Skeletal System
High-Yield Summary
- Humans have an endoskeleton. Axial skeleton (skull, vertebral column, rib cage, hyoid) = protection/support. Appendicular skeleton (limbs + pectoral/pelvic girdles) = motion.
- Long bone regions: diaphysis (shaft, compact bone), epiphyses (ends, spongy bone), metaphysis (contains epiphyseal plate during growth → epiphyseal line after), marrow cavity (red marrow = blood cells in children, yellow marrow = fat in adults), periosteum (outer CT layer, growth/repair, tendon/ligament attachment).
- Bone matrix: organic portion (collagen) = flexibility/tensile strength; inorganic portion (calcium + phosphate → hydroxyapatite) = hardness/compression resistance.
- Compact bone: osteons (Haversian systems) — Haversian canal (vessels/nerves) → lamellae (rings) → lacunae (house osteocytes) → canaliculi (connect lacunae). Volkmann's canals connect osteons to periosteum. Spongy bone: trabeculae lattice, no osteons, houses red marrow.
- Remodeling: osteoblasts build bone (differentiate into osteocytes once embedded); osteoclasts resorb bone. PTH (low blood Ca2+) → stimulates osteoclasts → raises blood Ca2+. Calcitonin (high blood Ca2+) → inhibits osteoclasts → lowers blood Ca2+.
- Cartilage: chondrocytes in matrix, avascular + not innervated (heals slowly). Endochondral ossification = cartilage model replaced by bone (most long bones). Intramembranous ossification = bone forms directly from CT, no cartilage step (flat skull bones).
- Fibrous joints (skull sutures) = immovable, protective. Synovial joints (elbow, knee, shoulder, hip) = articular cartilage + synovial capsule/fluid + ligaments = freely movable.
Compact Bone vs. Spongy Bone
| Compact Bone | Spongy Bone |
|---|---|
| Osteons (Haversian systems), dense | Trabeculae lattice, no osteons, porous/lightweight |
| Diaphysis wall | Interior of epiphyses |
| Mechanical strength, weight-bearing | Force distribution, houses red marrow |
Osteoblasts vs. Osteoclasts
| Osteoblasts | Osteoclasts |
|---|---|
| Build bone — secrete collagen matrix, promote mineralization | Resorb bone — secrete acids/enzymes that dissolve matrix |
| Become osteocytes once embedded in matrix (mechanosensors) | Large, multinucleated cells |
| Stimulated by calcitonin (high blood Ca2+) | Stimulated by PTH (low blood Ca2+); releases Ca2+/phosphate into blood |
Key Terms
- Osteon (Haversian system)
- Cylindrical structural unit of compact bone, aligned with the bone's long axis.
- Hydroxyapatite
- Calcium- and phosphate-based mineral crystal that gives bone matrix its hardness.
- Epiphyseal plate
- Cartilage growth plate in the metaphysis; site of longitudinal bone growth, ossifies into the epiphyseal line when growth ends.
- Origin / Insertion
- Origin = proximal, fixed muscle attachment; insertion = distal attachment that moves toward the origin on contraction.
- Antagonistic pair
- Two muscles with opposite actions at a joint (e.g., biceps/triceps) — one contracts while the other relaxes.
Movement Classification at Joints
- 1Flexor — decreases the angle between two bones at a joint
- 2Extensor — increases the angle between two bones at a joint
- 3Abductor — moves a limb away from the body's midline
- 4Adductor — moves a limb toward the body's midline
- 5Medial rotation — turns a limb toward the midline; Lateral rotation — turns a limb away from the midline
Common MCAT Trap
- PTH RAISES blood calcium (stimulates resorption); calcitonin LOWERS blood calcium (inhibits resorption) — the names don't hint at direction, memorize the mechanism.
- Cartilage is avascular AND not innervated — that's why cartilage injuries (e.g., torn meniscus) heal much slower than bone fractures.
- Osteoblasts 'build' and become osteocytes once trapped in matrix; osteoclasts are a separate, unrelated cell line (large, multinucleated) that resorbs bone — don't confuse osteoblast/osteocyte/osteoclast.
- Endochondral ossification (cartilage → bone) forms most long bones; intramembranous ossification (no cartilage intermediate) forms flat skull bones — don't reverse these.
Quick Recall
What hormone raises blood calcium by stimulating osteoclast activity, and where is it released from?
What connects lacunae in compact bone, and why does it matter?
Which muscle attachment point stays relatively fixed during contraction — origin or insertion?
What's the key structural difference between fibrous and synovial joints?