Chapter 3: Musculoskeletal System

The skeletal framework, consisting of over two hundred bones, maintains systemic mineral balance through tightly regulated calcium and phosphate metabolism governed by parathyroid hormone, calcitriol, and estrogen. Bone tissue undergoes continuous remodeling through the coordinated actions of osteoblasts, osteocytes, and osteoclasts, with disruptions leading to conditions such as rickets and osteomalacia. Joints, primarily synovial in structure and reinforced by ligaments and cartilage, are subject to degenerative and inflammatory pathologies including osteoarthritis, gout, and traumatic injuries like the unhappy triad involving the anterior cruciate ligament and medial meniscus. Skeletal muscle function depends on the sarcomeric apparatus, where actin and myosin filaments interact under neural control via acetylcholine signaling at the neuromuscular junction. Muscle fiber types diverge in their metabolic strategies, with oxidative Type I fibers supporting sustained low-intensity activity and glycolytic Type II fibers enabling rapid powerful contractions. Metabolic flexibility allows muscles to shift between glucose, fatty acids, and ketone bodies depending on nutritional state, though genetic defects in metabolic enzymes produce glycogen storage diseases. The integumentary system serves as a protective barrier with thermoregulatory and immunological functions, and its pathologies range from inflammatory responses like contact dermatitis and psoriasis to infectious and autoimmune conditions. Connective tissues derive their mechanical properties from Type I collagen and elastin, while adipose tissue functions in energy storage, insulation, and endocrine signaling. Multiple autoimmune disorders target musculoskeletal components through aberrant immune mechanisms, including myasthenia gravis affecting the neuromuscular junction, rheumatoid arthritis causing joint inflammation, and systemic lupus erythematosus producing systemic manifestations including skin involvement.