Key Takeaways
- The adult skeleton has 206 bones divided into axial (80) and appendicular (126) skeletons
- Osteoblasts build bone; osteoclasts break down bone; osteocytes maintain bone
- Three muscle types: skeletal (voluntary, striated), cardiac (involuntary, striated), smooth (involuntary, non-striated)
- Tendons connect muscle to bone; ligaments connect bone to bone
- Synovial joints are freely movable and contain synovial fluid for lubrication
Last updated: January 2026
The Musculoskeletal System
The musculoskeletal system provides structure, support, protection, and movement. It consists of bones, muscles, cartilage, tendons, ligaments, and joints.
Skeletal System Functions
| Function | Description |
|---|---|
| Support | Framework for body structure |
| Protection | Shields vital organs (skull protects brain) |
| Movement | Provides attachment points for muscles |
| Blood cell production | Hematopoiesis in red bone marrow |
| Mineral storage | Stores calcium and phosphorus |
Bone Classification
| Type | Shape | Examples |
|---|---|---|
| Long bones | Longer than wide | Femur, humerus, phalanges |
| Short bones | Roughly cube-shaped | Carpals, tarsals |
| Flat bones | Thin, flat | Skull, sternum, ribs, scapula |
| Irregular bones | Complex shapes | Vertebrae, pelvis |
| Sesamoid bones | Embedded in tendons | Patella |
Bone Structure
| Part | Description |
|---|---|
| Diaphysis | Shaft of long bone |
| Epiphysis | Ends of long bone |
| Epiphyseal plate | Growth plate (cartilage in children) |
| Periosteum | Outer membrane covering bone |
| Compact bone | Dense, hard outer layer |
| Spongy bone | Porous inner layer with trabeculae |
| Medullary cavity | Central cavity containing marrow |
Bone Cells
| Cell Type | Function |
|---|---|
| Osteoblasts | Build new bone (bone formation) |
| Osteocytes | Maintain bone tissue |
| Osteoclasts | Break down bone (resorption) |
Axial vs. Appendicular Skeleton
| Division | Bones | Count |
|---|---|---|
| Axial | Skull, vertebrae, ribs, sternum | 80 bones |
| Appendicular | Limbs, shoulder girdle, pelvic girdle | 126 bones |
| Total | 206 bones (adult) |
Joint Types
| Type | Movement | Examples |
|---|---|---|
| Fibrous | Immovable | Skull sutures |
| Cartilaginous | Slightly movable | Vertebral discs, pubic symphysis |
| Synovial | Freely movable | Knee, elbow, shoulder, hip |
Synovial Joint Features
| Feature | Function |
|---|---|
| Articular cartilage | Reduces friction between bones |
| Synovial membrane | Produces synovial fluid |
| Synovial fluid | Lubricates joint |
| Joint capsule | Encloses joint |
| Ligaments | Connect bone to bone |
Muscle Types
| Type | Location | Control | Appearance |
|---|---|---|---|
| Skeletal | Attached to bones | Voluntary | Striated |
| Cardiac | Heart | Involuntary | Striated |
| Smooth | Organs, blood vessels | Involuntary | Non-striated |
Muscle Structure
| Component | Description |
|---|---|
| Muscle | Entire organ |
| Fascicle | Bundle of muscle fibers |
| Muscle fiber | Single muscle cell |
| Myofibril | Contractile units within fiber |
| Sarcomere | Basic unit of contraction |
Muscle Contraction
Sliding Filament Theory:
- Nerve impulse triggers calcium release
- Calcium exposes binding sites on actin
- Myosin heads attach to actin (cross-bridges)
- Myosin pulls actin, sliding filaments together
- Sarcomere shortens → muscle contracts
Key proteins:
- Actin: Thin filaments
- Myosin: Thick filaments with heads
Key Connective Tissues
| Tissue | Connects |
|---|---|
| Tendons | Muscle to bone |
| Ligaments | Bone to bone |
| Cartilage | Cushions joints, provides flexibility |
Test Your Knowledge
Which bone cells are responsible for breaking down bone tissue?
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D
Test Your Knowledge
Which type of muscle tissue is found in the walls of blood vessels?
A
B
C
D
Test Your Knowledge
What connects muscle to bone?
A
B
C
D