Skeletal System: Structure, Function, and Classification Study Notes

Skeletal System Overview

Functions of the Skeletal System

The skeletal system provides the framework for the body and serves several essential functions necessary for survival and movement.

• Support: The skeleton supports the body and maintains its shape.

• Protection: Bones protect vital organs, such as the brain (skull) and heart/lungs (rib cage).

• Movement: Bones act as levers for muscles, enabling movement.

• Storage (Mineral & Fat): Bones store minerals (e.g., calcium, phosphorus) and fat in the marrow.

• Hematopoiesis: The production of blood cells occurs in the red bone marrow.

• Acid-Base Homeostasis: Bones help regulate blood pH by absorbing or releasing alkaline salts.

Bone Structure and Classification

Types of Bones

Bones are classified based on their shapes and functions.

• Long bones: Longer than they are wide (e.g., femur, humerus).

• Short bones: Cube-shaped (e.g., carpals, tarsals).

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

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

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

Macroscopic Parts of a Typical Long Bone

Long bones have distinct anatomical regions and features.

• Diaphysis: The shaft of the bone, composed mainly of compact bone.

• Epiphysis: The ends of the bone, containing spongy bone and red marrow.

• Articular cartilage: Covers the epiphyses, reducing friction at joints.

• Periosteum: A dense connective tissue membrane covering the bone's surface.

• Medullary cavity: The hollow interior of the diaphysis, containing yellow marrow.

• Epiphyseal line/plate: Site of bone growth in children and adolescents.

Bone Tissue Composition

Inorganic and Organic Matrix

Bones are composed of both inorganic and organic components, each contributing to their strength and flexibility.

• Inorganic matrix: Mainly hydroxyapatite (calcium phosphate crystals), providing hardness.

• Organic matrix: Primarily collagen fibers and ground substance, providing flexibility and tensile strength.

Equation:

$\text{Bone} = \text{Inorganic matrix} + \text{Organic matrix}$

Bone Cells

Types of Bone Cells

Bone tissue contains several specialized cell types, each with unique functions.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoblasts: Cells responsible for bone formation.

• Osteoclasts: Cells that break down bone tissue during remodeling.

Microscopic Structure of Bone

Compact vs. Spongy Bone

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

• Compact bone: Dense and forms the outer layer of bones; contains osteons (Haversian systems).

• Spongy bone: Porous and found at the ends of long bones; contains trabeculae and red marrow.

Osteon (Haversian System)

The osteon is the fundamental unit of compact bone.

• Central (Haversian) canal: Contains blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

Bone Formation and Growth

Ossification Processes

Bones form through two main processes: intramembranous and endochondral ossification.

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

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

Bone Remodeling and Repair

Bone tissue is continuously remodeled and repaired throughout life.

• Bone deposition: Addition of new bone by osteoblasts.

• Bone resorption: Removal of bone by osteoclasts.

Remodeling is influenced by hormones, diet, and mechanical stress.

Hormonal Regulation of Bone

Several hormones regulate bone growth and remodeling.

• Sex hormones: Estrogen and testosterone promote bone growth during puberty.

• Dietary factors: Calcium and vitamin D are essential for bone health.

• Age: Bone density decreases with age, increasing risk of osteoporosis.

• Calcium homeostasis: Parathyroid hormone (PTH) and calcitonin regulate blood calcium levels.

Classification of the Skeleton

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts: axial and appendicular.

Skeleton	Main Components	Examples
Axial	Skull, vertebral column, rib cage	Skull (28 bones), Sternum, Ribs, Vertebrae
Appendicular	Limb bones, girdles	Clavicle, Scapula, Humerus, Femur, Tibia, Fibula

Major Bones and Bone Markings

Each region of the skeleton contains specific bones and anatomical landmarks.

• Skull: Frontal, parietal, occipital, temporal bones; facial bones.

• Vertebral column: Cervical, thoracic, lumbar vertebrae; sacrum; coccyx.

• Thoracic cage: Sternum, ribs.

• Upper limb: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Lower limb: Femur, tibia, fibula, tarsals, metatarsals, phalanges.

• Pectoral girdle: Clavicle, scapula.

• Pelvic girdle: Hip bones (ilium, ischium, pubis).

Joints and Articulations

Classification of Joints

Joints are classified by their structure and function.

• Structural classification:

  • Fibrous: Bones joined by fibrous tissue (e.g., sutures in the skull).

  • Cartilaginous: Bones joined by cartilage (e.g., symphysis).

  • Synovial: Bones separated by a joint cavity filled with synovial fluid (e.g., knee, shoulder).

• Functional classification:

  • Synarthroses: Immovable joints (e.g., sutures).

  • Amphiarthroses: Slightly movable joints (e.g., symphysis).

  • Diarthroses: Freely movable joints (e.g., synovial joints).

Synovial Joints

Synovial joints are the most movable type of joint and have distinct anatomical features.

• Joint cavity: Space filled with synovial fluid.

• Articular cartilage: Covers bone surfaces.

• Synovial membrane: Produces synovial fluid.

• Ligaments: Connect bones and stabilize joints.

Movements at Synovial Joints

Synovial joints allow a variety of movements.

• Flexion vs. Extension

• Abduction vs. Adduction

• Dorsiflexion vs. Plantar flexion

• Rotation vs. Circumduction

• Pronation vs. Supination

• Inversion vs. Eversion

Comparison of Joint Classes

Joint Class	Movement	Examples
Synarthroses	Immovable	Sutures of the skull
Amphiarthroses	Slightly movable	Symphysis pubis
Diarthroses	Freely movable	Shoulder, knee

Additional info:

• Some content inferred from standard Anatomy & Physiology curriculum to ensure completeness and clarity.

• Tables reconstructed for comparison and classification purposes.