Skeletal and Muscular Systems

Core Review

The Skeletal System

Functions of bone: Support and structural framework, protection of vital organs (skull protects brain, rib cage protects heart/lungs), movement (attachment points for muscles), mineral storage (calcium and phosphate), blood cell production (hematopoiesis in red bone marrow), and energy storage (yellow bone marrow stores fat).

Bone Structure:

• Compact bone: Dense outer layer composed of osteons (Haversian systems) — concentric rings (lamellae) of mineralized matrix around a central Haversian canal containing blood vessels and nerves. Osteocytes sit in lacunae, connected by canaliculi for nutrient exchange.
• Spongy (cancellous) bone: Lighter interior with a honeycomb structure of trabeculae. Found in the epiphyses of long bones and flat bones. Contains red bone marrow for hematopoiesis.

Bone Cells:

• Osteoblasts: Build bone by secreting collagen and mineralizing the matrix. Found on bone surfaces.
• Osteoclasts: Large, multinucleated cells derived from monocytes/macrophages. They secrete acid and enzymes to resorb (break down) bone matrix, releasing Ca²⁺ into the blood.
• Osteocytes: Mature osteoblasts that become trapped within the mineralized matrix. They reside in lacunae and sense mechanical stress, signaling for remodeling.

Axial vs. Appendicular Skeleton:

• Axial: Skull, vertebral column, ribs, sternum (80 bones). Protects CNS and thoracic organs.
• Appendicular: Pectoral and pelvic girdles, limbs (126 bones). Enables locomotion and manipulation.

Calcium Homeostasis: Blood Ca²⁺ must be maintained within a narrow range (~9–10.5 mg/dL). Two hormones regulate this:

• Parathyroid hormone (PTH): Released by the parathyroid glands when blood Ca²⁺ is LOW. PTH stimulates osteoclast activity (bone resorption), increases kidney Ca²⁺ reabsorption, and activates vitamin D (which increases intestinal Ca²⁺ absorption). Net effect: increases blood Ca²⁺.
• Calcitonin: Released by the thyroid gland when blood Ca²⁺ is HIGH. It inhibits osteoclast activity and stimulates osteoblasts (bone deposition). Net effect: decreases blood Ca²⁺. (Calcitonin is less critical in adult humans; PTH is the dominant calcium regulator.)

The Muscular System

Three Types of Muscle:

Skeletal Muscle Organization: Muscle → fascicles → muscle fibers (cells) → myofibrils → sarcomeres. The sarcolemma (plasma membrane) has invaginations called T-tubules (transverse tubules) that propagate action potentials deep into the fiber. The sarcoplasmic reticulum (specialized smooth ER) stores and releases Ca²⁺.

The Sarcomere and Sliding Filament Mechanism: The sarcomere is the functional contractile unit, bounded by Z-lines. It contains:

• Thin filaments (actin): Anchored at Z-lines. Include tropomyosin (blocks myosin binding sites in resting muscle) and troponin (Ca²⁺-binding complex).
• Thick filaments (myosin): Contain myosin heads with ATPase activity that bind actin and perform the power stroke.

Steps of Muscle Contraction:

1. Action potential arrives at the neuromuscular junction, releasing acetylcholine (ACh), which depolarizes the sarcolemma.
2. Depolarization travels down T-tubules, triggering Ca²⁺ release from the sarcoplasmic reticulum.
3. Ca²⁺ binds troponin, causing a conformational change that moves tropomyosin, exposing myosin-binding sites on actin.
4. Myosin head (already "cocked" with ADP + Pᵢ from prior ATP hydrolysis) binds actin, forming a cross-bridge.
5. Power stroke: Myosin head pivots, pulling actin filament toward the M-line (center of the sarcomere). ADP and Pᵢ are released.
6. ATP binds to the myosin head, causing it to detach from actin.
7. ATP is hydrolyzed to ADP + Pᵢ, re-cocking the myosin head. The cycle repeats as long as Ca²⁺ remains elevated and ATP is available.
8. When the action potential ends, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum, tropomyosin re-covers myosin-binding sites, and the muscle relaxes.

Rigor mortis: After death, ATP production ceases. Without ATP, myosin heads cannot detach from actin, locking muscles in a contracted state. This resolves as proteins degrade several hours later.

Common Traps

• "Calcitonin is the most important calcium regulator": No — PTH is. Calcitonin plays a minor role in humans. The DAT may ask what hormone is released in response to HIGH Ca²⁺, which IS calcitonin, but PTH is the primary regulator overall.
• "Smooth muscle has sarcomeres": No. Smooth muscle contracts via a different mechanism (calmodulin → myosin light-chain kinase → phosphorylation). Sarcomeres are unique to skeletal and cardiac muscle.
• "ATP causes muscle contraction": This is backwards. ATP causes RELAXATION (allows myosin to detach from actin). ATP hydrolysis provides the energy for contraction (cocking the myosin head), but ATP binding is the detachment step.