Skeletal System: Structure, Function, and Articulations

Chapter 7: Skeletal System – Bone Structure and Function

Components of the Skeletal System

The skeletal system is composed of bones, cartilage, ligaments, and other connective tissues that stabilize or connect bones.

• Bones: Provide structural support and protection.

• Cartilage: Flexible connective tissue found in joints, rib attachments, and other areas.

• Ligaments: Connect bones to other bones.

• Other Connective Tissues: Include tendons (connect muscle to bone) and periosteum (bone covering).

Compact vs. Spongy Bone

Bones are made of two main types of tissue: compact and spongy bone.

• Compact Bone: Dense, forms the outer layer of bones, provides strength and protection.

• Spongy Bone: Porous, found inside bones, especially at the ends; contains trabeculae and bone marrow.

• Comparison: Compact bone is stronger and heavier; spongy bone is lighter and supports marrow.

Types and Locations of Cartilage

Cartilage is found in various locations and comes in three types:

• Hyaline Cartilage: Most common; found in articular surfaces, costal cartilage, nose, and trachea.

• Fibrocartilage: Found in intervertebral discs, pubic symphysis, and menisci.

• Elastic Cartilage: Found in the external ear and epiglottis.

General Functions of Bone

• Support: Framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain).

• Movement: Muscles attach to bones for movement.

• Mineral Storage: Stores calcium and phosphate.

• Blood Cell Production: Hematopoiesis in bone marrow.

• Energy Storage: Yellow marrow stores fat.

Classes of Bones by Shape

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

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

• Flat Bones: Thin, flat surfaces (e.g., skull, ribs).

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

Structural Components of a Long Bone

• Diaphysis: Shaft, provides leverage and weight support.

• Epiphysis: Ends, contain spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains epiphyseal plate.

• Periosteum: Outer covering, contains nerves and blood vessels.

• Endosteum: Lines internal bone surfaces.

Gross Anatomy of Other Bones

Short, flat, and irregular bones lack a shaft and epiphyses; they have compact bone surrounding spongy bone.

Blood Vessels and Nerves in Bone

• Blood Vessels: Supply nutrients and remove waste.

• Nerves: Sense pain and regulate blood flow.

Bone Marrow Types and Locations

• Red Bone Marrow: Hematopoietic; found in spongy bone of axial skeleton, pelvis, and proximal epiphyses of humerus/femur.

• Yellow Bone Marrow: Fat storage; found in medullary cavities of long bones.

Types of Bone Cells and Their Functions

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Build bone matrix.

• Osteocytes: Mature bone cells, maintain bone tissue.

• Osteoclasts: Break down bone matrix.

Bone Matrix Composition, Formation, and Resorption

• Composition: Organic (collagen, proteins) and inorganic (hydroxyapatite, calcium phosphate).

• Formation: Osteoblasts secrete matrix; mineralization occurs.

• Resorption: Osteoclasts dissolve matrix, releasing minerals.

Microscopic Structure of Compact vs. Spongy Bone

• Compact Bone: Organized into osteons (Haversian systems).

• Spongy Bone: Contains trabeculae; lacks osteons.

Components of Hyaline Cartilage

• Chondrocytes: Cartilage cells in lacunae.

• Matrix: Collagen fibers, ground substance.

• Perichondrium: Surrounds cartilage (except articular cartilage).

Interstitial vs. Appositional Growth of Cartilage

• Interstitial Growth: Growth from within; chondrocytes divide and secrete new matrix.

• Appositional Growth: Growth at surface; new layers added by chondroblasts.

Intramembranous Ossification

• Bones Produced: Flat bones of skull, mandible, clavicle.

• Steps:

  1. Ossification centers form in mesenchyme.

  2. Osteoid secreted, calcifies.

  3. Formation of woven bone and periosteum.

  4. Replacement by lamellar bone.

Endochondral Ossification

• Bones Produced: Most bones, especially long bones.

• Steps:

  1. Cartilage model forms.

  2. Cartilage calcifies, periosteal bone collar forms.

  3. Primary ossification center develops in diaphysis.

  4. Secondary ossification centers develop in epiphyses.

  5. Bone replaces cartilage except at articular cartilage and epiphyseal plate.

  6. Epiphyseal plate ossifies, forming epiphyseal line.

Epiphyseal Plate Zones and Growth in Length

• Five Zones: Resting, proliferating, hypertrophic, calcification, ossification.

• Growth: Chondrocyte proliferation and matrix production lead to bone elongation.

Appositional Growth Steps

• Osteoblasts in periosteum add new bone matrix to outer surface.

• Osteoclasts remove bone from inner surface, enlarging medullary cavity.

Bone Remodeling and Mechanical Stress

• Bone is continuously renewed by osteoblasts and osteoclasts.

• Mechanical stress increases bone density; lack of stress leads to bone loss.

Hormones Influencing Bone Growth and Remodeling

• Growth Hormone: Stimulates bone growth.

• Thyroid Hormone: Regulates metabolism.

• Sex Hormones: Promote growth and closure of epiphyseal plate.

• Parathyroid Hormone (PTH): Increases blood calcium.

• Calcitonin: Lowers blood calcium.

• Calcitriol: Active vitamin D, increases calcium absorption.

Activation of Vitamin D to Calcitriol

• Vitamin D is converted to calcitriol in the liver and kidneys.

• Calcitriol increases intestinal absorption of calcium.

Regulation of Blood Calcium Levels

• PTH and Calcitriol: Increase blood calcium by stimulating bone resorption, increasing absorption, and reducing excretion.

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts.

Age Influence on Bone Structure

• Bone density decreases with age.

• Risk of osteoporosis increases.

Fracture Healing Steps

• Hematoma formation

• Fibrocartilaginous callus formation

• Bony callus formation

• Bone remodeling

Chapter 8: Skeletal System – Axial and Appendicular Skeleton

Axial vs. Appendicular Skeleton

The skeleton is divided into axial and appendicular components.

• Axial Skeleton: Skull, vertebral column, thoracic cage; supports and protects organs.

• Appendicular Skeleton: Limbs and girdles; enables movement.

Common Bone Markings

• Foramen: Hole for vessels/nerves.

• Process: Projection for muscle attachment.

• Fossa: Depression.

• Condyle: Rounded articular surface.

• Tubercle/Tuberosity: Small/large rounded projection.

Cranial vs. Facial Bones

• Cranial Bones: Enclose brain (e.g., frontal, parietal, temporal, occipital).

• Facial Bones: Form face (e.g., maxilla, mandible, zygomatic).

Skull Bones and Features

• Each bone has unique landmarks (e.g., supraorbital foramen, mastoid process).

Cranial Fossae

• Anterior, Middle, Posterior Fossae: House different brain regions.

Cranial Sutures

• Coronal, Sagittal, Lambdoid, Squamous: Joints between cranial bones.

Orbital and Nasal Complexes

• Formed by multiple bones (e.g., maxilla, ethmoid, sphenoid).

Paranasal Sinuses

• Air-filled spaces; lighten skull, resonate voice.

Auditory Ossicles

• Malleus, Incus, Stapes: Transmit sound in middle ear.

Hyoid Bone

• Located in neck; supports tongue, not attached to other bones.

Sexually Dimorphic Features of Skull and Pelvis

• Male skulls: more robust, larger brow ridges.

• Female pelvis: wider, shallower, larger pelvic inlet/outlet.

Fetal, Child, and Adult Skulls

• Fontanelles present in infants; close with age.

Vertebrae Types and Locations

Type	Number	Location
Cervical	7	Neck
Thoracic	12	Upper back
Lumbar	5	Lower back
Sacral	5 (fused)	Pelvis
Coccygeal	4 (fused)	Tailbone

Vertebral Column Curvature Development

• Primary curves: thoracic and sacral (present at birth).

• Secondary curves: cervical and lumbar (develop after birth).

Parts of a Typical Vertebra

• Body: Weight-bearing.

• Vertebral Arch: Protects spinal cord.

• Processes: Muscle and ligament attachment.

Sternum and Ribs

• Sternum: Manubrium, body, xiphoid process.

• Ribs: True (1-7), false (8-12), floating (11-12).

Upper and Lower Limbs

• Both have similar bone patterns: one proximal bone, two distal bones, carpals/tarsals, metacarpals/metatarsals, phalanges.

• Upper limb specialized for mobility; lower limb for weight-bearing.

Clavicle, Scapula, Humerus, Radius, Ulna

• Each bone has distinct landmarks (e.g., acromion, olecranon).

Supination vs. Pronation

• Supination: Palm up.

• Pronation: Palm down.

Pelvis and Ossa Coxae

• Os Coxae: Ilium, ischium, pubis.

• True vs. False Pelves: True pelvis is below pelvic brim; false pelvis is above.

• Pelvic Inlet/Outlet: Boundaries for childbirth.

Femur, Patella, Tibia, Fibula, Foot Bones

• Femur: largest bone, articulates with pelvis.

• Patella: sesamoid bone in quadriceps tendon.

• Tibia: medial, weight-bearing; fibula: lateral, non-weight-bearing.

• Foot: tarsals, metatarsals, phalanges; arches provide support.

Chapter 9: Skeletal System – Articulations

Definition and Classification of Joints

• Joint (Articulation): Where two bones meet.

• Structural Classification: Fibrous, cartilaginous, synovial.

• Functional Classification: Synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable).

Mobility vs. Stability in Joints

• Inverse relationship: more mobility means less stability.

Types and Locations of Fibrous Joints

• Gomphoses: Teeth in sockets.

• Sutures: Between skull bones.

• Syndesmoses: Interosseous membranes (e.g., between radius and ulna).

Types and Locations of Cartilaginous Joints

• Synchondroses: Hyaline cartilage (e.g., epiphyseal plate).

• Symphyses: Fibrocartilage (e.g., pubic symphysis).

Synovial Joint Anatomy

• Articular cartilage, joint cavity, synovial membrane, fibrous capsule, ligaments.

Synovial Fluid Composition and Function

• Contains hyaluronic acid, lubricates, nourishes cartilage, absorbs shock.

Joint Movement and Axes

• Movements occur along three axes: transverse, sagittal, vertical.

Types of Synovial Joints

Type	Structure	Movement	Example
Plane	Flat surfaces	Gliding	Intercarpal joints
Hinge	Cylinder in trough	Flexion/extension	Elbow
Pivot	Rounded bone in ring	Rotation	Atlantoaxial joint
Condylar	Oval surface in depression	Flexion/extension, abduction/adduction	Metacarpophalangeal joints
Saddle	Concave/convex surfaces	Flexion/extension, abduction/adduction	Thumb
Ball-and-socket	Ball in socket	All movements	Shoulder, hip

Joint Motions

• Gliding: Flat surfaces slide (e.g., wrist).

• Angular: Flexion, extension, abduction, adduction.

• Rotational: Medial/lateral rotation (e.g., shoulder, hip).

• Special Movements: Elevation, depression, protraction, retraction, inversion, eversion.

Levers in the Human Body

• Three types: first-class, second-class, third-class.

• Most joints are third-class levers (effort between fulcrum and load).

Specific Joints and Their Features

• TMJ: Jaw joint; allows gliding and hinge movements.

• Shoulder: Glenohumeral, acromioclavicular, sternoclavicular joints; high mobility, low stability.

• Elbow: Hinge joint; flexion/extension.

• Hip: Ball-and-socket; more stable than shoulder.

• Knee: Complex hinge; flexion/extension, slight rotation.

• Ankle (Talocrural): Hinge; dorsiflexion/plantarflexion.

Joint Development and Age-Related Changes

• Joints form from mesenchyme in embryo.

• With age, cartilage thins, joint flexibility decreases, risk of arthritis increases.