Skeletal System and Articulations: Study Guide for Anatomy & Physiology

Skeletal System

Functions of the Skeletal System

The skeletal system provides the framework for the body, supports and protects organs, enables movement, stores minerals, and houses bone marrow for blood cell production.

• Support: Provides structural support for the entire body.

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

• Movement: Serves as levers for muscles to act upon.

• Mineral Storage: Stores calcium and phosphorus.

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

Axial vs. Appendicular Skeleton

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum).

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic girdles).

Classification of Bones

Bones are classified by shape:

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

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

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

• Irregular Bones: Complex shapes (e.g., vertebrae, some facial bones).

Structure of a Long Bone

• Diaphysis: Shaft of the bone.

• Epiphysis: Ends of the bone.

• Epiphyseal Plate: Growth plate between diaphysis and epiphysis.

• Periosteum: Outer fibrous covering.

• Endosteum: Membrane lining the medullary cavity.

• Medullary Cavity: Central cavity containing bone marrow.

• Articular Cartilage: Covers joint surfaces.

Bone Markings: Projections and Depressions

Bones have various markings for attachment and passage of structures.

• Projections: Sites for muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Depressions/Openings: Allow passage of nerves and blood vessels (e.g., foramen, fossa, groove).

Example: The greater trochanter of the femur is a projection for muscle attachment.

Bone as Connective Tissue

• Cells: Osteoblasts (form bone), osteocytes (maintain bone), osteoclasts (break down bone).

• Matrix: Composed of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components.

• Fibers: Collagen fibers provide tensile strength.

Organic matrix = collagen and ground substance; Inorganic matrix = mineral salts (mainly calcium phosphate).

Compact vs. Spongy Bone Structure

• Compact Bone: Dense, forms outer layer of bones; contains osteons (Haversian systems).

• Spongy Bone: Porous, found at ends of long bones and inside flat bones; contains trabeculae.

Osteon: Structural unit of compact bone, consisting of concentric lamellae around a central canal.

Bone Marrow: Red marrow (hematopoietic), yellow marrow (fat storage).

Location of Compact and Spongy Bone

• Compact bone: Outer surface of all bones, shaft of long bones.

• Spongy bone: Ends of long bones, inside flat bones.

Bone Tissue Membranes

• Periosteum: Outer membrane; contains nerves and blood vessels.

• Endosteum: Lines internal bone surfaces.

Ossification: Bone Formation

• Intramembranous Ossification: Bone develops from fibrous membrane (e.g., flat bones of skull).

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

Ossification Center: Site where bone formation begins.

Bone Remodeling and Calcium Homeostasis

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

• Calcium Regulation: Controlled by parathyroid hormone (PTH), calcitonin, and vitamin D.

• Blood Calcium Levels: PTH increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts.

• Wolff's Law: Bone grows/remodels in response to forces placed upon it.

Fractures and Osteoporosis

• Types of Fractures: Simple, compound, comminuted, greenstick, etc.

• Fracture Healing: Four steps: hematoma formation, fibrocartilaginous callus formation, bony callus formation, bone remodeling.

• Osteoporosis: Disease characterized by decreased bone mass and increased fracture risk.

Articulations (Joints)

Joint Classifications

• Functional Classification: Based on movement allowed.

  • Synarthrosis: Immovable (e.g., sutures of skull).

  • Amphiarthrosis: Slightly movable (e.g., intervertebral discs).

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

• Structural Classification: Based on material binding bones and presence of joint cavity.

  • Fibrous: Bones joined by dense connective tissue (e.g., sutures).

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

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

Binding Material and Joint Table

Joints can be classified by their structure, function, and the type of movement they allow. The following table summarizes these classifications:

Structural Type	Functional Type	Binding Material	Example
Fibrous	Synarthrosis	Dense connective tissue	Sutures of skull
Cartilaginous	Amphiarthrosis	Cartilage	Intervertebral discs
Synovial	Diarthrosis	Joint capsule, synovial fluid	Knee, shoulder

Synovial Joints

• Components: Articular cartilage, joint (synovial) cavity, articular capsule, synovial fluid, reinforcing ligaments, nerves, and blood vessels.

• Accessory Structures: Menisci, bursae, tendon sheaths, ligaments (e.g., anterior cruciate ligament in the knee).

Movements at Synovial Joints

• Flexion/Extension/Hyperextension

• Abduction/Adduction

• Medial/Lateral Rotation

• Protraction/Retraction

• Elevation/Depression

• Inversion/Eversion

• Plantar flexion/Dorsiflexion

• Opposition

Example: Flexion at the elbow decreases the angle between the forearm and upper arm.

Summary Table: Synovial Joint Movements

Movement	Description	Example
Flexion	Decreases angle between bones	Bending elbow
Extension	Increases angle between bones	Straightening knee
Abduction	Movement away from midline	Raising arm sideways
Adduction	Movement toward midline	Lowering arm to side
Rotation	Movement around longitudinal axis	Turning head side to side

Additional info:

• Some details, such as the specific steps of bone healing and the full list of hormones involved in calcium regulation, were expanded for academic completeness.

• Tables were inferred and constructed based on standard textbook content for joint classification and movement types.