The Skeletal System: Axial Skeleton

Introduction to the Skeletal System

  • The skeletal system consists of bones and cartilage, performing critical functions for the body:
  • Support: Provides a rigid structure to support body weight.
  • Movement: Serves as levers for muscle action.
  • Protection: Shields internal organs like the heart and lungs.
  • Blood Cell Production: Produces blood cells within the bone marrow.
  • Mineral Storage: Stores and releases minerals and lipids.

Major Divisions of the Skeleton

  • The human skeleton has 206 bones in adults, grouped into:
  • Axial Skeleton: Comprising 80 bones. Key components include:
    • Skull
    • Vertebral Column
    • Thoracic Cage
  • Appendicular Skeleton: Comprising 126 bones, including bones of the limbs and girdles.

Bone Classification by Shape

  • Bones are classified into five categories based on their shape:
  1. Long Bones:
    • Cylindrical, longer than wide (e.g., femur, humerus).
    • Function primarily as levers for movement.
  2. Short Bones:
    • Cube-shaped, equal in dimensions (e.g., carpals, tarsals).
    • Provide stability and some limited motion.
  3. Flat Bones:
    • Thin, often curved (e.g., skull, ribs, sternum).
    • Serve as points of attachment for muscles and protect organs.
  4. Irregular Bones:
    • Complex shapes (e.g., vertebrae).
    • Serve various functions and do not fit into other categories.
  5. Sesamoid Bones:
    • Small, round bones found in tendons (e.g., patella).
    • Protect tendons and improve mechanical advantage.

Structure of Long Bones

  • Key structures include:
  • Diaphysis: The tubular shaft filled with medullary cavity, lined with endosteum.
  • Epiphysis: The ends of the bone, covered with articular cartilage to reduce friction.
  • Periosteum: A fibrous membrane covering bone, containing blood vessels, nerves, and serves as an attachment site for tendons and ligaments.

Histology of Bone Tissue

  • Bone is a connective tissue composed of:
  • Bone Cells:
    • Osteoblasts: Form new bone.
    • Osteocytes: Maintain bone matrix.
    • Osteoclasts: Resorb bone tissue, crucial for remodeling.
  • Bone Matrix: Made of crystalline collagen fibers and inorganic salts.

Compact vs. Spongy Bone

  • Compact Bone:

  • Dense structure, found in the diaphysis of bones.

  • Organized into osteons, containing a central canal with blood vessels.

  • Spongy Bone (Trabecular Bone):

  • Lighter, less dense, characterized by a lattice of trabeculae that provide strength while minimizing weight.

  • Contains red marrow that produces blood cells.

The Axial Skeleton

  • Composed of:
  • Skull: Cranium and facial bones, 22 total bones.
  • Vertebral Column: 24 total vertebrae (7 cervical, 12 thoracic, 5 lumbar), plus sacrum and coccyx (fused vertebrae).
  • Thoracic Cage: 12 pairs of ribs and the sternum.
The Skull:
  • Comprised of cranial bones that protect the brain and facial bones for structure. Notable features:
  • Sutures: Immovable joints between skull bones (e.g., coronal, sagittal, lambdoid).
Vertebral Column:
  • Supports the head and houses the spinal cord.
  • Sections:
  • Cervical (C1-C7), Thoracic (T1-T12), Lumbar (L1-L5)
  • Special structures: Atlas (C1) and Axis (C2), allowing head rotation.

The Thoracic Cage

  • Protects the heart and lungs, consisting of 12 pairs of ribs:
  • True Ribs (1-7): Attach directly to the sternum.
  • False Ribs (8-10): Connect to the sternum via costal cartilage.
  • Floating Ribs (11-12): Lack anterior attachment.
  • Sternum Parts: Manubrium, body, xiphoid process.

Additional Notes

  • Hyoid Bone: U-shaped bone in the neck that aids in tongue and larynx movement.
  • Age Consideration: Certain bones fuse during development; hyoid can fracture easily in strangulations.

Summary

  • Understand the structure, classification, and function of the skeletal system and its components.