Skeletal System: Structure, Function, and Associated Tissues

Skeletal System

Main Functions and Overview

The skeletal system provides structural support, protection for internal organs, facilitates movement, stores minerals, and produces blood cells. The adult human skeleton consists of approximately 206 bones.

• Support: Framework for the body.

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

• Movement: Muscles attach to bones for locomotion.

• Mineral Storage: Calcium and phosphorus.

• Blood Cell Production: Hematopoiesis in bone marrow.

Axial vs. Appendicular Skeleton

The skeleton is divided into two main regions:

• Axial Skeleton: Skull, vertebral column, and thoracic cage (ribs and sternum).

• Appendicular Skeleton: Limbs and girdles (pectoral and pelvic).

Bone Classification by Shape

Bones are classified based on their shape:

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

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, often curved (e.g., skull, sternum).

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

Skull: Cranial vs. Facial Bones

The skull consists of:

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

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

Orbit and Nasal Cavity Bones

• Orbit Walls: Formed by frontal, sphenoid, zygomatic, maxilla, palatine, ethmoid, and lacrimal bones.

• Nasal Cavity: Walls formed by maxilla, palatine, ethmoid, and vomer; conchae are ethmoid and inferior nasal conchae; septum is vomer and perpendicular plate of ethmoid.

Key Foramina and Structures

• Supraorbital Foramen: Above orbit; passage for nerves/vessels.

• Infraorbital Foramen: Below orbit; passage for infraorbital nerve.

• Mandibular Foramen: Inside mandible; passage for nerves.

• Optic Foramen: Sphenoid bone; optic nerve passage.

• Foramen Magnum: Occipital bone; spinal cord passage.

• Jugular Foramen: Between temporal and occipital bones; jugular vein passage.

• Sella Turcica: Sphenoid bone; houses pituitary gland.

Sutures and Skull Development

• Sutures: Immovable joints between skull bones (e.g., coronal, sagittal, lambdoid).

• Fetal vs. Adult Skull: Fetal skull has fontanels (soft spots); adult skull has fused sutures.

Paranasal Sinuses

• Frontal Sinus

• Ethmoid Sinus

• Sphenoid Sinus

• Maxillary Sinus

Fontanels

• Anterior Fontanel: Largest, between frontal and parietal bones.

• Posterior Fontanel: Between parietal and occipital bones.

Vertebrae Structure and Regions

• Typical Vertebra: Body, vertebral arch, spinous process, transverse process, vertebral foramen.

• Transverse Foramina: Cervical vertebrae; passage for vertebral artery.

• Vertebral Canal: Contains spinal cord.

Vertebral Column Regions

• Cervical: 7 vertebrae (C1-C7); C1 (Atlas), C2 (Axis).

• Thoracic: 12 vertebrae (T1-T12).

• Lumbar: 5 vertebrae (L1-L5).

• Sacrum: 5 fused vertebrae.

• Coccyx: 3-5 fused vertebrae.

Unique Features

• Atlas (C1): No body; supports skull.

• Axis (C2): Dens (odontoid process) for rotation.

Curvature Differences

• Infant Spine: Single curve (C-shape).

• Adult Spine: Four curves (cervical, thoracic, lumbar, sacral).

Intervertebral Discs and Articulations

• Parts: Annulus fibrosus (outer), nucleus pulposus (inner).

• Function: Shock absorption, flexibility.

• Articulations: Facet joints between vertebrae.

Carpal Bones

• Number: 8 carpal bones.

• Names: Scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate.

Sternum and Rib Articulations

• Sternum Parts: Manubrium, body, xiphoid process.

• Articulations: Sternum articulates with clavicles and ribs.

Rib Classification and Articulations

• True Ribs: 1-7; direct attachment to sternum.

• False Ribs: 8-12; indirect or no attachment.

• Floating Ribs: 11-12; no anterior attachment.

• Articulations: Ribs articulate with thoracic vertebrae.

Bones of Limbs and Girdles

• Pectoral Girdle: Clavicle, scapula.

• Arm: Humerus.

• Forearm: Radius, ulna.

• Wrist: Carpals.

• Hand: Metacarpals, phalanges.

• Pelvic Girdle: Ilium, ischium, pubis.

• Thigh: Femur.

• Leg: Tibia, fibula.

• Ankle: Tarsals.

• Foot: Metatarsals, phalanges.

Principal Joints

• Ball-and-Socket Joints: Shoulder (glenohumeral), hip (acetabulofemoral).

• Hinge Joints: Elbow, knee, interphalangeal joints.

Tarsal Bones

• Number: 7 tarsal bones.

• Names: Talus, calcaneus, navicular, medial cuneiform, intermediate cuneiform, lateral cuneiform, cuboid.

Joint Types and Synovial Joint Structure

• Fibrous Joints: Bones joined by fibrous tissue (e.g., sutures).

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

• Synovial Joints: Freely movable, joint cavity with synovial fluid.

Structure of Synovial Joint:

• Articular cartilage

• Joint (synovial) cavity

• Articular capsule

• Synovial fluid

• Ligaments

Bone Markings

Bone markings include projections, depressions, and openings for muscle attachment, passage of nerves and blood vessels.

Cartilage and Bone Tissue

• Hyaline Cartilage: Most common; found in joints, nose, trachea.

• Fibrocartilage: Strong, found in intervertebral discs, pubic symphysis.

• Bone Tissue: Hard, supports body.

Cell Types: Chondrocytes (cartilage), osteocytes (bone).

Matrix: Collagen fibers, ground substance; bone matrix also contains hydroxyapatite (calcium phosphate).

Arthritis Classification and Pathologies

• Osteoarthritis: Degenerative, wear and tear.

• Rheumatoid Arthritis: Autoimmune, inflammation.

• Gouty Arthritis: Uric acid crystal deposition.

Bone Fractures and Healing

• Types: Simple (closed), compound (open).

• Healing: Hematoma formation, fibrocartilaginous callus, bony callus, bone remodeling.

Skeletal Changes Throughout Life

• Infant: More cartilage, fontanels.

• Childhood: Ossification, growth plates.

• Adult: Fully ossified, possible bone loss with aging.

Example Table: Classification of Bones by Shape

Type	Description	Example
Long	Longer than wide	Femur, humerus
Short	Cube-shaped	Carpals, tarsals
Flat	Thin, often curved	Skull, sternum
Irregular	Complex shape	Vertebrae, pelvis

Example Table: Types of Joints

Type	Composition	Mobility	Example
Fibrous	Fibrous tissue	Immovable	Sutures in skull
Cartilaginous	Cartilage	Slightly movable	Intervertebral discs
Synovial	Synovial cavity	Freely movable	Knee, shoulder

Example Table: Paranasal Sinuses

Sinus	Location
Frontal	Frontal bone
Ethmoid	Ethmoid bone
Sphenoid	Sphenoid bone
Maxillary	Maxilla

Example Table: Carpal Bones

Bone	Location
Scaphoid	Lateral, proximal row
Lunate	Medial to scaphoid
Triquetrum	Medial to lunate
Pisiform	Anterior to triquetrum
Trapezium	Lateral, distal row
Trapezoid	Medial to trapezium
Capitate	Medial to trapezoid
Hamate	Medial, distal row

Example Table: Tarsal Bones

Bone	Location
Talus	Superior, articulates with tibia
Calcaneus	Heel bone
Navicular	Anterior to talus
Medial cuneiform	Medial, anterior row
Intermediate cuneiform	Middle, anterior row
Lateral cuneiform	Lateral, anterior row
Cuboid	Lateral, anterior row

Example Table: Types of Cartilage

Type	Location	Function
Hyaline	Joints, nose, trachea	Support, flexibility
Fibrocartilage	Intervertebral discs	Strength, shock absorption

Bone Healing Process (Stages)

Stage	Description
Hematoma Formation	Blood clot at fracture site
Fibrocartilaginous Callus	Soft tissue repair
Bony Callus	Hard bone replaces callus
Bone Remodeling	Restores bone shape

Key Formula: Bone Matrix Composition

The bone matrix is composed of collagen fibers and hydroxyapatite crystals:

$\text{Bone Matrix} = \text{Collagen Fibers} + \text{Hydroxyapatite} (\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2)$

Key Formula: Calcium Homeostasis

Calcium levels are regulated by hormones:

$\text{Parathyroid Hormone} \uparrow \Rightarrow \text{Blood Calcium} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Blood Calcium} \downarrow$