Module 7: Axial Skeleton

Axial Skeleton Study Notes

Introduction to the Skeletal System
  • **Two Major Divisions:
    • Axial Skeleton:
    • Includes:
      • Skull
      • Vertebral column
      • Thoracic cage
    • Appendicular Skeleton
  • Composed of bones, cartilage, joints, and ligaments
  • Represents about 20% of body mass
  • Provides support, protects internal organs, allows for movement, stores minerals, and is the site of blood cell formation.
The Skull
  • Formed by two sets of bones:
    • **Cranial Bones:**
    • Enclose the brain in the cranial cavity
    • Consist of:
      • Cranial vault
      • Cranial base (anterior, middle, and posterior cranial fossae - shallow depressions)
    • Provide sites for muscle attachment (head and neck muscles)
    • **Facial Bones:**
    • Form the framework of the face
    • Cavities for special sense organs (sight, taste, smell)
    • Openings for air and food passage
    • Sites of attachment for teeth and muscles
Geography of the Skull
  • Composed of various cavities:
    • Cranial cavity
    • Middle and internal ear cavities
    • Nasal cavity
    • Orbits (eye sockets)
  • Contains **85 named openings:**
    • **Foramina:** Holes or openings in a bone
    • **Canals:** Longer, tube-like passageways through a bone
    • **Fissures:** Narrow slit-like openings between bones
  • All three structures allow for the passage of nerves and blood vessels
Cranial Bones
  • **Frontal Bone:**
    • Located at the anterior portion of the cranium
    • Forms the superior wall of the orbits
    • Contributes to the forehead and anterior cranial fossa
    • Contains the air-filled frontal sinus, which helps lighten the skull and enhance voice resonance
  • **Parietal Bones (2):**
  • **Occipital Bone:**
  • **Temporal Bones (2):**
  • **Sphenoid Bone:**
  • **Ethmoid Bone:**
Parietal Bones and Major Associated Sutures
  • **Sutures:**
    • **Immovable, interlocking joints**
    • Four major sutures mark the articulations of parietal bones with other skull bones:
    • **Coronal Suture:** Between the parietal and frontal bone
    • **Sagittal Suture:** Between the right and left parietal bones
    • **Lambdoid Suture:** Between the parietal and occipital bones
    • **Squamousal Suture:** Between the parietal and temporal bones on each side of the skull
Important Features of the Skull
  • **Cranial Features:**
    • Occipital bone, external occipital protuberance, sella turcica of the sphenoid bone, and more
  • **Mandibular Foramen:** Important for dental procedures, located on the mandible
Details on the Occipital Bone
  • **Role and Location:**
    • Forms most of the skull’s posterior wall and posterior fossa
    • Articulates with the 1st vertebra (atlas) via the occipital condyles
    • Sites for attachment of the ligamentum nuchae and neck/back muscles
    • Provides support and stability to the cervical spine
    • Limits forward flexion of the head and neck
Temporal Bones
  • **Location:**
    • Inferolateral aspects of the skull and parts of the cranial base
  • **Major Regions:**
    • **Squamous Region:**
    • Attachment for the temporal muscle (chewing)
    • Forms the superior part of the lateral skull wall.
    • Contains the zygomatic process that articulates with the zygomatic bone.
    • **Tympanic Region:**
    • Surrounds the external auditory canal
    • Features the styloid process, a needle-like projection for muscle and ligament attachment.
    • **Mastoid Region:**
    • Protects the inner ear
    • Regulates pressure in the middle ear
    • Contains the mastoid process, an attachment site for several neck muscles.
    • **Petrous Region:**
    • Houses inner ear components
    • A strong, wedge-shaped part that forms the middle and posterior cranial fossae floors.
Sphenoid Bone
  • **Description:**
    • Complex, bat-shaped and known as the keystone bone
  • **Articulation:**
    • Articulates with all other cranial bones
  • **Major Features:**
    • Contains three pairs of processes:
    • Greater wings
    • Lesser wings
    • Pterygoid processes
    • Contains the sella turcica, a saddle-shaped depression housing the pituitary gland.
Ethmoid Bone
  • **Characteristics:**
    • Deepest skull bone
    • Forms superior part of nasal septum and roof of nasal cavities
    • Contributes to medial wall of orbits
    • Contains cribriform plates with olfactory foramina for the passage of olfactory nerves.
  • **Crista Galli:**
    • Serves as attachment for dura mater (protective brain cover)
Sutural Bones
  • **Description:**
    • Tiny, irregularly-shaped bones appearing within sutures
  • **Significance:**
    • Their function is unknown as their number varies per individual
Facial Bones
  • **Overview:**
    • **Fourteen Facial Bones:**
    • Mandible
    • Maxillary bones (2)
    • Zygomatic bones (2)
    • Nasal bones (2)
    • Lacrimal bones (2)
    • Palatine bones (2)
    • Vomer
    • Inferior nasal conchae (2)
Maxillary Bones
  • **Description:**
    • Medially fused to form the upper jaw and central portion of facial skeleton
  • **Keystone Bones:**
    • Articulates with all other facial bones except the mandible
  • **Contain Maxillary Sinuses:**
    • Largest of the paranasal sinuses
    • Form part of the hard palate.
Zygomatic Bones
  • **Characterization:**
    • Form the inferolateral margins of the orbits
    • Known as cheekbones
Nasal and Lacrimal Bones
  • **Nasal Bones:**
    • Form the bridge of the nose
  • **Lacrimal Bones:**
    • Found in medial walls of the orbits
    • Contains lacrimal fossa housing the lacrimal sac, which allows tears to drain from the eye surface to the nasal cavity
Palatine Bones, Vomer and Inferior Nasal Conchae
  • **Palatine Bones:**
    • Form the posterior 1/3 of the hard palate and contribute to the posterolateral walls of the nasal cavity and part of orbits
  • **Vomer:**
    • Plow-shaped bone forming the inferior part of the nasal septum
  • **Inferior Nasal Conchae:**
    • Largest of the three pairs of conchae, contributing to lateral walls of the nasal cavity
The Orbits
  • **Function:**
    • Cavities that encase eyes and lacrimal glands
    • Sites of attachment for eye muscles
  • **Formed by:**
    • Parts of seven bones: frontal, sphenoid, maxilla, palatine, lacrimal, ethmoid, and zygomatic bones
Nasal Cavity
  • **Composition:**
    • Roof, lateral walls, and floor formed by parts of four bones:
    • Ethmoid bone
    • Palatine bones
    • Maxillary bones
    • Inferior nasal conchae
  • **Nasal Septum:**
    • Comprised of bone and hyaline cartilage, formed by:
    • Perpendicular plate of the ethmoid bone
    • Vomer
    • Anterior septal cartilage
Paranasal Sinuses
  • **Definition:**
    • Mucosa-lined air-filled sacs that lighten the skull
    • Enhance voice resonance
    • Warm and humidify air breathed in
  • **Located in:**
    • Frontal, sphenoid, ethmoid, and maxillary bones
Hyoid Bone
  • **Characteristics:**
    • Not a bone of the skull and does not articulate directly with any other bone
    • Movable base for the tongue
    • Site of attachment for muscles associated with swallowing and speech
Vertebral Column
  • **Functionality:**
    • Transmits weight of the trunk to lower limbs
    • Surrounds and protects the spinal cord
    • Flexible curved structure containing 26 irregular vertebrae in five major regions:
    • Cervical vertebrae (7)
    • Thoracic vertebrae (12)
    • Lumbar vertebrae (5)
    • Sacrum
    • Coccyx
Curvature of the Vertebral Column
  • **Importance:**
    • Four main curvatures (cervical, thoracic, lumbar, sacral) increase resilience and flexibility of the spine:
    • Cervical and lumbar curvatures: concave posteriorly
    • Thoracic and sacral curvatures: convex posteriorly
  • **Abnormal Spine Curvatures:**
    • **Scoliosis:** Abnormal lateral curve
    • **Kyphosis:** Exaggerated thoracic curvature (hunchback)
    • **Lordosis:** Accentuation of lumbar curvature (swayback)
Ligaments of the Vertebral Column
  • **Anterior and Posterior Longitudinal Ligaments:**
    • Extend from neck to sacrum
  • **Ligamentum Flavum:**
    • Connects adjacent vertebrae
  • **Short Ligaments:**
    • Connect each vertebra to those above and below
Intervertebral Discs
  • **Structure:**
    • Cushion-like pads composed of two parts, acting as shock absorbers during movement:
    • **Nucleus Pulposus:** Inner gelatinous nucleus providing elasticity and compressibility
    • **Anulus Fibrosus:** Outer collar composed of collagen and fibrocartilage, surrounding the nucleus pulposus
General Structure of Vertebrae
  • Contains:
    • **Body (Centrum):** Anterior weight-bearing region
    • **Vertebral Arch:** Composed of pedicles and laminae that enclose the vertebral foramen
    • **Vertebral Foramina:** Together form the vertebral canal for the spinal cord
    • **Intervertebral Foramina:** Lateral openings between adjacent vertebrae for spinal nerves
    • **Seven Processes per Vertebra:**
    • Spinous process (projects posteriorly)
    • Transverse processes (2, project laterally)
    • Superior articular processes (2, protrude superiorly)
    • Inferior articular processes (2, protrude inferiorly)
Cervical Vertebrae
  • **Characteristics:**
    • C1 to C7: Smallest and lightest vertebrae
    • C3 to C7 features:
    • Oval body
    • Bifid spinous processes
    • Large, triangular vertebral foramen
    • Transverse foramen in each transverse process for vertebral arteries
    • C7 known as the vertebra prominens
Unique Features of Cervical Vertebrae
  • **Atlas (C1):**
    • No body or spinous process
    • Consists of anterior and posterior arches, and two lateral masses
    • Superior surface of lateral masses articulate with occipital condyles (enabling 'Yes' movement)
  • **Axis (C2):**
    • Dens projects superiorly into the anterior arch of atlas
    • No body of atlas present
    • Dens serves as a pivot for rotation of atlas (enabling 'No' movement)
Thoracic Vertebrae
  • **Description:**
    • T1 to T12
    • Articulate with ribs at facets and demifacets
    • Possess costal facets on the body and transverse processes for articulation with the ribs.
    • Long spinous processes pointing inferiorly
    • Circular vertebral foramen
    • Location of articular facets allows rotation of this spine section
Lumbar Vertebrae
  • **Description:**
    • L1 to L5
    • Receives the most stress from weight
    • Features include:
    • Short, thick pedicles and laminae
    • Flat, hatchet-shaped spinous processes pointing posteriorly
    • Triangular vertebral foramen
    • Orientation of articular facets locks lumbar vertebrae together to prevent rotation
Sacrum and Coccyx
  • **Sacrum:**
    • Composed of 5 fused vertebrae (S1 to S5)
    • Forms posterior wall of pelvis
    • Articulates with L5 superiorly and with hip bones at sacroiliac joints
  • **Coccyx:**
    • Composed of 3-5 fused vertebrae
    • Articulates superiorly with the sacrum
Thoracic Cage
  • **Functions:**
    • Protects vital organs of the thoracic cavity
    • Supports shoulder girdles and upper limbs
    • Provides attachment for neck, back, chest, and shoulder muscles
  • **Composition:**
    • Thoracic vertebrae (posteriorly)
    • Sternum and costal cartilages (anteriorly)
    • Ribs (laterally)
Sternum (Breastbone)
  • **Structure:**
    • Composed of three fused bones:
    • **Manubrium:** Superior portion; articulates with clavicles and ribs 1 and 2
    • **Body:** Articulates with costal cartilages of ribs 2 through 7
    • **Xiphoid Process:** Inferior end; a muscle attachment site, not ossified until about age 40
  • **Landmarks:**
    • **Jugular Notch:** Central indentation at the superior border of manubrium
    • **Sternal Angle:** Horizontal ridge across the front of the sternum
    • **Xiphisternal Joint:** Point where the sternal body and xiphoid process fuse
Ribs and Their Attachments
  • **Total:**
    • Twelve pairs of ribs
    • All attach posteriorly to the thoracic vertebrae (bodies and transverse processes)
  • **Classification:**
    • **True (vertebrosternal) ribs:** Pairs 1 through 7; attach directly to the sternum through their own individual costal cartilages.
    • **False ribs:**
    • Pairs 8 through 10 (vertebrochondral ribs); attach indirectly to the sternum by fusing their costal cartilages with that of the rib above.
    • Pairs 11-12 (floating or vertebral ribs); have no anterior attachment to the sternum or other costal cartilages.
Rib Structure
  • **Head:**
    • Articulates with facets (demifacets) on bodies of two adjacent vertebrae
  • **Neck:**
    • Constricted portion beyond the head
  • **Tubercle:**
    • Lateral to neck; articulates posteriorly with transverse costal facet of the same-numbered thoracic vertebra
  • **Shaft:**
    • Most of the rib, typically featuring a costal groove for nerves and blood vessels on its inferior border.
      **End of Module