Skeletal System - Axial Skeleton: Comprehensive Study Notes

Skeletal System - Axial Skeleton

• Learning objectives

  • State the functions of bone.
  • Identify the basic structural features of a bone.
  • Explain the relationship between the structure of bone and the functions of bone.
  • Distinguish between the various types of bones.
  • Identify axial skeleton, and list the bones and bony processes found on each.

• Bone tissue: cells and matrix

  • The tissue that makes up bone comprises four main types of bone cells: osteogenic cells, osteoblasts, osteocytes, and osteoclasts.
  • These cells help with the production and maintenance of the mineral extracellular matrix that surrounds them.

• Osseous tissue types and bone mass distribution

  • There are two types of osseous tissue that contribute to the formation of bone: compact bone and spongy bone.
  • Approximately $80\%$ of the skeleton is formed by compact bone, with the remaining $20\%$ formed by spongy bone.
  • Spongy bone (also called cancellous or trabecular bone) is found in the long bones and is surrounded by compact bone.
  • Compact bone (also called cortical bone) surrounds spongy bone; they are heavy, tough, and compact in nature.

• Cartilage and embryology of the skeleton

  • The bones of the skeletal system are intimately linked with cartilage.
  • During embryological development, the whole skeleton is initially comprised of cartilage, which gradually ossifies (is converted into bone).
  • Even in adults, bones interacting at joints are often capped by cartilage.
  • Types of cartilage include: hyaline cartilage, fibrocartilage, and elastic cartilage, each with specific structure and function at different joints.

• Hyaline cartilage

  • Locations: Between tips of ribs and sternum; covering bone surfaces at synovial joints; supporting larynx (voice box), trachea, and bronchi; forming part of nasal septum.
  • Function: Provides flexible support; reduces friction between bony surfaces.
  • Structure: Chondrocytes in lacunae; matrix; (in diagrams) hyaline matrix with chondrocytes in lacunae.

• Elastic cartilage

  • Locations: Auricle of external ear; epiglottis; auditory tube; cuneiform cartilages of larynx.
  • Function: Provides support, tolerates distortion without damage and returns to original shape.
  • Structure: Chondrocyte in lacuna; elastic fibers in matrix.

• Fibrocartilage

  • Locations: Pads within knee joint; between pubic bones of pelvis; intervertebral discs.
  • Function: Resists compression; prevents bone-to-bone contact; limits relative movement.

• Anatomy of a long bone

  • The central shaft is the diaphysis.
  • The epiphyses (two) are the bulky ends on both sides.
  • Metaphysis is the narrow section between the diaphysis and epiphysis.
  • Periosteum covers the bone.

• Examples of long bones

  • In the arms: Clavicle, Humerus, Radius, Ulna, Metacarpals, Phalanges.
  • In the legs: Femur, Tibia, Fibula, Metatarsals, Phalanges.

• Medullary cavity and bone marrow

  • The central part of the diaphysis is hollowed to form the medullary (marrow) cavity.
  • Two main types of bone marrow: red marrow (blood cell production) and yellow marrow (fat cells).
  • At birth, all marrow is red; with age, marrow becomes yellow in the interior of most long bones.
  • Some bones (e.g., pelvic bones) continue to produce blood throughout life.

• Red vs. yellow marrow aging and grafting notes

  • Infants: marrow is predominantly red (blood formation).
  • With age: red marrow decreases; yellow marrow increases (fatty).
  • Hip (pelvic) bones remain red marrow-producing into adulthood, which is why bone marrow is often sampled from the hip for grafts or blood cell analyses.
  • Red marrow is mainly found in flat bones such as the hip and vertebrae; marrow remains red until around the age of $7$ years because the need for new blood formation is high.

• Anatomy of flat bones

  • Flat bones consist of two thin plates of periosteum-covered compact bone, separated by a thin layer of endosteum-covered spongy bone.
  • In flat bones, the trabeculae sandwiched between compact bone are termed diploe.
  • Flat bones are slightly curved to protect underlying soft tissue and provide a large surface area for muscle attachment.
  • Examples: frontal, hip (pelvis), lacrimal, nasal, occipital, parietal, ribs, scapula, sternum, vomer.

• Examples of flat bones

  • In the skull: Frontal, Parietal, Lacrimal, Nasal, Occipital, Vomer.
  • In the arm: Scapula.
  • In the chest: Sternum, Ribs.
  • In the pelvis: Ilium, Pubis, Ischium.

• Classification of bones by shape

  • Five types: long, short, flat, irregular, and sesamoid.
  • Long bones: e.g., Femur, Tibia, Fibula, Humerus; primarily for mechanical strength.
  • Short bones: cube-shaped; mostly in carpal and tarsal bones; provide multi-directional motion.
  • Flat bones: thin and flat; provide protection and large muscle attachment surfaces; e.g., many skull bones, sternum, ribs, scapulae.
  • Irregular bones: complex shapes (e.g., vertebrae).
  • Sesamoid bones: most unnamed; the patella is the named standard example; provide mechanical support and reduce friction.
  • Summary of examples (from the slide):
  • Long: Ulna, Radius, Carpal bones (hands/wrists), Tarsal bones (feet/ankles), Femur, etc.
  • Short: Carpals, Tarsals.
  • Flat: Cranial bones, Sternum, Ribs, Scapula, etc.
  • Irregular: Vertebrae, Hyoid bone, Sphenoid bone, Facial bones.
  • Sesamoid: Patella (the only universally named one in a normal skeleton).

• Bone surface markings

  • Two types: projections (processes) and depressions (cavities).
  • Projections (where muscles, tendons, ligaments attach):
  • Tuberosity (large rounded projection)
  • Spinous process (spine; sharp, slender projection)
  • Trochanter (very large, blunt irregular projection)
  • Crest (narrow ridge of bone)
  • Examples: Iliac crest, Ischial spine, Ischial tuberosity.
  • Depressions and openings for vessels and nerves to pass through:
  • Foramen (round or oval opening)
  • Meatus (canal-like opening)
  • Fossa (shallow depression; often forms joints)
  • Notch, Groove
  • Examples: Inferior orbital fissure, Foramen, Skull sinuses.
  • Additional labeled terms often seen in diagrams: Head, Trochanter, Condyle, Tubercle, Sinus, Fissure, Process, Ramus, Epicondyle, Alveolus, Facet, Sulcus, Line, etc.

• Skull and hyoid bone: cranial, facial bones, and associated bones

  • There are 22 bones in the skull (cranial + facial bones).
  • Cranial bones (8 total): Frontal (unpaired), Occipital (unpaired), Parietal (paired x2), Sphenoid (unpaired), Ethmoid (unpaired), Temporal (paired x2).
  • Facial bones (14 total): Maxillae (2), Lacrimal (2), Zygomatic (2), Palatine (2), Nasal (2), Inferior nasal conchae (2), Vomer (1), Mandible (1).
  • The hyoid bone is a horseshoe-shaped bone at the front of the neck. It is counted in skull bones, but it is NOT included in the cranial or facial bones.
  • The cranium includes the skull bones; the hyoid bone is considered part of the skull but is not a cranial or facial bone.

• Skull landmarks and features (selected exemplars)

  • External auditory meatus (ear canal)
  • Supraorbital foramen (eye socket region)
  • Orbits (eye sockets)
  • Zygomatic process, Coronoid process, Mandibular condyle, Mastoid process, Styloid process, Mental foramen, Inferior orbital fissure, Alveolar margins.
  • These landmarks support muscle attachment, articulation, and passage of nerves/vessels.

• The ear bones and hearing

  • The three tiny middle-ear bones (ossicles) transmit sound from the eardrum to the inner ear.
  • The smallest bones in the body:
  • Malleus (hammer) – two in number
  • Incus (anvil) – two in number
  • Stapes (stirrup) – two in number
  • Order of vibration: Malleus → Incus → Stapes.
  • Location: middle ear within the tympanic cavity of the temporal bone.
  • Function: carry sound vibrations to the inner ear for neural interpretation.

• Vertebral column overview

  • The vertebral (spinal) column comprises $24\, irregular\, bones$ (vertebrae) stacked, plus fused sacrum and coccyx.
  • Primary functions: protect the spinal cord and allow passage of the spinal cord along the body.
  • Section counts:
  • Cervical vertebrae: $7$ (C1–C7)
  • Thoracic vertebrae: $12$ (T1–T12)
  • Lumbar vertebrae: $5$ (L1–L5)
  • Sacrum: $5$ fused vertebrae
  • Coccyx: $4$ fused bones

• Cervical vertebrae details

  • The cervical region consists of 7 vertebrae (C1–C7).
  • C1 (atlas) and C2 (axis) are specialized; C1 supports the head and lacks a vertebral body and a spinous process; its superior articular facets contact the occipital condyles.
  • C2 (axis) has a dens (odontoid process) that articulates with C1 and allows rotation of the head.
  • C7 (vertebra prominens) may be considered atypical; it has two notable features: the transverse foramen is not used by the vertebral artery in C7 (instead it contains accessory veins), and it has a long spinous process.
  • Typical cervical vertebrae (C3–C6) features include: vertebral body (load-bearing), vertebral arch, pedicles, laminae, and spinous process; intervertebral discs lie between adjacent vertebral bodies and cushion movement.

• Thoracic vertebrae details

  • The thoracic region contains $12$ vertebrae (T1–T12).
  • Typical vertebrae have a vertebral body, a vertebral arch, and seven processes.
  • They articulate with the ribs via costal facets on the body and the transverse processes.
  • The intervertebral discs, laminae, pedicles, and articular processes create the exit path for spinal nerves.
  • Thoracic vertebrae provide attachments for the ribs, forming part of the thoracic cage.
  • The T5–T8 vertebrae are particularly representative of thoracic features.

• Lumbar vertebrae details

  • The lumbar region consists of $5$ vertebrae (L1–L5).
  • They are the largest vertebrae and bear the most body weight.
  • They provide flexibility and movement, while supporting the upper body's weight.

• Sacrum and coccyx

  • The sacrum and coccyx are formed by fusion of sacral and coccygeal vertebrae, typically completing fusion by about age $30$.
  • Sacrum is composed of five fused vertebrae (S1–S5).
  • Coccyx typically consists of 3–5 small bones that fuse to form a single tailbone.
  • Both structures are weight-bearing and contribute to posture and locomotion (walking, standing, sitting).

• Thoracic cage (rib cage)

  • The thoracic cage consists of $12$ thoracic vertebrae, $12$ pairs of ribs, and the sternum (manubrium, body, and xiphoid process).
  • Function: protects the heart and lungs and provides origin/insertion points for muscles of respiration, back, and upper limbs.

• Anatomy of a typical rib

  • Features: head, neck, shaft.
  • The rib has a tubercle for articulation with the transverse process of the corresponding thoracic vertebra.
  • The rib has costal facets on the head for articulation with vertebral bodies and a costal facet on the neck for articulation with the thoracic vertebra's body.
  • The costal groove contains vessels and nerves, and the rib articulates anteriorly via costal cartilage.

• Atypical ribs

  • Ribs 1, 2, 10, 11, and 12 are considered atypical due to unique features:
  • Rib 1: shorter and wider than other ribs.
  • Rib 2: thinner and longer than rib 1; has two articular facets on the head; roughened area for origin of serratus anterior.
  • Rib 10: only one facet.
  • Ribs 11 and 12: no neck; only one facet.

• Rib articulations (posterior and anterior)

  • Posterior articulation: each rib forms an articulation with a facet on the body of its corresponding vertebra.
  • Ribs 1–10 form a secondary articulation with the transverse process of the same-numbered vertebra.
  • Ribs 2–10 articulate with a facet on the vertebra above them.
  • Anterior attachments vary:
  • Ribs 1–7 attach independently to the sternum via costal cartilage (true ribs).
  • Ribs 8–10 attach to the costal cartilage of the rib above (false ribs).
  • Ribs 11–12 do not attach to the sternum (floating ribs).

• Ribs: summary

  • There are $12$ pairs of ribs, each articulating with facets on the thoracic vertebrae.
  • True ribs: $1-7$ attach to the sternum via costal cartilage.
  • False ribs: $8-10$ attach to the costal cartilage of the rib above.
  • Floating ribs: $11-12$ do not attach to the sternum.

• Connection to bone names and functions

  • The axial skeleton (skull, vertebral column, thoracic cage) forms the central axis of the body and protects vital organs, provides attachment sites for limbs, and supports the body.
  • The appendicular skeleton includes the bones of the limbs and girdles (not covered in detail here, but referenced in the broader study of the skeletal system).

• Quick reference counts and terms (summary)

  • Axial skeleton: $80$ bones total.
  • Skull bones: cranial $8$ + facial $14$ = $22$ total.
  • Vertebral column: $24$ vertebrae + fused sacrum and coccyx (S1–S5 + coccygeal segments).
  • Hyoid bone: counted among skull bones but not cranial or facial bones.
  • Ossicles: malleus, incus, stapes (middle ear).

• Connections to previous and real-world relevance

  • Understanding bone classifications and markings helps in radiology, anatomy labs, and clinical assessments (e.g., fracture sites, joint mechanics).
  • Knowledge of marrow types is relevant for hematology, bone marrow transplantation, and aging processes.
  • The vertebral column organization underpins posture, movement, and potential injury patterns (e.g., cervical rotation, thoracic rib articulation, lumbar weight-bearing).

• Ethical, philosophical, or practical implications

  • Bone marrow harvesting for grafts raises ethical considerations about donor consent and risk-benefit analysis.
  • Understanding age-related marrow changes relates to discussions about aging, healthspan, and interventions for hematopoietic health.

• Key equations and numerical references

  • Number of bones and vertebrae (summary):
  • Total skeletal bones: $206$
  • Axial skeleton bones: $80$
  • Cranial bones: $8$
  • Facial bones: $14$
  • Vertebrae by region: cervical $7$, thoracic $12$, lumbar $5$
  • Ribs: $12\times 2 = 24$ ribs

• Summary note

  • The axial skeleton forms the central core of the skeleton, providing protection and support, while the vertebral column and thoracic cage form critical protective and functional structures. The interplay between bone tissue types, cartilage, and bone markings supports both movement and stability across the body.