Exercise 9 The Axial Skeleton Review Sheet

The axial skeleton, the central pillar of our body, provides the scaffolding that supports our frame, protects vital organs, and enables us to maintain an upright posture. Understanding its intricacies is crucial for anyone studying anatomy, physiology, or related fields. This comprehensive review sheet will guide you through the components of the axial skeleton, their functions, and key anatomical features.

Unveiling the Axial Skeleton: An Introduction

The axial skeleton forms the longitudinal axis of the body, extending from the skull to the coccyx. It consists of the skull, vertebral column, and thoracic cage. Unlike the appendicular skeleton, which includes the limbs and girdles, the axial skeleton is primarily responsible for protection, support, and maintaining the body's central structure.

• Components: Skull, vertebral column, and thoracic cage
• Functions: Protection of vital organs, support of body weight, and maintenance of posture
• Importance: Understanding the axial skeleton is fundamental to grasping human anatomy and physiology.

The Skull: A Bony Fortress

The skull, the most complex part of the axial skeleton, protects the brain and sensory organs. It's composed of 22 bones, divided into the cranial and facial bones.

Cranial Bones: Enclosing the Brain

The cranial bones form the cranium, the protective vault surrounding the brain. These bones are joined by sutures, immovable joints that fuse during development.

1. Frontal Bone: Forms the forehead and the superior part of the orbit.
   • Key features: Supraorbital foramen (or notch), frontal sinuses
2. Parietal Bones (2): Form the superior and lateral walls of the cranium.
   • Key features: Sagittal suture (joins the two parietal bones), coronal suture (joins the parietal bones to the frontal bone)
3. Temporal Bones (2): Form the inferior lateral aspects of the skull and part of the cranial base.
   • Key features: External acoustic meatus, mastoid process, styloid process, zygomatic process, mandibular fossa
4. Occipital Bone: Forms the posterior aspect and most of the base of the cranium.
   • Key features: Foramen magnum, occipital condyles, external occipital protuberance
5. Sphenoid Bone: A complex, bat-shaped bone that articulates with all other cranial bones.
   • Key features: Sella turcica (houses the pituitary gland), greater wings, lesser wings, pterygoid processes, optic canal
6. Ethmoid Bone: Lies between the sphenoid and nasal bones; forms part of the nasal septum and the medial walls of the orbits.
   • Key features: Cribriform plate, crista galli, perpendicular plate, superior and middle nasal conchae

Facial Bones: Shaping the Face

The facial bones form the framework of the face, provide attachments for facial muscles, and house the teeth.

1. Mandible: The lower jawbone; the only movable bone of the skull.
   • Key features: Body, ramus, angle, coronoid process, mandibular condyle, mental foramen, mandibular foramen
2. Maxillae (2): Form the upper jaw and central part of the facial skeleton.
   • Key features: Alveolar processes, palatine processes, infraorbital foramen, maxillary sinuses
3. Zygomatic Bones (2): Form the cheekbones.
   • Key features: Temporal process
4. Nasal Bones (2): Form the bridge of the nose.
5. Lacrimal Bones (2): Located in the medial orbital walls.
   • Key features: Lacrimal fossa
6. Palatine Bones (2): Form the posterior part of the hard palate.
   • Key features: Horizontal plate
7. Inferior Nasal Conchae (2): Thin, curved bones in the nasal cavity.
8. Vomer: Forms the inferior part of the nasal septum.

Hyoid Bone: The Lone Wolf

Although not technically part of the skull, the hyoid bone is often discussed in conjunction with it due to its proximity and functional relationship. It's a U-shaped bone located in the neck, inferior to the mandible. It doesn't articulate with any other bone and is suspended by ligaments from the styloid processes of the temporal bones. The hyoid bone serves as an attachment point for muscles involved in swallowing and speech.

The Vertebral Column: The Body's Backbone

The vertebral column, or spine, is a flexible, curved structure that supports the head and trunk, protects the spinal cord, and provides attachment points for ribs and back muscles. It's composed of 26 irregular bones called vertebrae, separated by intervertebral discs.

Regions of the Vertebral Column

The vertebral column is divided into five regions:

1. Cervical Vertebrae (7): Located in the neck; the smallest and most mobile vertebrae.
   • Key features: Transverse foramina (for vertebral arteries), bifid spinous processes (C2-C6), atlas (C1), axis (C2)
2. Thoracic Vertebrae (12): Located in the upper back; articulate with the ribs.
   • Key features: Costal facets (for rib articulation), long, downward-pointing spinous processes
3. Lumbar Vertebrae (5): Located in the lower back; the largest and strongest vertebrae.
   • Key features: Thick, block-like bodies, short, hatchet-shaped spinous processes
4. Sacrum: A triangular bone formed by the fusion of five sacral vertebrae.
   • Key features: Sacral promontory, alae, sacral foramina, median sacral crest, sacral hiatus
5. Coccyx: The tailbone; a small, triangular bone formed by the fusion of three to five coccygeal vertebrae.

General Structure of a Vertebra

While vertebrae vary in size and shape depending on their location, they share a common structural plan:

• Body (Centrum): The weight-bearing region of the vertebra.
• Vertebral Arch: Formed by the laminae and pedicles; encloses the vertebral foramen.
• Vertebral Foramen: The opening through which the spinal cord passes.
• Spinous Process: A posterior projection from the vertebral arch; serves as an attachment point for muscles and ligaments.
• Transverse Processes: Lateral projections from the vertebral arch; serve as attachment points for muscles and ligaments.
• Superior and Inferior Articular Processes: Paired projections that articulate with adjacent vertebrae.
• Intervertebral Foramina: Openings formed by the notches on the superior and inferior borders of adjacent pedicles; allow passage of spinal nerves.

Special Vertebrae

• Atlas (C1): The first cervical vertebra; articulates with the occipital condyles of the skull, allowing for nodding movements. It lacks a body and a spinous process.
• Axis (C2): The second cervical vertebra; has a prominent superior projection called the dens (odontoid process), which articulates with the atlas, allowing for rotational movements.

Curvatures of the Vertebral Column

The vertebral column exhibits four normal curvatures: cervical, thoracic, lumbar, and sacral. These curvatures increase the spine's resilience and flexibility, allowing it to absorb shock and distribute weight more effectively. The cervical and lumbar curvatures are concave posteriorly (lordosis), while the thoracic and sacral curvatures are convex posteriorly (kyphosis).

The Thoracic Cage: Protecting the Chest

The thoracic cage, or rib cage, protects the vital organs of the thorax, including the heart, lungs, and major blood vessels. It's composed of the sternum, ribs, and thoracic vertebrae.

Sternum: The Breastbone

The sternum is a flat bone located in the anterior midline of the thorax. It consists of three parts:

1. Manubrium: The superior part of the sternum; articulates with the clavicles and the first pair of ribs.
   • Key features: Jugular notch, clavicular notches
2. Body: The middle part of the sternum; articulates with ribs 2-7.
3. Xiphoid Process: The inferior part of the sternum; a small, cartilaginous projection that ossifies during adulthood.

Ribs: Bony Arches

Twelve pairs of ribs form the lateral walls of the thoracic cage. They articulate posteriorly with the thoracic vertebrae and curve anteriorly towards the sternum.

• True Ribs (1-7): Attach directly to the sternum via their own costal cartilages.
• False Ribs (8-12): Attach indirectly to the sternum via the costal cartilage of rib 7 or not at all.
• Floating Ribs (11-12): Do not attach to the sternum.

Rib Structure

A typical rib consists of the following:

• Head: Articulates with the vertebral body.
• Neck: Connects the head to the tubercle.
• Tubercle: Articulates with the transverse process of the vertebra.
• Body (Shaft): The main part of the rib.
• Costal Groove: Located on the inferior border of the rib; provides a passage for intercostal nerves and vessels.

Common Injuries and Conditions Affecting the Axial Skeleton

The axial skeleton, due to its crucial role in support and protection, is susceptible to various injuries and conditions.

• Fractures: Skull fractures, vertebral fractures (compression fractures, burst fractures), and rib fractures are common, often resulting from trauma.
• Herniated Discs: Occur when the nucleus pulposus of an intervertebral disc protrudes through the annulus fibrosus, compressing spinal nerves.
• Scoliosis: An abnormal lateral curvature of the spine.
• Kyphosis: An exaggerated thoracic curvature (hunchback).
• Lordosis: An exaggerated lumbar curvature (swayback).
• Osteoporosis: A condition characterized by decreased bone density, making bones more susceptible to fractures.
• Arthritis: Inflammation of the joints, affecting the vertebral column and potentially causing pain and stiffness.
• Spinal Stenosis: Narrowing of the spinal canal, compressing the spinal cord and nerves.
• Whiplash: A neck injury caused by a sudden, forceful back-and-forth movement of the head.

Clinical Significance and Diagnostic Imaging

Understanding the anatomy of the axial skeleton is crucial for diagnosing and treating various conditions. Medical imaging techniques such as X-rays, CT scans, and MRI are commonly used to visualize the bones and soft tissues of the axial skeleton.

• X-rays: Useful for detecting fractures and dislocations.
• CT Scans: Provide detailed cross-sectional images of the bones, useful for assessing complex fractures and spinal stenosis.
• MRI: Excellent for visualizing soft tissues, including the spinal cord, intervertebral discs, and ligaments.

Axial Skeleton: Frequently Asked Questions

• What is the function of the axial skeleton?

  The axial skeleton provides support, protects vital organs, and allows for movement. It forms the central axis of the body and includes the skull, vertebral column, and thoracic cage.

• How many bones are in the axial skeleton?

  The adult axial skeleton typically consists of 80 bones: 22 in the skull, 26 in the vertebral column (including the sacrum and coccyx), and 25 in the thoracic cage (including the sternum).

• What are the cranial bones?

  The cranial bones are the frontal, parietal (2), temporal (2), occipital, sphenoid, and ethmoid bones.

• What are the facial bones?

  The facial bones are the mandible, maxillae (2), zygomatic bones (2), nasal bones (2), lacrimal bones (2), palatine bones (2), inferior nasal conchae (2), and vomer.

• What are the regions of the vertebral column?

  The vertebral column is divided into five regions: cervical (7 vertebrae), thoracic (12 vertebrae), lumbar (5 vertebrae), sacrum (5 fused vertebrae), and coccyx (3-5 fused vertebrae).

• What is the difference between true ribs and false ribs?

  True ribs (1-7) attach directly to the sternum via their own costal cartilages, while false ribs (8-12) attach indirectly to the sternum via the costal cartilage of rib 7 or not at all. Floating ribs (11-12) do not attach to the sternum.

• What is the function of the intervertebral discs?

  Intervertebral discs act as shock absorbers between the vertebrae, allowing for movement and flexibility of the spine.

• What is scoliosis?

  Scoliosis is an abnormal lateral curvature of the spine.

• What is the significance of the foramen magnum?

  The foramen magnum is a large opening in the occipital bone through which the spinal cord passes.

• What is the sella turcica?

  The sella turcica is a saddle-shaped depression in the sphenoid bone that houses the pituitary gland.

Conclusion: The Importance of Understanding the Axial Skeleton

The axial skeleton is a fundamental component of human anatomy, providing the framework for our bodies and protecting our vital organs. A thorough understanding of its structure, function, and common conditions is essential for healthcare professionals, students of anatomy and physiology, and anyone interested in learning more about the human body. By studying the components of the axial skeleton – the skull, vertebral column, and thoracic cage – we gain a deeper appreciation for the intricate design and remarkable resilience of the human form.