Skeletal System Notes

Skeletal System

Classification of Bones

  • The human skeleton contains 206 named bones.
  • These bones are divided into two main groups:
    • Axial skeleton: Includes bones along the long axis of the body.
      • Skull
      • Vertebral column
      • Rib cage
    • Appendicular skeleton: Includes bones of the upper and lower limbs and the girdles that attach them to the axial skeleton.

Cartilage

  • Hyaline cartilage
  • Elastic cartilages
  • Fibrocartilages

Bone Classification by Shape

  • Long bones
    • Longer than they are wide.
    • Examples: Limb bones, wrist bones (metacarpals, radius, ulna), ankle bones (metatarsals, tibia, fibula).
  • Short bones
    • Cube-shaped.
    • Examples: Wrist and ankle bones.
    • Sesamoid bones are a type of short bone found within tendons (e.g., patella).
    • Vary in size and number between individuals.
  • Flat bones
    • Thin, flat, and slightly curved.
    • Examples: Sternum, scapulae, ribs, and most skull bones.
  • Irregular bones
    • Complicated shapes.
    • Examples: Vertebrae, coxal (hip) bones.

Functions of Bones

  • Seven important functions:
    • Support: Provides support for the body and soft organs.
    • Protection: Protects vital organs such as the brain, spinal cord, and organs in the rib cage.
    • Movement: Bones act as levers for muscle action, enabling body movement.
    • Mineral and growth factor storage: Bones serve as a reservoir for minerals like calcium and phosphorus, as well as growth factors.
    • Blood cell formation (hematopoiesis): Occurs in the red marrow cavities of certain bones.
    • Triglyceride (fat) storage: Fat is stored in bone cavities as an energy source.
    • Hormone production: Bones produce hormones like osteocalcin, which regulates bone formation and protects against obesity, glucose intolerance, and diabetes mellitus.

Bone Structure

  • Bones are organs containing different types of tissues:
    • Osseous (bone) tissue
    • Nervous tissue
    • Cartilage
    • Fibrous connective tissue
    • Muscle and epithelial cells in blood vessels
  • Three levels of structure:
    • Gross anatomy
    • Microscopic anatomy
    • Chemical composition

Gross Anatomy

  • Bone Textures
    • Compact bone: Dense outer layer that appears smooth and solid.
    • Spongy bone (cancellous or trabecular bone): Honeycomb-like structure of flat pieces of bone deep to the compact bone, called trabeculae.

Bone Markings

  • Sites for muscle, ligament, and tendon attachment on external surfaces.
  • Joint surfaces.
  • Conduits for blood vessels and nerves.
  • Types of Bone Markings
    • Projections: Indicate stresses created by muscle pull or joint modifications.
    • Depressions and openings: Allow nerves and blood vessels to pass through.

Bone Markings - Terminology

  • Projections (Sites of Muscle and Ligament Attachment)
    • Tuberosity: Large rounded projection; may be roughened.
    • Crest: Narrow ridge of bone; usually prominent.
    • Trochanter: Very large, blunt, irregularly shaped process (only on the femur).
    • Line: Narrow ridge of bone; less prominent than a crest.
    • Tubercle: Small rounded projection or process.
    • Epicondyle: Raised area on or above a condyle.
    • Spine: Sharp, slender, often pointed projection.
    • Process: Any bony prominence.
  • Projections (Help to Form Joints)
    • Head: Bony expansion carried on a narrow neck.
    • Facet: Smooth, nearly flat articular (joint) surface.
    • Condyle: Rounded articular projection.
    • Ramus: Armlike bar of bone.
  • Depressions and Openings (For Passage of Blood Vessels and Nerves)
    • Groove: Furrow.
    • Fissure: Narrow, slit-like opening.
    • Foramen: Round or oval opening through a bone.
    • Notch: Indentation at the edge of a structure.
    • Meatus: Canal-like passageway.
    • Sinus: Cavity within a bone, filled with air and lined with mucous membrane.
    • Fossa: Shallow, basin-like depression in a bone, often serving as an articular surface.

Bone Development

  • Ossification (osteogenesis): Process of bone tissue formation.
    • Formation of the bony skeleton begins in the 2nd month of development.
    • Postnatal bone growth occurs until early adulthood.
    • Bone remodeling and repair are lifelong processes.

Bone Stress

  • Bones are subjected to bending stress due to body weight and other loads.
  • Compression occurs on one side of the bone, while tension occurs on the opposite side.
  • Vigorous exercise can strengthen bone by adding bone matrix to counteract added stress.

Age-Related Changes in Bone

  • Children and adolescents: Bone formation exceeds bone resorption.
  • Young adults: Bone formation and resorption are in balance; males generally have greater bone mass.
  • Bone density changes over a lifetime are largely determined by genetics.
    • The gene for Vitamin D's cellular docking influences bone mass early in life and osteoporosis risk as one ages.
  • Bone mass, mineralization, and healing ability decrease with age beginning in the 4th decade (30s).
    • Except for bones of the skull.
    • Bone loss is greater in whites and in females.
    • Electrical stimulation and daily ultrasound treatments can hasten bone repair.

Skeletal System Overview

  • Composed of bones, cartilages, joints, and ligaments.
  • Accounts for approximately 20% of body mass.
  • Two major divisions:
    • Axial
    • Appendicular

The Axial Skeleton

  • Consists of 80 bones.
  • Three major regions:
    • Skull
    • Vertebral column
    • Thoracic cage

The Skull

  • Formed by two sets of bones:
    • Cranial bones (cranium): Enclose the brain in the cranial cavity.
      • Cranial vault (calvaria)
      • Cranial base, featuring the anterior, middle, and posterior cranial fossae.
      • Provide attachment sites for head and neck muscles.
    • Facial bones
      • Form the framework of the face.
      • Provide cavities for special sense organs (sight, taste, smell).
      • Provide openings for air and food passage.
      • Provide attachment sites for teeth and muscles of facial expression.

Skull Geography

  • Cranial cavity
  • Middle and internal ear cavities
  • Nasal cavity
  • Orbits (eye sockets)
  • 85 named openings (foramina, canals, fissures) for passage of nerves and blood vessels

Paranasal Sinuses

  • Frontal sinus
  • Ethmoidal air cells (sinus)
  • Sphenoidal sinus
  • Maxillary sinus