Exercise 9 Review Sheet The Axial Skeleton

The axial skeleton, a fundamental component of the human skeletal system, forms the central axis of the body and provides protection for vital organs. This leads to understanding its structure and function is crucial for anyone studying anatomy, physiology, or related fields. This review sheet will cover the major components of the axial skeleton, their key features, and clinical relevance.

Introduction to the Axial Skeleton

The axial skeleton consists of the bones of the skull, vertebral column, ribs, and sternum. Its primary functions include:

• Supporting the body's upright posture.
• Protecting delicate organs such as the brain, spinal cord, heart, and lungs.
• Providing attachment points for muscles that support movement and respiration.

The axial skeleton contrasts with the appendicular skeleton, which includes the bones of the limbs and girdles that attach them to the axial skeleton. Together, these two divisions form the complete skeletal system.

The Skull: Cranium and Facial Bones

The skull, the most complex part of the axial skeleton, protects the brain and houses sensory organs. It is composed of two main sets of bones: the cranium and the facial bones.

Cranium

The cranium, or braincase, is formed by eight bones that enclose and protect the brain. These bones are connected by immovable joints called sutures. The cranial bones include:

1. Frontal Bone: Forms the anterior part of the cranium and the forehead. Key features include:

   • Supraorbital margin: The bony ridge above the eye sockets.
   • Glabella: The smooth area between the eyebrows.
   • Frontal sinuses: Air-filled spaces within the bone.

2. Parietal Bones (2): Form the superior and lateral walls of the cranium. They articulate with each other at the sagittal suture and with the frontal bone at the coronal suture.

3. Temporal Bones (2): Form the lateral and inferior walls of the cranium. Key features include:

   • External acoustic meatus: The opening of the ear canal.
   • Mastoid process: A bony projection behind the ear, serving as an attachment site for muscles.
   • Styloid process: A slender projection below the ear, serving as an attachment site for muscles and ligaments.
   • Zygomatic process: A projection that articulates with the zygomatic bone to form part of the cheekbone.
   • Mandibular fossa: A depression that articulates with the mandible (lower jaw).

4. Occipital Bone: Forms the posterior part of the cranium and the base of the skull. Key features include:

   • Foramen magnum: A large opening through which the spinal cord passes.
   • Occipital condyles: Oval processes that articulate with the first vertebra (atlas).
   • External occipital protuberance: A bony projection on the posterior surface, serving as an attachment site for muscles.

5. Sphenoid Bone: A complex, bat-shaped bone that forms part of the base of the skull. It articulates with all other cranial bones and contributes to the orbits. Key features include:

   • Sella turcica: A saddle-shaped depression that houses the pituitary gland.
   • Greater and lesser wings: Lateral projections that contribute to the orbits and cranial floor.
   • Pterygoid processes: Inferior projections that serve as attachment sites for muscles of mastication.
   • Sphenoid sinuses: Air-filled spaces within the bone.

6. Ethmoid Bone: An irregularly shaped bone located between the orbits. It contributes to the nasal cavity and orbits. Key features include:

   • Cribriform plate: A horizontal plate with numerous perforations (olfactory foramina) for the passage of olfactory nerves.
   • Crista galli: A vertical projection that serves as an attachment site for the falx cerebri, a membrane that separates the cerebral hemispheres.
   • Perpendicular plate: A vertical plate that forms part of the nasal septum.
   • Ethmoid sinuses: Air-filled spaces within the bone (also called ethmoid air cells).
   • Superior and middle nasal conchae: Thin, scroll-shaped plates that project into the nasal cavity, increasing its surface area.

Facial Bones

The facial bones form the framework of the face, provide attachment points for facial muscles, and house the teeth. The facial bones include:

1. Mandible: The lower jawbone, the only movable bone of the skull. Key features include:

   • Body: The horizontal portion that forms the chin.
   • Ramus: The vertical portion that articulates with the temporal bone.
   • Condylar process: Articulates with the mandibular fossa of the temporal bone.
   • Coronoid process: An anterior projection that serves as an attachment site for the temporalis muscle (a muscle of mastication).
   • Alveolar processes: Sockets for the teeth.
   • Mental foramen: An opening on the anterior surface for the passage of nerves and blood vessels.

2. Maxillae (2): The upper jawbones, which form part of the hard palate, nasal cavity, and orbits. Key features include:

   • Alveolar processes: Sockets for the teeth.
   • Palatine process: Forms the anterior part of the hard palate.
   • Maxillary sinuses: Air-filled spaces within the bone.
   • Infraorbital foramen: An opening below the orbit for the passage of nerves and blood vessels.

3. Zygomatic Bones (2): The cheekbones, which form part of the lateral walls of the orbits. They articulate with the zygomatic processes of the temporal bones and the maxillae.

4. Nasal Bones (2): Form the bridge of the nose.

5. Lacrimal Bones (2): Small bones located in the medial walls of the orbits. They contain the lacrimal fossa, which houses the lacrimal sac (part of the tear drainage system).

6. Palatine Bones (2): Form the posterior part of the hard palate and contribute to the nasal cavity and orbits.

7. Inferior Nasal Conchae (2): Thin, scroll-shaped bones that project into the nasal cavity from the lateral walls. They increase the surface area of the nasal cavity.

8. Vomer: A single bone that forms the inferior part of the nasal septum.

Sutures of the Skull

Sutures are immovable joints that connect the bones of the cranium. The major sutures of the skull include:

• Coronal Suture: Connects the frontal bone to the parietal bones.
• Sagittal Suture: Connects the two parietal bones along the midline of the skull.
• Lambdoid Suture: Connects the parietal bones to the occipital bone.
• Squamous Suture: Connects the temporal bones to the parietal bones.

The Vertebral Column

The vertebral column, or spine, is a flexible, curved structure that supports the head, neck, and trunk. It consists of 33 vertebrae, although some fuse during development. The vertebral column is divided into five regions:

1. Cervical Vertebrae (7): Located in the neck region. They are the smallest and most mobile vertebrae. Key features include:

   • Transverse foramina: Openings in the transverse processes for the passage of vertebral arteries and veins.
   • Bifid spinous processes: Spinous processes that are split at the tip (except for C7).
   • Atlas (C1): The first cervical vertebra, which articulates with the occipital condyles of the skull. It lacks a body and spinous process.
   • Axis (C2): The second cervical vertebra, which has a prominent dens (odontoid process) that articulates with the atlas, allowing for rotation of the head.

2. Thoracic Vertebrae (12): Located in the chest region. They articulate with the ribs. Key features include:

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   • Costal facets: Articular surfaces for the ribs on the vertebral bodies and transverse processes.
   • Long, downward-pointing spinous processes.

3. Lumbar Vertebrae (5): Located in the lower back region. They are the largest and strongest vertebrae. Key features include:

   • Large, kidney-shaped bodies.
   • Short, thick spinous processes.

4. Sacrum: A triangular bone formed by the fusion of five sacral vertebrae. It articulates with the hip bones to form the pelvis.

5. Coccyx: The tailbone, formed by the fusion of four coccygeal vertebrae.

General Structure of a Vertebra

A typical vertebra consists of the following components:

• Body: The weight-bearing, cylindrical portion of the vertebra.
• Vertebral Arch: Formed by the pedicles and laminae, which enclose the vertebral foramen.
• Vertebral Foramen: The opening through which the spinal cord passes.
• Spinous Process: A posterior projection that serves as an attachment site for muscles and ligaments.
• Transverse Processes: Lateral projections that serve as attachment sites for muscles and ligaments.
• Superior and Inferior Articular Processes: Projections with articular facets that articulate with adjacent vertebrae.
• Intervertebral Foramina: Openings formed between adjacent vertebrae through which spinal nerves exit the spinal cord.

Intervertebral Discs

Intervertebral discs are fibrocartilaginous pads located between the vertebral bodies. They act as shock absorbers and allow for flexibility of the vertebral column. Each disc consists of:

• Anulus fibrosus: A tough, outer ring of fibrocartilage.
• Nucleus pulposus: A gelatinous, inner core.

The Thoracic Cage

The thoracic cage protects the heart, lungs, and other organs in the chest. It is formed by the thoracic vertebrae, ribs, and sternum.

Ribs

There are 12 pairs of ribs, which articulate with the thoracic vertebrae posteriorly. The ribs are classified as:

• 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 the 7th rib (ribs 8-10) or do not attach to the sternum at all (floating ribs 11-12).
• Floating Ribs (11-12): Do not attach to the sternum.

A typical rib consists of:

• Head: Articulates with the vertebral body.
• Neck: Connects the head to the body.
• Tubercle: Articulates with the transverse process of the vertebra.
• Body (shaft): The main portion of the rib.

Sternum

The sternum, or breastbone, is a flat bone located in the midline of the anterior chest wall. It consists of three parts:

• Manubrium: The superior portion, which articulates with the clavicles and the first pair of ribs.
• Body: The middle portion, which articulates with ribs 2-7.
• Xiphoid Process: The inferior portion, which is a small, cartilaginous projection that ossifies during adulthood.

Clinical Relevance

Understanding the anatomy of the axial skeleton is essential for diagnosing and treating a variety of clinical conditions. Some examples include:

• Fractures: Skull fractures can result from trauma and may lead to brain injury. Vertebral fractures can cause spinal cord damage and neurological deficits. Rib fractures can result from chest trauma and may lead to lung injury.
• Herniated Discs: Occur when the nucleus pulposus of an intervertebral disc protrudes through the anulus fibrosus, compressing spinal nerves and causing pain, numbness, or weakness.
• Scoliosis: An abnormal lateral curvature of the vertebral column.
• Kyphosis: An exaggerated thoracic curvature of the vertebral column (hunchback).
• Lordosis: An exaggerated lumbar curvature of the vertebral column (swayback).
• Osteoporosis: A condition characterized by decreased bone density, which increases the risk of fractures, particularly in the vertebrae.
• Arthritis: Inflammation of the joints, which can affect the vertebrae and ribs, causing pain and stiffness.
• Sinusitis: Inflammation of the sinuses, which can be caused by infection or allergies.

Frequently Asked Questions (FAQ)

1. How many bones are in the axial skeleton?
   • The axial skeleton typically consists of 80 bones: 22 in the skull (8 cranial and 14 facial), 26 in the vertebral column (including the sacrum and coccyx), and 25 in the thoracic cage (1 sternum and 24 ribs).
2. What is the function of the foramen magnum?
   • The foramen magnum is a large opening in the occipital bone through which the spinal cord passes to connect with the brain.
3. What is the purpose of the intervertebral discs?
   • Intervertebral discs act as shock absorbers between the vertebrae and allow for flexibility of the vertebral column.
4. What are the differences between true ribs, false ribs, and floating ribs?
   • 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 the 7th rib (ribs 8-10) or do not attach to the sternum at all (floating ribs 11-12). Floating ribs (11-12) do not attach to the sternum.
5. What is the sella turcica?
   • The sella turcica is a saddle-shaped depression in the sphenoid bone that houses the pituitary gland.
6. What are the nasal conchae?
   • The nasal conchae are thin, scroll-shaped bones that project into the nasal cavity from the lateral walls. They increase the surface area of the nasal cavity, which helps to warm and humidify the air before it enters the lungs. The ethmoid bone contributes to the superior and middle nasal conchae, while the inferior nasal conchae are separate bones.
7. What is the hyoid bone, and why isn't it part of the axial skeleton?
   • The hyoid bone is a small, U-shaped bone located in the neck, below the mandible. It does not articulate with any other bone and is suspended by ligaments and muscles. It supports the tongue and provides attachment points for muscles involved in swallowing and speech. Although located in the axial region of the body, it's not considered part of the axial skeleton because it doesn't directly articulate with the skull or vertebral column.
8. What is the difference between the axial and appendicular skeleton?
   • The axial skeleton forms the central axis of the body and includes the bones of the skull, vertebral column, ribs, and sternum. The appendicular skeleton includes the bones of the limbs and girdles that attach them to the axial skeleton.
9. What is the clinical significance of the fontanelles in infants?
   • Fontanelles are soft spots in the skull of infants where the cranial bones have not yet fully fused. They allow for growth of the brain and skull during infancy and enable passage through the birth canal during delivery. The fontanelles typically close by 18-24 months of age.
10. What are sinuses, and what is their function?
    • Sinuses are air-filled spaces located within certain bones of the skull, including the frontal, ethmoid, sphenoid, and maxillary bones. They are lined with mucous membrane and connect to the nasal cavity. The functions of the sinuses include:
      • Reducing the weight of the skull.
      • Providing resonance for the voice.
      • Warming and humidifying the air before it enters the lungs.
      • Acting as shock absorbers in the event of facial trauma.

Conclusion

The axial skeleton is a complex and vital part of the human skeletal system. Its bones provide support, protection, and attachment points for muscles, enabling a wide range of movements and functions. That said, a thorough understanding of the axial skeleton is essential for healthcare professionals and anyone interested in human anatomy and physiology. This review sheet provides a comprehensive overview of the axial skeleton, covering its major components, key features, and clinical relevance. By studying the axial skeleton, you gain valuable insight into the structure and function of the human body.