Skeletal System

Functions of the Skeletal System

The skeletal system provides the structural framework for the body and serves several vital functions.

• Support: Maintains body shape and supports soft tissues.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores calcium and phosphorus.

• Blood Cell Production: Houses red bone marrow for hematopoiesis.

• Energy Storage: Yellow bone marrow stores lipids.

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions.

• Long Bones: Longer than wide (e.g., femur, humerus).

• Short Bones: Nearly equal length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., sternum, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

• Sesamoid Bones: Embedded in tendons (e.g., patella).

Histology of Bone Tissue and Osteon Structure

Bones are dynamic organs composed of several tissue types, primarily osseous tissue. The osteon is the fundamental unit of compact bone.

• Osteon (Haversian System): Cylindrical structures containing concentric lamellae, central canal, osteocytes in lacunae, and canaliculi.

• Gross Anatomy: Includes diaphysis, epiphysis, metaphysis, medullary cavity, and periosteum.

Compact vs. Spongy Bone

Bone tissue is organized into two main types: compact and spongy bone.

• Compact Bone: Dense, forms the outer layer, composed of osteons.

• Spongy Bone: Porous, found at ends of long bones, composed of trabeculae.

Types and Functions of Cartilage

Cartilage is a flexible connective tissue found in various parts of the body.

• Hyaline Cartilage: Most common, found in joints, nose, trachea.

• Elastic Cartilage: Flexible, found in ear and epiglottis.

• Fibrocartilage: Tough, found in intervertebral discs, pubic symphysis.

Bone Formation: Endochondral vs. Intramembranous Ossification

Bone develops through two main processes:

• Endochondral Ossification: Bone forms from cartilage templates (most bones).

• Intramembranous Ossification: Bone forms directly from mesenchymal tissue (flat bones of skull).

Epiphyseal Plate and Bone Growth

The epiphyseal plate (growth plate) is responsible for longitudinal bone growth during development.

• Layers: Resting, proliferative, hypertrophic, calcification, ossification zones.

• Appositional Growth: Increase in bone diameter by adding new bone tissue to the surface.

Types of Fractures

Fractures are classified based on their characteristics.

• Simple (Closed): Bone breaks but does not pierce skin.

• Compound (Open): Bone breaks and pierces skin.

• Comminuted: Bone shatters into multiple pieces.

• Greenstick: Incomplete break, common in children.

Phases of Bone Repair

Bone repair occurs in several stages:

• Hematoma Formation

• Fibrocartilaginous Callus Formation

• Bony Callus Formation

• Bone Remodeling

Calcium, Vitamin D, PTH, and Calcitonin

Calcium homeostasis is regulated by several hormones.

• Vitamin D: Increases calcium absorption from the gut.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

Muscular System

Gross and Microanatomy of Skeletal Muscle

Skeletal muscle is composed of bundles of fibers organized for contraction.

• Gross Anatomy: Muscle belly, tendons, fascicles.

• Microanatomy: Muscle fibers, myofibrils, sarcomeres.

• Appearance: Striated due to arrangement of actin and myosin filaments.

Components of Skeletal and Muscle System

The musculoskeletal system includes bones, muscles, tendons, ligaments, and connective tissues.

• Bones: Provide structure.

• Muscles: Enable movement.

• Tendons: Connect muscle to bone.

• Ligaments: Connect bone to bone.

Sources of Energy for Muscle Contraction

Muscle cells utilize both aerobic and anaerobic pathways for energy.

• Aerobic Respiration: Uses oxygen, produces ATP efficiently.

• Anaerobic Respiration: No oxygen, produces ATP quickly but less efficiently, leads to lactic acid buildup.

Energy Yield:

• Aerobic: Up to 36 ATP per glucose molecule.

• Anaerobic: 2 ATP per glucose molecule.

Functional Unit of Muscle Fiber

The sarcomere is the basic contractile unit of muscle fiber.

• Composed of actin (thin) and myosin (thick) filaments.

• Responsible for muscle contraction via the sliding filament mechanism.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber.

• Transmits nerve impulses to muscle, triggering contraction.

• Uses acetylcholine as the neurotransmitter.

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates.

• Small motor units: Fine control (e.g., eye muscles).

• Large motor units: Gross movements (e.g., leg muscles).

Action Potential and Excitation-Contraction Coupling

Muscle contraction is initiated by an action potential that leads to calcium release and interaction of actin and myosin.

• Action Potential: Electrical signal travels along sarcolemma.

• Excitation-Contraction Coupling: Links electrical signal to muscle contraction.

Equation:

Sliding Filament and Cross-Bridge Cycle

Muscle contraction occurs as myosin heads bind to actin, forming cross-bridges and pulling filaments past each other.

• ATP binds to myosin, allowing detachment from actin.

• Hydrolysis of ATP powers the next cycle.

Effects of Exercise and Aging on Muscle and Bones

Physical activity and aging have significant impacts on musculoskeletal health.

• Exercise: Increases bone density, muscle mass, and strength.

• Aging: Leads to bone loss (osteoporosis), muscle atrophy, and decreased strength.

Microanatomy and Function of Smooth Muscle

Smooth muscle is found in walls of hollow organs and differs from skeletal and cardiac muscle.

• Non-striated: Lacks sarcomeres.

• Involuntary control: Regulated by autonomic nervous system.

• Location: Blood vessels, digestive tract, respiratory tract.

Microanatomy of Cardiac Muscle and Syncytium

Cardiac muscle cells are striated, branched, and interconnected by intercalated discs, allowing the heart to contract as a unit (syncytium).

• Intercalated Discs: Specialized junctions for electrical and mechanical connection.

• Syncytium: Coordinated contraction of the entire heart muscle.

Equation:

Additional info: Some details, such as the specific zones of the epiphyseal plate and the phases of bone repair, were expanded for academic completeness.