Functions and Structure of the Skeletal System

Functions of the Skeleton

  • Support: Acts as the internal framework of the body, providing shape and stability.

  • Protection: Shields vital organs and plays a role in the immune response by producing white blood cells.

  • Movement: Muscles pull on bones allowing for a variety of movements.

  • Storage: Stores calcium and fat within the medullary cavity of bones.

  • Hematopoiesis: The process of blood cell formation occurs in red marrow present in bones.

Bone Structure

Types of Bones

  • Long Bones: Characterized by a lengthy shaft and typically found in appendages (e.g., femur).

  • Short Bones: Cube-shaped bones found in the wrist and ankle.

  • Flat Bones: Thin bones providing protection (e.g., skull, sternum).

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

  • Sesamoid Bones: Embedded within a tendon; an example is the patella.

Long Bone Anatomy

  • Diaphysis: The shaft portion of long bones, primarily made of compact bone.

  • Medullary Cavity: The hollow space inside the diaphysis that stores yellow marrow.

  • Epiphyses: The ends of long bones, made of spongy bone containing red marrow.

  • Articular Cartilage: Cushions the joints where bones meet.

  • Periosteum: A dense outer covering that protects bones, containing blood vessels and nerves.

  • Endosteum: A thin membrane lining the medullary cavity.

Microscopic Structure

  • Compact Bone: Composed of structural units called osteons, which consist of concentric lamellae surrounding a central canal.

  • Spongy Bone: Contains trabeculae, which are lattice-like beams that house marrow-filled spaces.

  • Osteocytes: Mature bone cells located within lacunae; interconnected via canaliculi to communicate and exchange nutrients.

Bone Remodeling

  • Osteoblasts: Cells responsible for building new bone.

  • Osteoclasts: Cells that break down old or damaged bone, essential for remodeling.

Bone Growth and Development

  • Epiphyseal Plate: Growth cartilage that becomes the epiphyseal line once growth is complete.

  • Endochondral Ossification: The process in which cartilage is replaced with bone at growth plates.

  • Intramembranous Ossification: The formation of flat bones, like those of the skull, directly from mesenchymal tissue; includes fontanels in newborns.

Divisions of the Skeleton

  • Axial Skeleton: Comprising 80 bones including the skull, vertebral column, ribs, sternum, and hyoid bone.

  • Appendicular Skeleton: Comprising 126 bones including the limbs and girdles (shoulder and pelvic).

Axial Skeleton Highlights

Skull

  • Composed of 8 cranial bones and 14 facial bones.

  • Paranasal Sinuses: Include frontal, maxilla, sphenoid, and ethmoid sinuses.

  • Sutures: Fibrous joints connecting the cranial bones (coronal, sagittal, lambdoidal, squamous).

  • Hyoid Bone: Unique as it is not attached to any other bone.

Vertebral Column

  • Composed of 5 regions: C (cervical) - 7 vertebrae, T (thoracic) - 12 vertebrae, L (lumbar) - 5 vertebrae, plus sacrum and coccyx.

  • Atlas (C1): Allows nodding of the head (yes motion).

  • Axis (C2): Contains the dens, allowing pivoting of the head (no motion).

  • Normal Curves:

    • Convex: Thoracic & sacral.

    • Concave: Cervical & lumbar.

Thoracic Cage

  • Comprises 12 pairs of ribs and the sternum.

  • True Ribs: Ribs 1–7 directly attached to the sternum via cartilage.

  • False Ribs: Ribs 8–10 indirectly attached to the sternum.

  • Floating Ribs: Ribs 11–12 not attached to the sternum.

Appendicular Skeleton Highlights

Upper Limb

  • Pectoral Girdle: Comprising the clavicle and scapula.

  • Arm Bones: Humerus, radius, ulna.

  • Wrist Bones: Carpals (8), metacarpals (5), phalanges (14).

Lower Limb

  • Coxal Bones: Comprising ilium, ischium, and pubis.

  • Leg Bones: Femur (the longest bone), tibia (weight-bearing), fibula (not weight-bearing).

  • Foot Bones: Tarsals (7), metatarsals (5), phalanges (14), calcaneus.

Male vs Female Pelvis

  • Male Pelvis: Narrow V shape; funnel-shaped physiology.

  • Female Pelvis: Wider V shape; basin-shaped for childbirth.

Joints (Articulations)

Classification by Movement

  • Synarthroses: Joints with no movement (e.g., skull sutures).

  • Amphiarthroses: Joints allowing slight movement (e.g., pubic symphysis, vertebrae).

  • Diarthroses: Freely movable synovial joints containing synovial fluid.

Types of Synovial Joints

  • Ball and Socket Joints: Located at shoulders and hips.

  • Hinge Joints: Located at elbows, knees, and fingers.

  • Pivot Joints: Atlas/axis joint allowing rotation.

  • Saddle Joints: Found in the thumb.

  • Gliding Joints: Found between vertebrae.

  • Condyloid Joints: Found in wrist (radiocarpal joint).

Skeletal Disorders

Bone Disorders

  • Osteoporosis: Condition leading to low bone density and increasing fracture risk.

  • Rickets/Osteomalacia: Resulting from vitamin D deficiency, leading to softened bones.

  • Paget Disease: Disorder characterized by abnormal bone remodeling.

  • Osteogenesis Imperfecta: Genetic disorder resulting in brittle bones.

Bone Fractures

  • Simple: Bone is broken but skin remains intact.

  • Compound: Bone break that pierces the skin.

  • Complete: Bone is broken into two or more pieces.

  • Incomplete: Partial fracture, such as a greenstick fracture common in children.

  • Other Fracture Types: Comminuted, impacted, spiral, transverse, and oblique fractures.

Joint Disorders

  • Osteoarthritis: Degeneration of cartilage resulting in bone spurs and joint pain.

  • Rheumatoid Arthritis: An autoimmune disorder resulting in synovial inflammation and resultant joint deformity.

  • Gouty Arthritis: Formed by deposits of uric acid crystals in the joints.

  • Infectious Arthritis: Caused by infections, such as Lyme disease, affecting the joints.