Musculoskeletal System
The musculoskeletal system encompasses 206 bones, ~600 skeletal muscles (~40% of body mass), tendons, ligaments, ~400 joints, and the connective tissue matrices binding these structures. Far from merely providing mechanical support, it is a major metabolic, endocrine, haematopoietic, and thermogenic organ: the skeleton holds 99% of body calcium in hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂] and houses red bone marrow; osteocalcin from osteoblasts promotes insulin secretion; contracting muscle secretes myokines (IL-6, irisin, BDNF, VEGF, FGF21) coordinating whole-body metabolism; shivering thermogenesis in cold is almost entirely skeletal muscle-driven.
Overview
The musculoskeletal system's non-mechanical roles are often underappreciated. Bone is not inert mineral — it is a dynamic endocrine organ undergoing continuous remodelling (~10% of skeleton renewed per year) by coupled osteoclast–osteoblast activity, regulated by PTH, oestrogen, calcitriol, and mechanical loading (Wolff's law). The osteoblast-derived hormone osteocalcin enters the circulation to stimulate insulin secretion and muscle glucose uptake — a bone-to-pancreas-to-muscle endocrine axis. Skeletal muscle is the dominant site of insulin-stimulated glucose disposal (80% of postprandial glucose uptake) and, through exercise-induced myokine secretion, communicates with adipose tissue (irisin → browning), the brain (BDNF → hippocampal neurogenesis), liver (IL-6 → gluconeogenesis + anti-inflammatory IL-10), and the vasculature (VEGF → angiogenesis).
Bone Biology
| Cell | Origin | Function |
|---|---|---|
| Osteoblasts | Mesenchymal stem cells | Synthesise osteoid (type I collagen, osteocalcin, osteopontin, bone sialoprotein); mineralisation; regulate HSC niche (CXCL12, SCF, Angpt-1) |
| Osteocytes | Embedded osteoblasts | Mechanosensing via canalicular network; sclerostin (Wnt/LRP5 antagonist → ↓formation); FGF-23 (phosphatonin → kidney Pi excretion) |
| Osteoclasts | Monocyte-macrophage lineage (RANKL-driven) | Bone resorption via ruffled border V-ATPase (pH ~4.5) + cathepsin K + MMPs; Howship's lacunae; regulated by RANKL/RANK/OPG triad |
| Bone lining cells | Quiescent osteoblasts | Surface coverage of resting bone; activated by PTH or mechanical stimuli |
Remodelling cycle (BMU — basic multicellular unit): Activation (osteocyte mechanosensing or systemic signals → RANKL/OPG ratio ↑ → osteoclast recruitment) → Resorption (2–4 weeks) → Reversal (coupling factors TGF-β, IGF-1, BMP released → recruit osteoblasts) → Formation (3–4 months) → Mineralisation and rest. Oestrogen normally suppresses RANKL-driven osteoclastogenesis; menopause → rapid bone loss.
Skeletal Muscle — Excitation-Contraction Coupling
Motor neuron action potential
│ ACh released at NMJ → nAChR (Nm)
▼
End-plate potential → sarcolemmal AP propagates
│ enters T-tubules
▼
DHPR (L-type Ca²⁺ channel) in T-tubule
│ mechanical gating of RyR1 (sarcoplasmic reticulum)
▼
Massive SR Ca²⁺ release (100 nM → 10–100 µM cytosolic)
│
▼
Ca²⁺ binds Troponin C → conformational change
→ Tropomyosin shifts off actin myosin-binding sites
→ Myosin S1 binds actin
→ ATP hydrolysis → POWER STROKE (sarcomere shortens)
→ ADP+Pi release → rigor → new ATP resets cycle
│
RELAXATION: SERCA1a pumps Ca²⁺ back into SR
(PLN modulates SERCA; ATP-dependent)
HENNEMAN'S SIZE PRINCIPLE:
Low-threshold slow type I fibres recruited first
(fatigue-resistant, oxidative) → type IIa → type IIx last
(explosive, glycolytic, fatigable)
Key Functions
Mineral homeostasis: Skeleton holds 99% of body calcium (~1 kg) in hydroxyapatite. Serum ionised Ca²⁺ maintained at 1.1–1.3 mM by coupled PTH (↑osteoclastogenesis via ↑RANKL/↓OPG → bone resorption → ↑Ca²⁺), calcitriol (↑intestinal absorption), and calcitonin (↓osteoclasts). Osteocytes produce FGF-23 (phosphatonin) to regulate kidney phosphate excretion.
Haematopoiesis: Red bone marrow in axial skeleton and proximal long bones produces ~500 billion blood cells per day. Osteoblasts are essential HSC niche cells regulating quiescence and mobilisation via CXCL12/CXCR4, SCF/c-Kit, Angpt-1/Tie2. G-CSF mobilises HSCs by disrupting CXCL12/CXCR4.
Myokines — muscle as endocrine organ:
| Myokine | Trigger | Key systemic effects |
|---|---|---|
| IL-6 | Muscle contraction (AMPK), glycogen depletion | Hepatic gluconeogenesis; induces anti-inflammatory IL-10 and IL-1RA; insulin-sensitising; lipolysis in adipose |
| Irisin (FNDC5 cleavage) | Exercise via PGC-1α | Browning of white adipose (↑UCP1); ↑BDNF in hippocampus → memory + neuroplasticity; ↑bone density |
| BDNF | Endurance exercise | Hippocampal neurogenesis; memory consolidation; motor learning |
| VEGF | Hypoxia, exercise | Angiogenesis; ↑capillary density in trained muscle; ↓O₂ diffusion distance to mitochondria |
| FGF21 | Prolonged exercise, fasting | ↑fatty acid oxidation; ↑ketogenesis; ↑insulin sensitivity |
Thermogenesis: Shivering (involuntary rapid oscillatory contraction; anterior hypothalamic cold sensors → dorsomedial hypothalamus → motor neurons) generates heat by ATP hydrolysis without net mechanical work — the major heat source during cold exposure in adults (non-shivering thermogenesis via BAT/UCP1 dominates in neonates).
Exercise Adaptation
Resistance training
Mechanical stretch + mTORC1 activation → ↑muscle protein synthesis; satellite cell (muscle stem cell) activation → myonuclei addition → myofibre hypertrophy. ↑GLUT4 protein expression → greater insulin-stimulated glucose uptake at rest.
Endurance training
Ca²⁺ transients + AMPK → PGC-1α → TFAM → ↑mitochondrial biogenesis; ↑oxidative capacity; ↑VO₂max. VEGF → angiogenesis → ↑capillary density. Weight-bearing → osteocyte mechanosensing → ↓sclerostin → ↑Wnt → ↑bone formation.
Pathology
Osteoporosis
Imbalanced remodelling (net resorption > formation) → ↓BMD → fragility fractures. Leading cause: oestrogen deficiency at menopause (↓OPG → ↑RANKL → ↑osteoclasts). Secondary causes: glucocorticoid excess (↓OPG, ↓collagen synthesis, ↓gut Ca²⁺ absorption), hyperparathyroidism, hypogonadism, malabsorption. BMD T-score ≤−2.5 = osteoporosis; −1.0 to −2.5 = osteopenia. Antiresorptives: bisphosphonates (↑osteoclast apoptosis), denosumab (anti-RANKL mAb). Anabolics: teriparatide (intermittent PTH 1-34), abaloparatide, romosozumab (anti-sclerostin).
Rheumatoid Arthritis (RA)
Autoimmune synovitis: anti-citrullinated protein antibodies (ACPA) + rheumatoid factor → Th17 and macrophage-driven pannus → cartilage and bone erosion via RANKL-mediated osteoclast activation + MMPs. Systemic effects: ↑CVD risk, anaemia of chronic disease. DMARDs: methotrexate (anchor drug), hydroxychloroquine. Biologics: anti-TNF (etanercept, adalimumab), anti-IL-6R (tocilizumab), abatacept (CTLA4-Ig), rituximab (anti-CD20).
Osteoarthritis (OA)
Progressive articular cartilage breakdown (↑MMP-1/3/13, ADAMTS-4/5 → ↓aggrecan + ↓type II collagen), subchondral bone sclerosis + osteophyte formation, synovial inflammation (↑IL-1β, TNF-α, IL-6). Risk factors: ageing, obesity (mechanical + adipokine-mediated), female sex, prior joint injury. Management: exercise (most effective), weight loss, NSAIDs, intra-articular corticosteroids, total joint arthroplasty for end-stage.
Sarcopenia
Age-related muscle mass and function loss (~1%/year after age 30; accelerates after 60). Pathophysiology: ↓satellite cell pool, ↑anabolic resistance (to protein/insulin), ↑myostatin (TGF-β family → Smad2/3 → ↓protein synthesis), inflammaging (IL-6, TNF-α → ubiquitin-proteasome degradation), ↓motor unit innervation. Treatment: resistance exercise + adequate protein (1.2–1.6 g/kg/day) + vitamin D; investigational myostatin inhibitors.
Duchenne Muscular Dystrophy (DMD)
X-linked frameshift mutation in dystrophin (Xp21.2, largest human gene). Dystrophin connects actin to ECM (laminin) via DAPC. Absence → mechanical membrane fragility → Ca²⁺ overload → necrosis → fibrotic/fatty replacement. Progressive: loss of ambulation ~12 years; respiratory failure by 20s without ventilatory support. Exon-skipping therapies (eteplirsen, golodirsen), micro-dystrophin gene therapy, glucocorticoids (slow progression).
Rhabdomyolysis
Massive skeletal muscle necrosis releasing myoglobin → precipitates in renal tubules (at acid pH) + direct tubular toxicity → AKI. Causes: extreme exertion, crush injury, seizures, statins (CYP3A4 drug interactions), viral myositis. Hallmark: CK >5,000 IU/L, myoglobinuria (dipstick blood+, microscopy RBCs−). Treatment: aggressive IV fluid resuscitation → alkalinise urine.
Cross-Atlas Connections
References
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021. elsevier.com
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022. NCBI Bookshelf
- Pedersen BK, Febbraio MA. Muscles, exercise and obesity: skeletal muscle as a secretory organ. Nat Rev Endocrinol. 2012;8(8):457-465. doi:10.1038/nrendo.2012.49
- Kanis JA, et al. FRAX and the assessment of fracture probability in men and women from the UK. Osteoporos Int. 2008;19(4):385-397. doi:10.1007/s00198-007-0543-5
- Cruz-Jentoft AJ, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing. 2019;48(1):16-31. doi:10.1093/ageing/afy169