MR

CH8 BONES PT4 - AXIAL SKELETON -

Axial vs. Appendicular Skeleton

  • Start by comparing the axial and appendicular skeletons as two main groups of bones. When given a bone name, first classify it as axial or appendicular.
  • The lecture emphasizes using the two-column organization (axial vs. appendicular) both in lab and in lectures.

Axial Skeleton Overview

  • Includes:
    • Skull (cranium and facial bones)
    • Vertebral column (cervical, thoracic, lumbar vertebrae; sacrum; coccyx)
    • Rib cage (thoracic cage) including sternum
    • Hyoid bone (a small, independent bone located anterior to the cervical vertebrae)
  • The axial skeleton forms the central axis of the body and protects the brain, spinal cord, and thoracic organs, and supports the body’s posture.

Appendicular Skeleton Overview

  • Includes all bones of the limbs and girdles:
    • Upper limb: clavicle, scapula, arm, forearm, hand
    • Lower limb: os coxae (hip bones), femur, leg, foot
  • The appendicular skeleton begins with the clavicle, which forms a joint with the sternum (sternoclavicular joint) and thus connects to the axial skeleton.
  • The os coxae (hip bones) form a joint with the sacrum (sacroiliac joint), where the appendicular and axial skeletons meet.
  • The sternum is attached to the ribs; the ribs articulate with the thoracic vertebrae posteriorly.
  • Summary tip for exams: if you hear the name of a bone, decide if it belongs to the axial or the appendicular skeleton.

The Skull: Cranial and Facial Bones

  • The skull contains two major groups: cranial bones (cranium) and facial bones.
  • The skull has a total of 8 cranial bones and 14 facial bones.
  • Cranial bones (8): 2 ext{ parietal}, 2 ext{ temporal}, ext{frontal}, ext{occipital}, ext{sphenoid}, ext{ethmoid}
  • Facial bones (14): 2 ext{ maxillae}, 2 ext{ zygomatic}, 2 ext{ nasal}, 2 ext{ lacrimal}, 2 ext{ palatine}, 2 ext{ inferior nasal conchae}, ext{mandible}, ext{vomer}
  • The sphenoid bone is centrally located and articulates with all the other cranial bones.
  • The ethmoid bone is located anteriorly and forms part of the nasal cavity and the orbit.

Hands-on Lab Tips ( Skull Anatomy )

  • In the lab, examine real bones to understand fit and function, not just pictures:
    • Identify the sphenoid bone in the facial bone box; understand its base-of-skull placement and relation to the pituitary gland sits in the sella turcica (pituitary saddle).
    • The ethmoid bone sits anterior to the sphenoid and helps form the anterior base of the skull.
    • The base of the skull and articulations: sphenoid fits into the skull base; the ethmoid sits in front of it.
  • Two useful skull demonstrations:
    • A skull with yellow sutures highlights the base of the skull and how sphenoid fits into it (sella turcica region).
    • An open-top skull shows the brain sitting on the sphenoid bone and the ethmoid bone anterior to it.

Facial Bones: Details and Functions

  • Maxillae (upper jaw bones):
    • Contain alveolar sockets for teeth.
    • Form the anterior two-thirds of the hard palate (the two maxillae fuse along the midline during development; failure to fuse can cause a cleft palate, which has cosmetic and functional consequences such as potential feeding difficulties and infection risk).
  • Hard palate composition:
    • Anterior two-thirds: maxillae (purple in the model)
    • Posterior one-third: palatine bones (orange in the model)
    • Together, they form the hard palate.
  • Palatine bones and maxillae together create the hard palate; the fusion of maxillae is critical for normal feeding development.
  • Zygomatic bones, lacrimal bones, nasal bones, and vomer:
    • Nasal bones form the bridge of the nose; nasal bones are easily identifiable and two in number.
    • Lacrimal bones contain the tear ducts; tears drain through a small opening into the nasal cavity (nasolacrimal canal).
    • Vomer: a small triangular bone that forms part of the nasal septum.
  • Inferior nasal conchae:
    • Cone-shaped bones in the nasal cavity; the inferior conchae look like a taco and are the two most inferior conchae.
    • They form turbinates that increase surface area for air flow, aiding in warming, humidifying, and filtering air; middle and superior conchae are parts of the ethmoid bone.
  • Orbit (eye socket) anatomy:
    • Frontal (superior), sphenoid (posterior), zygomatic (lateral), maxilla (inferior), lacrimal (medial), ethmoid (medial), and palatine bones contribute to the orbital walls.
    • A fracture of the orbit is often complex due to involvement of multiple bones.
  • Nasal structure details:
    • Nasal bones are the bridge of the nose.
    • The tiny lacrimal hole is where tears flow into the nasal cavity.
  • Hyoid bone (axial skeleton):
    • A horseshoe-shaped bone located anterior to the larynx (voice box).
    • It does not articulate with any other bone; it is connected by ligaments and muscles to the mandible and sternum/larynx.
    • Provides attachment for muscles of the tongue and throat and protects the larynx.
    • Forensic note: fractures of the hyoid bone are indicative of strangulation.

Paranasal Sinuses: Locations and Functions

  • Paranasal sinuses are air-filled spaces within bones lined by mucous membranes. They reduce skull weight, assist resonance of voice, and help warm/moisten inspired air.
  • Major sinuses:
    • Frontal sinus (frontal bone)
    • Sphenoid sinus (sphenoid bone)
    • Ethmoid sinus (ethmoid bone)
    • Maxillary sinus (maxilla)
  • Palatine bone can also contribute a sinus space, but this is not covered in depth in the current class.
  • Why sinuses matter clinically:
    • Sinus infections (sinusitis) occur when sinus mucosa becomes inflamed and drainage ducts are blocked.
    • Common pain locations by sinus type:
    • Frontal sinus: frontal headaches or pain over the forehead, especially when bending over.
    • Maxillary sinus: cheek/face pressure.
    • Ethmoid sinus: behind the eyes and between the temples.
    • Sphenoid sinus: behind the eyes, sometimes felt as deep headache.
  • Example pathophysiology:
    • Normally, mucus drains from sinuses into the nasal cavity. In infection, drainage can be blocked, mucus accumulates, and pressure/pain results.
  • The frontal sinus is located within the frontal bone; the sphenoid sinus is located within the body of the sphenoid bone; the ethmoid sinuses are within the ethmoid bone; the maxillary sinuses lie within the maxilla.

The Cervical Spine and the Joints with the Skull

  • The occipital bone articulates with the first cervical vertebra (the atlas, C1).
  • Atlas (C1) characteristics:
    • No body; has two flat articular surfaces called condyles that articulate with the occipital condyles of the skull to form a hinge-like joint that allows nodding (yes).
  • The second cervical vertebra (C2) is the axis and features the dens (odontoid process).
  • Atlas and axis articulation:
    • The joint between C1 (atlas) and C2 (axis) is a pivot joint that enables rotation (no).
  • Mnemonic to remember order: atlas comes before axis (C1 before C2). The atlas articulates with the occipital bone; the axis provides rotation via the dens.

The Vertebral Column: Segments and Numbers

  • Segments:
    • 7 cervical vertebrae (C1–C7)
    • 12 thoracic vertebrae (T1–T12)
    • 5 lumbar vertebrae (L1–L5)
    • Sacrum (fused from 5 originally, in adults)
    • Coccyx (fused from 4 originally, in adults)
  • The ribs articulate with the thoracic vertebrae and contribute to the thoracic cage.
  • The intervertebral discs sit between vertebral bodies and consist of two components:
    • Outer ring: annulus fibrosus (fibrocartilage)
    • Inner gel center: nucleus pulposus
  • Intervertebral discs provide cushioning and support as the spine bears load.
  • Anatomical drawing reminder: if viewed from the superior aspect, each intervertebral disc forms a ring of fibrocartilage with a gel nucleus pulposus in the center.

Intervertebral Discs: Structure, Function, and Pathology

  • Structure:
    • Outer ring: annulus fibrosus (fibrocartilage)
    • Center: nucleus pulposus (gel-like core)
  • Function:
    • Cushions vertebral bodies and allows movement while distributing load.
  • Daily height changes:
    • Intervertebral discs contribute to a slight decrease in height by the end of the day due to compression of the nucleus pulposus.
  • Aging:
    • With age, nucleus pulposus dehydrates, reducing disc height and contributing to overall height loss.
  • Herniated disc (herniation):
    • Degeneration or injury can cause the nucleus pulposus to bulge or rupture through the annulus fibrosus.
    • The nucleus can herniate posteriorly toward the spinal canal, potentially compressing the spinal cord, or laterally toward a spinal nerve exiting via the intervertebral foramen.
  • Clinical relevance:
    • Herniation can cause significant pain and neurological symptoms depending on the nerve roots affected.
  • Spinal nerves exit the vertebral column through the intervertebral foramina at each vertebral level.

Spinal Curvatures: Normal and Abnormal

  • Normal curves (as viewed laterally):
    • Cervical curve (secondary)
    • Thoracic curve (primary)
    • Lumbar curve (secondary)
    • Sacral curve (primary)
  • Developmental note:
    • An infant’s spine initially has a single C-shaped curve. As the child develops and holds up the head, the cervical curve forms (secondary). As the child sits and walks, the lumbar curve forms (secondary).
  • Secondary curves are those developed after birth; primary curves are the original fetal curves.
  • Abnormal curvatures:
    • Scoliosis: lateral (sideways) curvature of the spine. Severe thoracic scoliosis can impair lung expansion; scoliosis elsewhere can alter gait and weight distribution on the hip/knee/ankle.
    • Kyphosis: excessive thoracic curvature (often called a hunchback in common language). Commonly associated with osteoporosis; vertebral bodies may collapse and take a triangular shape.
    • Lordosis: exaggerated lumbar curvature (swayback).
  • Practical implications:
    • Abnormal curvatures can affect posture, balance, respiratory efficiency, and joint loading over time.

Sacrum, Coccyx, and Pelvic Joins

  • Sacrum:
    • Forms a joint with the ilium (sacroiliac joint), connecting the axial skeleton to the appendicular skeleton in the pelvic region.
  • Coccyx (tailbone):
    • Fuse from the coccygeal vertebrae in adulthood.
  • Pelvic connections:
    • The sacrum and ilium form the sacroiliac joint, a key link between the spine and pelvis.

Thoracic Cage: Ribs and Sternum

  • Sternoclavicular joint: the joint between the clavicle and the sternum (manubrium) that anchors the upper limb to the axial skeleton.
  • Ribs (12 pairs) attach to the thoracic vertebrae posteriorly and connect to the sternum anteriorly via costal cartilages.
  • Ribs classification by anterior attachment:
    • True ribs: ribs with their own costal cartilage attaching directly to the sternum (ribs 1-7).
    • False ribs: ribs whose costal cartilages connect to the cartilage of the rib above them (ribs 8-10).
    • Floating ribs: ribs that do not attach anteriorly to the sternum or to the cartilage of another rib (ribs 11-12 with no anterior attachment).
  • The ribs and sternum together form the thoracic cage, which protects thoracic organs and assists with respiration.
  • Important note: the posterior attachments involve costovertebral joints with the thoracic vertebrae.

The Hyoid Bone: Unique Features and Clinical Significance

  • Location: anterior to the larynx (voice box).
  • Unique status: no direct articulation with any other bone; connected via ligaments and muscles.
  • Functions: site for attachment of muscles associated with the tongue, pharynx, and larynx; provides a midline anchor for neck muscles.
  • Forensic relevance: a fractured hyoid bone can indicate strangulation.

Integrating the Material for Exam Preparation

  • Key exam focus points include:
    • Distinguishing axial vs. appendicular bones and recognizing which bones belong to each group.
    • Naming and locating the cranial (8) and facial (14) bones and understanding their articulations and functions (e.g., sphenoid’s central position; ethmoid’s location).
    • Understanding the sutures, TMJ (mandible–temporal joint), and the orbit’s multi-bone composition.
    • Explaining paranasal sinuses: locations (frontal, sphenoid, ethmoid, maxillary), functions (sound resonance, skull lightening, drainage), and clinical relevance of sinusitis.
    • Recognizing the relationships of the skull to the vertebral column: atlas (C1) and axis (C2); atlas articulates with the occipital bone; axis bears the dens; pivot vs hinge joint.
    • Memorizing vertebral counts: 7 cervical, 12 thoracic, 5 lumbar, plus sacrum and coccyx and their fusion timelines.
    • Intervertebral discs: composition (annulus fibrosus and nucleus pulposus), function, and consequences of degeneration/herniation (nerve compression and potential spinal cord involvement).
    • Understanding the spinal curvature: primary (thoracic and sacral) vs. secondary (cervical and lumbar); recognizing scoliosis, kyphosis, and lordosis and their clinical implications (e.g., osteoporosis-related kyphosis).
    • Identifying rib types (true, false, floating) and the sternoclavicular and sternocostal joints; understanding how the thoracic cage supports respiration.
    • Recognizing the hyoid’s special status and forensic significance.

Quick Reference: Key Terms and Concepts

  • Axial skeleton, Appendicular skeleton
  • Cranium, Facial bones, Sutures
  • Mandible, Temporomandibular joint (TMJ)
  • Orbit, Superior/Inferior/middle conchae, Lacrimal, Nasal, Vomer, Palatine, Zygomatic, Maxilla
  • Sphenoid bone, Ethmoid bone, Sella turcica (pituitary sits in this area)
  • Frontal sinus, Maxillary sinus, Ethmoid sinus, Sphenoid sinus
  • Atlas (C1), Axis (C2), Dens (Odontoid process)
  • Intervertebral disc, Annulus fibrosus, Nucleus pulposus
  • Kyphosis, Lordosis, Scoliosis
  • Sacrum, Coccyx, Sacroiliac joint
  • Ribs, True ribs, False ribs, Floating ribs, Costal cartilage
  • Sternoclavicular joint, Sternum, Rib articulations
  • Hyoid bone, Forensic significance of hyoid fracture

Formulas and numeric references (examples):

  • There are 8 cranial bones and 14 facial bones.
  • Vertebral counts: 7 cervical, 12 thoracic, 5 lumbar vertebrae.
  • True ribs: 7 pairs; False ribs: 3 pairs (ribs 8-10); Floating ribs: 2 pairs (ribs 11-12).
  • Intervertebral disc anatomy: outer annulus fibrosus + inner nucleus pulposus.
  • Secondary curves develop after birth (cervical and lumbar); primary curves present at birth (thoracic and sacral).

If you want, I can tailor these notes to a specific exam format (e.g., fill-in-the-blank, labeling diagrams, or a concise cheat sheet).