutrients from synovial fluid. This limits its regenerative capacity after injury.