Skeletal System Overview

Subdivisions of the Skeleton: Axial vs. Appendicular

The human skeleton is divided into two main subdivisions: the axial skeleton and the appendicular skeleton. Each subdivision has distinct anatomical components and functions.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum). It forms the central axis of the body and protects vital organs.

• Appendicular Skeleton: Includes the bones of the limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton. It is responsible for movement and manipulation of the environment.

Functions and Classifications of Bones

Functions of the Skeletal System

The skeletal system performs several essential functions for the human body:

• Support: Provides a structural framework for the body.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

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

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

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

Main Classifications of Bones

Bones are classified according to their shapes and functions:

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

• Short Bones: Cube-shaped and mostly spongy bone (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and usually curved (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, pelvis).

Bone Anatomy and Structure

Anatomical Areas of a Long Bone

Long bones have distinct anatomical regions:

• Diaphysis: The shaft or central part of a long bone.

• Epiphyses: The ends of the bone, usually wider than the diaphysis.

• Metaphysis: The region between the diaphysis and epiphysis, containing the growth plate.

• Medullary Cavity: The hollow interior of the diaphysis, containing bone marrow.

• Periosteum: A dense connective tissue membrane covering the outer surface of the bone.

• Endosteum: A thin membrane lining the medullary cavity.

Microscopic Structure of Compact Bone

Compact bone is organized into structural units called osteons (Haversian systems):

• Osteon: Cylindrical structures containing concentric lamellae (layers of bone matrix).

• Central (Haversian) Canal: Runs longitudinally through the osteon, containing blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes (bone cells).

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

Bone Salts and Organic Matrix

Bones are both hard and flexible due to their composition:

• Bone Salts: Mainly hydroxyapatite (calcium phosphate), providing hardness and strength.

• Organic Matrix: Primarily collagen fibers, giving bones flexibility and resistance to tension.

Bone Development and Remodeling

Bone Formation in the Fetus

Bone formation (ossification) begins in the fetus through two main processes:

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue (e.g., flat bones of the skull).

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

Throughout life, bones undergo remodeling, a process involving resorption by osteoclasts and formation by osteoblasts, allowing bones to adapt to stress and repair damage.

Bone Fractures

Types of Fractures

Fractures are classified based on their characteristics:

• Simple (Closed) Fracture: Bone breaks but does not penetrate the skin.

• Compound (Open) Fracture: Bone breaks and penetrates the skin.

• Comminuted Fracture: Bone is shattered into several pieces.

• Greenstick Fracture: Bone bends and cracks, common in children.

• Spiral Fracture: Caused by twisting forces.

• Compression Fracture: Bone is crushed, often in vertebrae.

Additional info: The original notes mention "there are 6" types; the above list includes six common types.

Skull Anatomy

Bones and Sutures of the Skull

The skull consists of several bones joined by sutures:

• ajor Bones: FrontalM, parietal, temporal, occipital, sphenoid, ethmoid.

• Major Sutures:

  • Coronal Suture: Between frontal and parietal bones.

  • Sagittal Suture: Between the two parietal bones.

  • Lambdoid Suture: Between parietal and occipital bones.

  • Squamous Suture: Between parietal and temporal bones.

Vertebral Column

Parts of a Typical Vertebra and Regional Differences

A typical vertebra consists of:

• Body: Main weight-bearing portion.

• Vertebral Arch: Encloses the vertebral foramen.

• Processes: Spinous and transverse processes for muscle attachment.

Regional differences:

• Cervical Vertebrae: Small, have transverse foramina; C1 (atlas) and C2 (axis) are specialized.

• Thoracic Vertebrae: Larger, articulate with ribs, have long spinous processes.

• Lumbar Vertebrae: Largest, thick bodies, short spinous processes.

Intervertebral Discs and Spinal Curvatures

Intervertebral discs are fibrocartilaginous pads between vertebrae that:

• Absorb shock

• Allow flexibility

• Prevent vertebrae from rubbing together

Spinal curvatures (cervical, thoracic, lumbar, sacral) help distribute mechanical stress during movement.

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature of the spine.

• Lordosis: Exaggerated inward curvature of the lumbar region.

• Kyphosis: Exaggerated outward curvature of the thoracic region.

Thoracic Cage

Components of the Thoracic Cage

• Sternum: Breastbone

• Ribs: 12 pairs

• Thoracic Vertebrae: Posterior attachment

True vs. False Ribs

Appendicular Skeleton

Bones of Shoulder and Pelvic Girdles and Limbs

• Shoulder Girdle: Clavicle and scapula

• Upper Limb: Humerus, radius, ulna, carpals, metacarpals, phalanges

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

• Lower Limb: Femur, tibia, fibula, patella, tarsals, metatarsals, phalanges

Differences Between Male and Female Pelvis

Joints

Structural Categories of Joints and Movement

Developmental Anatomy

Skull of Newborn vs. Adult and Fontanels

• Newborn Skull: Bones are not fully fused; contains fontanels (soft spots).

• Adult Skull: Bones are fused; sutures are immovable.

• Function of Fontanels: Allow for brain growth and facilitate passage through the birth canal.