Study Notes: The Skeletal System (Anatomy & Physiology)

The Skeletal System

Overview and Components

The skeletal system is a fundamental organ system in human anatomy, providing structure, protection, and facilitating movement. It consists of bones, joints, cartilages, and ligaments, and is subdivided into the axial and appendicular skeletons.

• Bones (skeleton): Rigid organs that form the framework of the body.

• Joints: Articulations where two or more bones meet, allowing movement.

• Cartilages: Flexible connective tissue found in joints and other areas.

• Ligaments: Bands of connective tissue connecting bones at joints.

Subdivisions of the Skeleton

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Comprises the limbs and girdles (pectoral and pelvic).

Functions of Bones

Major Roles

Bones serve several essential functions in the human body:

• Support: Provide structural support for the body.

• Protection: Safeguard soft organs (e.g., skull protects the brain, rib cage protects thoracic organs).

• Movement: Enable movement through attachment to muscles.

• Storage: Store minerals (calcium, phosphorus) and fats (in marrow).

• Blood cell formation: Hematopoiesis occurs in bone marrow.

Tissues of Bones

Types of Osseous Tissue

• Compact bone: Dense, smooth, and homogeneous tissue providing strength.

• Spongy bone: Composed of small, needlelike pieces (trabeculae) and many open spaces, contributing to lightweight structure and housing marrow.

Classification of Bones

Bone Shapes

Bones are classified by shape into four groups:

• Long bones: Longer than wide, with a shaft and enlarged ends. Mostly compact bone. Examples: Femur, Humerus

• Short bones: Cube-shaped, mostly spongy bone with a thin outer layer of compact bone. Includes sesamoid bones (e.g., patella). Examples: Carpals (wrist), Tarsals (ankle)

• Flat bones: Thin, flattened, usually curved. Two layers of compact bone sandwiching spongy bone. Examples: Skull bones, Ribs, Sternum

• Irregular bones: Complex shapes not fitting other categories. Examples: Vertebrae, Hip bones

Structure of Bone

Long Bone Anatomy

• Diaphysis (shaft): Main length of bone, composed of compact bone.

• Periosteum: Outer covering of diaphysis, made of fibrous connective tissue; secured by Sharpey's fibers.

• Epiphysis (ends): Mostly spongy bone enclosed by compact bone.

• Articular cartilage: Hyaline cartilage covering epiphyses, reducing friction at joints.

• Endosteum: Connective tissue lining the inner shaft surface.

• Medullary cavity: Central cavity containing yellow marrow (fat) in adults and red marrow (hematopoiesis) in children.

Bone Markings

Bone markings are features on bones serving as attachment sites or passageways for nerves and vessels.

• Projections/processes: Grow out from bone surface; all begin with "T"

  • e.g., tuberosity, trochanter, tubercle, crest, spine, epicondyle, condyle

• Depressions/cavities: Indentations; all begin with "F"

  • e.g., fossa, foramen, meatus, fissure, sinus

Name of bone marking	Description	Illustration
Trochanter	Very large, blunt, irregularly shaped process	Femur
Condyle	Rounded articular projection	Mandible
Foramen	Round or oval opening through a bone	Skull
Fossa	Shallow depression in a bone	Skull

Microscopic Anatomy of Bone

Spongy Bone

• Composed of trabeculae (needlelike pieces) and open spaces filled with marrow, blood vessels, and nerves.

Compact Bone

• Osteocytes: Mature bone cells in bone matrix.

• Lacunae: Cavities housing osteocytes.

• Lamellae: Concentric circles of lacunae around the central (Haversian) canal.

• Central (Haversian) canal: Runs lengthwise, carrying blood vessels and nerves.

• Osteon (Haversian system): Structural unit containing central canal and matrix rings.

• Canaliculi: Tiny canals radiating from central canal to lacunae, connecting bone cells to nutrient supply.

• Perforating (Volkmann's) canal: Perpendicular canal carrying blood vessels and nerves.

Cartilage Microscopic Anatomy

• Chondrocytes: Cartilage cells located in lacunae.

• Fibers in extracellular matrix provide flexibility.

• No blood vessels, resulting in slow growth and repair.

Bone Composition and Properties

• Bones are lightweight yet strong, resisting tension and compression.

• Collagen fibers: Provide flexibility and tensile strength.

• Calcium salts: Make bone hard and resistant to compression.

Bone Formation, Growth, and Remodeling

Ossification and Growth

• Ossification: Process of bone formation, occurring on hyaline cartilage or fibrous membranes.

• Long bone growth involves two phases:

  1. Osteoblasts cover cartilage model with bone matrix (embryo).

  2. Cartilage is replaced by bone, opening the medullary cavity (fetus).

Growth and Remodeling

• By birth, most cartilage is converted to bone except articular cartilages and epiphyseal plates.

• New cartilage forms on external faces; old cartilage is replaced by bone matrix.

• Appositional growth: Bones grow in width via osteoblasts (periosteum) and osteoclasts (endosteum).

• Growth is hormonally regulated (growth hormone, sex hormones).

Remodeling Factors

• Calcium ion level: Parathyroid hormone (PTH) activates osteoclasts when calcium is low; hypercalcemia prompts calcium storage by osteoblasts.

• Mechanical stress: Gravity and muscle pull determine where bone is formed or broken down.

Bone Fractures

Types and Treatment

• Fracture: Break in a bone.

• Closed (simple) fracture: Does not penetrate skin.

• Open (compound) fracture: Penetrates skin.

Fracture Type	Description	Comment
Comminuted	Bone breaks into three or more fragments	Common in older, brittle bones
Compression	Bone is crushed	Common in porous bones (osteoporotic)
Depressed	bone Pressed inward	Typical skull fracture
Impacted	Bone force into each other	Typical when breaking a fall
Spiral	Twisting applied to bone	Sports Injury
Greenstick	Incomplete break	Common in child

Repair Process

1. Hematoma formation

2. Fibrocartilage callus formation

3. Bony callus formation

4. Bone remodeling