Skeletal System

Bone Shapes and Examples

The human skeleton is composed of bones with various shapes, each adapted for specific functions.

• Long bones: e.g., femur, humerus. Function: support weight and facilitate movement.

• Short bones: e.g., carpals, tarsals. Function: provide stability and some movement.

• Flat bones: e.g., sternum, ribs, skull. Function: protect internal organs and provide surfaces for muscle attachment.

• Irregular bones: e.g., vertebrae, pelvis. Function: protect nervous tissue, anchor points for skeletal muscle.

• Sesamoid bones: e.g., patella. Function: protect tendons from stress and wear.

Structure of Bone

Bones have a complex structure with several regions and tissue types.

• Diaphysis: Shaft of a long bone, composed mainly of compact bone.

• Medullary cavity: Central cavity containing yellow bone marrow (fat storage).

• Epiphysis: Ends of long bones, contain spongy bone and red marrow (hematopoiesis).

• Articular cartilage: Hyaline cartilage covering joint surfaces, reduces friction.

• Periosteum: Dense connective tissue covering bone surface, contains nerves and blood vessels.

• Endosteum: Thin membrane lining the medullary cavity.

Bone Marrow

Bone marrow is a soft tissue found in the cavities of bones.

• Red bone marrow: Site of blood cell production (hematopoiesis), found in flat bones and the epiphyses of long bones.

• Yellow bone marrow: Stores fat, found in the medullary cavity of long bones.

• Most red marrow in adults: Found in flat bones (e.g., pelvis, sternum).

• Function of red bone marrow: Produces red blood cells, white blood cells, and platelets.

Bone Matrix Composition

The bone matrix is a composite material providing strength and flexibility.

• Organic portion: Mainly collagen fibers, provides tensile strength and flexibility.

• Inorganic salts: Primarily hydroxyapatite (calcium phosphate), provides hardness and resistance to compression.

• Calcium phosphate: Main mineral salt in bone.

• Calcium carbonate: Another mineral component.

• Bone cells: Osteoblasts (build bone), osteocytes (maintain bone), osteoclasts (break down bone).

Microscopic Anatomy of Bone

Bone tissue can be classified as compact or spongy, each with distinct microscopic features.

• Compact bone: Dense, organized into osteons (Haversian systems).

• Spongy (cancellous) bone: Lattice-like trabeculae, contains red marrow.

• Osteon (Haversian system): Structural unit of compact bone, consists of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allow for nutrient and waste exchange.

Bone Formation (Ossification)

Ossification is the process of bone tissue formation, occurring via two main pathways.

• Intramembranous ossification: Bone develops directly from mesenchyme (e.g., flat bones of the skull).

• Endochondral ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

• Epiphyseal plate: Growth plate where new bone is produced during childhood and adolescence.

• Bone remodeling: Ongoing replacement of old bone tissue by new bone tissue.

Hormonal Regulation of Bone

Bone growth and remodeling are regulated by several hormones.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclast activity.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Growth hormone, estrogen, testosterone: Stimulate bone growth.

• Vitamin D: Promotes calcium absorption in the gut.

Bone Fractures and Healing

Fractures are breaks in bone continuity, classified by their characteristics.

• Types of fractures: Simple (closed), compound (open), comminuted, greenstick, spiral, impacted, etc.

• Healing process: Hematoma formation → fibrocartilaginous callus → bony callus → bone remodeling.

Osteoporosis

Osteoporosis is a disease characterized by decreased bone density and increased fracture risk.

• Causes: Aging, hormonal changes (especially postmenopausal women), lack of calcium/vitamin D, sedentary lifestyle.

• Diagnosis: Bone density measurement (DEXA scan).

• Treatment: Medications (bisphosphonates), calcium/vitamin D supplementation, weight-bearing exercise.

Joints and Articulations

Joint Structure and Function

Joints (articulations) are connections between bones, allowing for movement and flexibility.

• Types of joints: Fibrous (immovable), cartilaginous (slightly movable), synovial (freely movable).

• Synovial joints: Characterized by a joint cavity filled with synovial fluid.

• Movements: Nonaxial (gliding), uniaxial (hinge, pivot), biaxial (condyloid, saddle), multiaxial (ball-and-socket).

• Examples: Shoulder (ball-and-socket), elbow (hinge), wrist (condyloid), thumb (saddle), vertebral (gliding).

Joint Stability and Injuries

• Stabilizing structures: Ligaments, tendons, joint capsule, menisci, bursae.

• Common injuries: Sprains (ligament injury), strains (muscle/tendon injury), dislocations, bursitis, tendonitis.

Muscular System

Types of Muscle Tissue

There are three types of muscle tissue, each with unique structure and function.

• Skeletal muscle: Voluntary, striated, attached to bones, responsible for body movement.

• Cardiac muscle: Involuntary, striated, found only in the heart.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscle Anatomy and Contraction

• Muscle fiber: Single muscle cell, contains myofibrils composed of sarcomeres.

• Sarcomere: Functional unit of muscle contraction, composed of actin and myosin filaments.

• Sliding filament theory: Explains muscle contraction as actin and myosin filaments slide past each other.

• Neuromuscular junction: Synapse between a motor neuron and muscle fiber; acetylcholine (ACh) is the neurotransmitter.

• Excitation-contraction coupling: Sequence of events from nerve impulse to muscle contraction.

Muscle Metabolism and Fatigue

• ATP production: Required for muscle contraction; produced via aerobic and anaerobic metabolism.

• Creatine phosphate: Rapidly regenerates ATP in muscle cells.

• Muscle fatigue: Inability to maintain force of contraction; caused by depletion of ATP, accumulation of lactic acid, ionic imbalances.

Muscle Soreness and Disorders

• DOMS (Delayed Onset Muscle Soreness): Muscle pain and stiffness occurring 24-72 hours after intense exercise; caused by microtrauma to muscle fibers.

• Muscular atrophy: Decrease in muscle mass due to disuse or disease.

• Fibromyalgia: Chronic widespread musculoskeletal pain, cause is not fully understood.

• Torn muscle: Muscle fibers are overstretched or torn; concern is loss of function and possible long-term damage.

Motor Units and Muscle Control

• Motor unit: A motor neuron and all the muscle fibers it innervates.

• Recruitment: Increasing the number of active motor units to increase muscle force.

• Isotonic contraction: Muscle changes length (concentric or eccentric).

• Isometric contraction: Muscle length does not change.

Muscle Fiber Types

• Type I (slow-twitch): Fatigue-resistant, aerobic, used for endurance.

• Type II (fast-twitch): Fatigue quickly, anaerobic, used for short bursts of power.

Table: Comparison of Muscle Tissue Types

Additional info:

• Bone remodeling is influenced by mechanical stress and hormones.

• Osteoporosis prevention includes adequate calcium/vitamin D intake and regular exercise.

• Muscle soreness (DOMS) is not caused by lactic acid but by microscopic damage to muscle fibers.