Study Notes: The Skeletal System (Anatomy & Physiology)

UNIT 10: SKELETAL SYSTEM

Introduction to the Skeletal System

The skeletal system provides the structural framework for the human body, supports movement, and protects vital organs. It is composed of bones, cartilage, ligaments, and joints, each with specialized functions.

• Functions of the Skeletal System:

  • 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, enabling movement.

  • Mineral Storage: Stores minerals such as calcium and phosphorus.

  • Blood Cell Production: Houses bone marrow, which produces blood cells (hematopoiesis).

  • Energy Storage: Yellow bone marrow stores lipids as an energy reserve.

Bone Tissue & Cartilage

Bone and cartilage are specialized connective tissues essential for the structure and function of the skeletal system.

• Bone Tissue:

  • Compact Bone: Dense, strong outer layer of bone.

  • Spongy Bone: Porous, inner layer containing trabeculae and red marrow.

• Cartilage:

  • Hyaline Cartilage: Most common type; found at joints, nose, and trachea.

  • Elastic Cartilage: Flexible; found in the ear and epiglottis.

  • Fibrocartilage: Tough; found in intervertebral discs and pubic symphysis.

Example: The articular cartilage at the ends of long bones is hyaline cartilage, reducing friction in joints.

Classification of Bones

Bones are classified by shape and structure, which relate to their 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, sternum).

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

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

Structure of a Typical Long Bone

Long bones have a characteristic structure that supports their function in movement and weight-bearing.

• Diaphysis: Shaft of the bone; composed mainly of compact bone.

• Epiphyses: Ends of the bone; contain spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis; includes the epiphyseal plate (growth plate).

• Medullary Cavity: Central cavity containing yellow marrow in adults.

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Endosteum: Thin membrane lining the medullary cavity.

Microscopic Structure of Bone

Bone tissue is organized into structural units that provide strength and facilitate metabolic functions.

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

• Lamellae: Layers of bone matrix.

• Lacunae: Small spaces housing osteocytes.

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

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

Bone Cells

Several specialized cells are involved in bone formation, maintenance, and remodeling.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Bone Formation and Growth

Bone develops 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).

• Epiphyseal Plate: Site of longitudinal bone growth during childhood and adolescence.

Bone Remodeling and Repair

Bone is a dynamic tissue that undergoes continuous remodeling and can repair itself after injury.

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

• Repair: Involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Axial and Appendicular Skeleton

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

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

• Appendicular Skeleton: Includes the pectoral girdle, pelvic girdle, and limbs.

Major Bones of the Human Skeleton

The skeleton contains 206 bones in the adult human body, each with specific names and locations.

• Skull: Frontal, parietal, temporal, occipital, sphenoid, ethmoid, mandible, maxilla, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal concha.

• Vertebral Column: Cervical (7), thoracic (12), lumbar (5), sacrum, coccyx.

• Thoracic Cage: Sternum, ribs (12 pairs).

• Pectoral Girdle: Clavicle, scapula.

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

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

• Lower Limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Joints (Articulations)

Joints are connections between bones that allow for movement and provide mechanical support.

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures of the skull).

  • Cartilaginous Joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

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

• Functional Classification:

  • Synarthrosis: Immovable joints.

  • Amphiarthrosis: Slightly movable joints.

  • Diarthrosis: Freely movable joints.

Types of Synovial Joints

Synovial joints are classified based on the shapes of their articulating surfaces and the types of movement they allow.

• Plane (Gliding) Joints: Allow sliding movements (e.g., intercarpal joints).

• Hinge Joints: Permit flexion and extension (e.g., elbow, knee).

• Pivot Joints: Allow rotation (e.g., atlantoaxial joint in the neck).

• Condyloid (Ellipsoidal) Joints: Permit movement in two planes (e.g., wrist joint).

• Saddle Joints: Allow movement in two planes (e.g., thumb joint).

• Ball-and-Socket Joints: Allow movement in multiple axes and planes (e.g., shoulder, hip).

Selected Table: Comparison of Joint Types

The following table summarizes the main types of joints, their structure, and movement capabilities.

Joint Type	Structural Features	Movement	Example
Fibrous	Dense connective tissue	Immovable or slight movement	Sutures of skull
Cartilaginous	Cartilage connection	Slight movement	Intervertebral discs
Synovial	Joint cavity with synovial fluid	Freely movable	Knee, shoulder

Bone Markings

Bones have various surface features that serve as attachment points for muscles, ligaments, and tendons, or as passageways for nerves and blood vessels.

• Projections: Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process.

• Depressions and Openings: Foramen, groove, fissure, notch, fossa, meatus, sinus.

Clinical Correlations

Understanding the skeletal system is essential for diagnosing and treating musculoskeletal disorders such as fractures, osteoporosis, arthritis, and congenital abnormalities.

• Fractures: Breaks in bone due to trauma or disease.

• Osteoporosis: Decreased bone mass and increased fracture risk.

• Arthritis: Inflammation of joints, leading to pain and reduced mobility.

Additional info:

• Bone remodeling is regulated by hormones such as parathyroid hormone (PTH) and calcitonin.

• Calcium homeostasis is critical for muscle contraction, nerve function, and blood clotting.