The Skeletal System

Overview and Functions

The skeletal system is a complex framework of bones and connective tissues that provides structural support, protection, and facilitates movement. It also serves as a reservoir for minerals and houses bone marrow for blood cell production.

• Bone Tissue: Bone is a specialized connective tissue composed of cells (osteocytes, osteoblasts, osteoclasts) and a mineralized matrix.

• Functions: Support, protection of internal organs, movement (with muscles), mineral storage (calcium, phosphate), and blood cell formation (hematopoiesis).

Divisions of the Skeletal System

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

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

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

• Total Bones: The adult human skeleton contains 206 bones.

Classification of Bones

Bones are classified based on their shape and function.

• Long Bones: e.g., femur, humerus

• Short Bones: e.g., carpals, tarsals

• Flat Bones: e.g., skull, sternum

• Irregular Bones: e.g., vertebrae, pelvis

Surface Markings and Parts of a Long Bone

Surface markings on bones serve as attachment points for muscles, tendons, and ligaments, and as passageways for nerves and blood vessels. The structure of a typical long bone includes several distinct regions.

• Diaphysis: Shaft of the bone

• Epiphysis: Ends of the bone

• Metaphysis: Region between diaphysis and epiphysis

• Medullary Cavity: Central cavity containing bone marrow

• Periosteum: Outer covering of bone

• Endosteum: Inner lining of the bone

Microscopic Structure of Bone

Bones are composed of compact and spongy tissue, each with unique structural features.

• Compact Bone: Dense, forms the outer layer

• Spongy Bone: Porous, found at the ends of long bones

• Osteon: Structural unit of compact bone

• Canaliculi: Small channels for nutrient and waste exchange

Bone Cells and Growth

Bone maintenance and growth are regulated by three main cell types and processes.

• Osteocytes: Mature bone cells, maintain bone matrix

• Osteoblasts: Bone-forming cells

• Osteoclasts: Bone-resorbing cells

• Appositional Growth: Increase in bone diameter by adding new layers to the surface

Cartilage vs. Bone

Cartilage and bone are both connective tissues but differ in structure and function.

• Cartilage: Flexible, avascular, found in joints, ear, nose

• Bone: Rigid, vascular, provides structural support

Skull Bones and Sutures

The adult skull consists of several bones joined by sutures, which are immovable joints.

• Major Bones: Frontal, parietal, occipital, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, nasal, lacrimal

• Sutures: Coronal (frontal-parietal), squamous (temporal-parietal), lambdoid (parietal-occipital)

Vertebral Column and Spinal Curves

The vertebral column is divided into regions, each with distinct structural characteristics and functions. Spinal curves are classified as primary (present at birth) and secondary (develop after birth).

• Cervical (7 vertebrae): Secondary curve, develops as infant balances head

• Thoracic (12 vertebrae): Primary curve, present at birth

• Lumbar (5 vertebrae): Secondary curve, develops with standing

• Sacral and Coccygeal: Primary curves, present at birth

• Function: Protects spinal cord, supports head and trunk, allows flexibility

Classification of Ribs

Ribs are classified based on their articulation with the sternum.

• True Ribs: Directly attached to sternum (1-7)

• False Ribs: Indirectly attached or not attached (8-12)

• Floating Ribs: Not attached to sternum (11-12)

Movements of the Body

Body movements are produced by the interaction of bones, muscles, and joints. Specific terms describe these movements.

• Flexion/Extension: Decrease/increase angle between bones

• Abduction/Adduction: Move limb away/toward midline

• Circumduction: Circular movement

• Rotation: Movement around longitudinal axis

Joints: Classification and Structure

Joints are classified by their structure and function. The main types are fibrous, cartilaginous, and synovial joints.

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

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

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

Synovial Joints: Structure and Types

Synovial joints are characterized by a joint capsule, synovial fluid, and accessory structures. They allow free movement and are classified by the shape of their articulating surfaces.

• Plane (Gliding) Joint: Flat surfaces, slight movement

• Hinge Joint: Angular movement in one plane

• Condylar (Ellipsoidal) Joint: Angular movement in two planes

• Saddle Joint: Angular movement and circumduction

• Pivot Joint: Rotation in one plane

• Ball-and-Socket Joint: Angular, rotational, and circumduction movement

Parts of a Synovial Joint

Each synovial joint contains specific structures that contribute to its function.

• Joint Capsule: Encloses the joint

• Synovial Membrane: Produces synovial fluid

• Articular Cartilage: Reduces friction

• Ligaments: Stabilize the joint

• Menisci: Pads for shock absorption

Summary Table: Types of Synovial Joints

Additional info:

• Fractures are classified by their pattern and location; common types include transverse, oblique, spiral, and comminuted.

• Surface markings include foramina (holes), processes (projections), and fossae (depressions).

• Bone remodeling is a continuous process involving osteoblasts and osteoclasts.