💀Anatomy and Physiology I Unit 7 – Axial Skeleton

Overview and Function

• The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage
• Provides support and protection for vital organs such as the brain, spinal cord, heart, and lungs
• Serves as an attachment point for muscles, allowing for movement and stability
• Plays a crucial role in maintaining posture and balance
• Facilitates breathing by providing a framework for the respiratory muscles
• Houses and protects sensory organs, including the eyes, ears, and nose
• Contributes to the production of blood cells through the bone marrow found in certain axial bones (sternum, ribs, and vertebrae)

Major Components

• The axial skeleton consists of 80 bones, which are divided into three main regions: the skull, vertebral column, and thoracic cage
• The skull is composed of 22 bones, including the cranium and the facial bones
  • The cranium houses and protects the brain
  • The facial bones provide structure and support for the eyes, nose, and mouth
• The vertebral column, also known as the spine, consists of 26 bones called vertebrae
  • The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal
  • The vertebral column supports the head, neck, and trunk, and protects the spinal cord
• The thoracic cage is made up of 12 pairs of ribs, the sternum, and the thoracic vertebrae
  • The ribs attach to the thoracic vertebrae posteriorly and to the sternum anteriorly, forming a protective cage around the heart and lungs
  • The sternum, also known as the breastbone, is a flat bone located in the center of the chest

Skull Structure

• The skull is divided into two main parts: the neurocranium (cranial vault) and the viscerocranium (facial skeleton)
• The neurocranium is composed of 8 bones that form a protective cavity for the brain
  • The frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones make up the neurocranium
  • The frontal bone forms the forehead and the roof of the orbits (eye sockets)
  • The parietal bones form the sides and roof of the cranium
  • The temporal bones house the structures of the middle and inner ear
  • The occipital bone forms the base and back of the cranium and has a large opening (foramen magnum) for the spinal cord
• The viscerocranium is composed of 14 bones that form the facial skeleton
  • The maxillae, zygomatic, nasal, lacrimal, palatine, inferior nasal conchae, and vomer bones make up the viscerocranium
  • The maxillae form the upper jaw and support the upper teeth
  • The mandible, the only movable bone of the skull, forms the lower jaw and supports the lower teeth

Vertebral Column

• The vertebral column is composed of 33 vertebrae, which are divided into five regions: cervical (7), thoracic (12), lumbar (5), sacral (5 fused), and coccygeal (4 fused)
• Each vertebra consists of a body, a vertebral arch, and various processes for muscle attachment and articulation
  • The body is the weight-bearing portion of the vertebra
  • The vertebral arch surrounds and protects the spinal cord
  • The processes, such as the spinous process and transverse processes, serve as attachment points for muscles and ligaments
• The cervical vertebrae support the head and allow for a wide range of motion
  • The first cervical vertebra (atlas) articulates with the occipital bone of the skull
  • The second cervical vertebra (axis) allows for rotation of the head
• The thoracic vertebrae articulate with the ribs and provide attachment points for the muscles of the back and chest
• The lumbar vertebrae are the largest and most weight-bearing vertebrae, supporting the weight of the upper body
• The sacrum is a triangular-shaped bone formed by the fusion of five sacral vertebrae, and it articulates with the hip bones to form the pelvis
• The coccyx, or tailbone, is a small bone formed by the fusion of four coccygeal vertebrae and serves as an attachment point for ligaments and muscles of the pelvic floor

Thoracic Cage

• The thoracic cage, also known as the ribcage, is formed by 12 pairs of ribs, the sternum, and the thoracic vertebrae
• The ribs are curved, flat bones that attach posteriorly to the thoracic vertebrae and anteriorly to the sternum or to each other
  • The first seven pairs of ribs (true ribs) attach directly to the sternum via costal cartilage
  • The 8th, 9th, and 10th pairs of ribs (false ribs) attach indirectly to the sternum via the costal cartilage of the rib above
  • The 11th and 12th pairs of ribs (floating ribs) do not attach to the sternum anteriorly
• The sternum is a flat bone located in the center of the chest and consists of three parts: the manubrium, body, and xiphoid process
  • The manubrium is the uppermost portion of the sternum and articulates with the clavicles (collarbones) and the first pair of ribs
  • The body is the longest portion of the sternum and provides attachment points for the 2nd through 7th pairs of ribs
  • The xiphoid process is the smallest and most inferior portion of the sternum and serves as an attachment point for some abdominal muscles
• The thoracic cage protects vital organs, such as the heart and lungs, and plays a crucial role in respiration by providing attachment points for the respiratory muscles (intercostal muscles and diaphragm)

Bone Composition and Growth

• Bones are composed of both organic and inorganic materials, which contribute to their strength and flexibility
  • The organic component consists primarily of collagen fibers, which provide flexibility and tensile strength
  • The inorganic component is made up of hydroxyapatite crystals, which are composed of calcium and phosphate ions and provide compressive strength and rigidity
• Bone growth occurs through two main processes: intramembranous ossification and endochondral ossification
  • Intramembranous ossification involves the direct formation of bone from mesenchymal tissue and occurs primarily in the flat bones of the skull and the clavicles
  • Endochondral ossification involves the replacement of a cartilage model with bone tissue and occurs in the long bones, vertebrae, and ribs
• Bones undergo continuous remodeling throughout life, with old bone tissue being resorbed by osteoclasts and new bone tissue being formed by osteoblasts
  • This process allows bones to adapt to mechanical stress, repair micro-damage, and maintain calcium homeostasis
• The rate of bone growth and remodeling is influenced by various factors, including hormones (e.g., growth hormone, thyroid hormone, and sex hormones), nutrition (e.g., calcium and vitamin D intake), and mechanical stress (e.g., weight-bearing exercise)

Clinical Applications

• Fractures of the axial skeleton can have serious consequences due to the proximity of vital organs and structures
  • Skull fractures can lead to brain injury, intracranial hemorrhage, and cerebrospinal fluid leaks
  • Vertebral fractures can cause spinal cord compression, leading to neurological deficits or paralysis
  • Rib fractures can result in pneumothorax (collapsed lung) or hemothorax (blood in the pleural cavity)
• Osteoporosis is a common condition characterized by decreased bone mass and increased risk of fractures, particularly in the vertebrae, hip, and wrist
  • Risk factors for osteoporosis include advanced age, female sex, low body weight, smoking, excessive alcohol consumption, and prolonged use of certain medications (e.g., glucocorticoids)
  • Prevention and treatment strategies for osteoporosis include adequate calcium and vitamin D intake, weight-bearing exercise, and medications such as bisphosphonates and denosumab
• Scoliosis is an abnormal lateral curvature of the spine that can cause pain, respiratory difficulties, and cosmetic concerns
  • Scoliosis can be congenital, neuromuscular, or idiopathic (unknown cause)
  • Treatment options for scoliosis depend on the severity of the curvature and may include observation, bracing, or surgical correction (spinal fusion)
• Temporomandibular joint (TMJ) disorders can cause pain, clicking, and limited movement of the jaw
  • TMJ disorders can be caused by factors such as teeth grinding (bruxism), jaw clenching, arthritis, or trauma
  • Treatment options for TMJ disorders may include jaw exercises, pain relief medications, bite guards, or surgery in severe cases

Key Terms and Concepts

• Axial skeleton: The central portion of the skeleton, consisting of the skull, vertebral column, and thoracic cage
• Appendicular skeleton: The peripheral portion of the skeleton, consisting of the bones of the upper and lower limbs and their associated girdles (pectoral and pelvic girdles)
• Sutures: Fibrous joints between the bones of the skull that allow for growth during development and provide stability in adulthood
• Intervertebral discs: Fibrocartilaginous structures located between the vertebral bodies that act as shock absorbers and allow for flexibility of the spine
• Foramen magnum: The large opening at the base of the occipital bone through which the spinal cord passes
• Costal cartilage: The cartilage that connects the ribs to the sternum anteriorly
• Osteoblasts: Cells responsible for the formation of new bone tissue
• Osteoclasts: Cells responsible for the resorption (breakdown) of old bone tissue
• Wolff's law: The principle that bones adapt to the mechanical stresses placed upon them, becoming stronger in response to increased stress and weaker in response to decreased stress
• Kyphosis: An exaggerated forward curvature of the thoracic spine, commonly known as "hunchback"
• Lordosis: An exaggerated inward curvature of the lumbar spine, commonly known as "swayback"