Support and Movement

Skeletons across the animal kingdom

Support systems fall into three patterns: hydrostatic (annelids, cnidarians) using fluid pressure against muscular walls; exoskeletons (arthropods, molluscs) made of chitin or calcium carbonate; and endoskeletons (echinoderms, vertebrates). Punjab Textbook Board Biology XII Chapter 17 and Campbell 12e Chapter 50 stress that each design trades off mass, growth strategy, and protection. Vertebrate endoskeletons grow continuously without moulting; arthropod exoskeletons require ecdysis. The human skeleton has 206 bones in adulthood (newborns have ≈270; many fuse postnatally).

Bone as a living tissue

Compact bone is built of osteons (Haversian systems): central canal carrying vessels and nerves, concentric lamellae of mineralised matrix, and lacunae housing osteocytes connected by canaliculi. The matrix is ~70 % hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ and ~30 % organic, mostly type I collagen. Spongy (cancellous) bone has trabeculae oriented along stress lines (Wolff's law, 1892). Three cell types remodel bone: osteoblasts deposit matrix, osteocytes maintain it, and osteoclasts (multinucleate, derived from monocyte lineage) resorb it via H⁺ secretion and cathepsin K. Parathyroid hormone raises plasma Ca²⁺ (normal range 8.5–10.5 mg/dL) by stimulating osteoclasts; calcitonin from thyroid C-cells inhibits them.

Skeletal muscle ultrastructure

A skeletal muscle fibre is a single multinucleate cell containing myofibrils built of repeating sarcomeres. The sarcomere extends from Z-disc to Z-disc and contains thick filaments of myosin II in the A-band, and thin filaments of actin, troponin (T, I, C subunits), and tropomyosin in the I-band. The H-zone (centre of A-band) holds only myosin; the M-line anchors thick filaments. Each thick filament has ~300 myosin heads. The transverse tubule (T-tubule) system invaginates the sarcolemma; the sarcoplasmic reticulum stores Ca²⁺ at concentrations near 1 mM (resting cytosolic Ca²⁺ is ~100 nM, a 10 000-fold gradient).

Sliding-filament mechanism

Huxley and Hanson's 1954 sliding-filament model — confirmed independently by A. F. Huxley and H. E. Huxley — explains contraction without filament shortening. Excitation–contraction coupling: action potential travels down the T-tubule, opens DHPR which mechanically gates RyR1, releasing Ca²⁺ from SR. Ca²⁺ binds troponin C, shifting tropomyosin to expose myosin-binding sites on actin. The cross-bridge cycle: ATP-bound myosin head detaches; ATP hydrolysis cocks the head; binding to actin releases Pᵢ and triggers the power stroke (≈10 nm displacement, generating ~5 pN force per head); ADP release leaves the rigor-bound state until a new ATP arrives. Rigor mortis reflects ATP depletion locking heads to actin.

Joints, fibre types, and disorders

Synovial joints (shoulder, knee, hip) have hyaline cartilage and a fluid-filled capsule; cartilaginous joints (intervertebral discs) and fibrous joints (skull sutures) permit limited or no motion. Skeletal-muscle fibres divide into Type I (slow oxidative, fatigue-resistant, myoglobin-rich, postural muscles), Type IIa (fast oxidative-glycolytic), and Type IIb/IIx (fast glycolytic, sprinter-type). Common MDCAT clinical vignettes include osteoporosis (BMD > 2.5 SD below young-adult mean, post-menopausal oestrogen loss accelerates osteoclast activity), Duchenne muscular dystrophy (X-linked dystrophin mutation), and rickets (vitamin-D deficiency impairing matrix mineralisation, still prevalent in low-sunlight Punjabi winter cohorts).