Chapter 5: The Skeletal System

Main Types of Bone Tissue

The skeletal system is composed of specialized cells and matrix that provide structure, protection, and support for the body. The main types of bone tissue cells are:

• Osteoblasts: "Bone builders." These immature cells secrete the bone matrix, initiating bone formation.

• Osteocytes: "Bone maintainers." Mature bone cells that maintain bone tissue and regulate calcium levels.

Bone Matrix Composition

The bone matrix consists of organic and inorganic components that contribute to bone strength and resilience.

• Organic Component: Collagen fibers provide tensile strength, allowing bones to resist stretching and twisting.

• Inorganic Component: Hydroxyapatite crystals (calcium phosphate) provide hardness and resistance to compression.

Main Functions of Bone

• Support & Protection: Forms the framework of the body, protecting vital organs.

• Movement: Serves as an anchor for muscles, enabling body movement.

• Blood Formation: Red bone marrow produces red blood cells, white blood cells, and platelets.

• Mineral Storage: Stores essential minerals such as calcium and phosphate, regulating mineral balance.

Bone Structure

The functional unit of compact bone is the Osteon (Haversian system). Each osteon contains a central canal surrounded by concentric layers of bone matrix.

• Central (Haversian) Canal: The center of an osteon, containing blood vessels and nerves.

Three Types of Lamellae (Layers of Bone Matrix)

• Concentric Lamellae: Layers that encircle the osteon canal.

• Circumferential Lamellae: Layers that encircle the entire bone shaft.

• Interstitial Lamellae: Layers that fill spaces between osteons.

Two Types of Bone Tissue

• Compact Bone: Dense, solid outer layer. Arranged in osteons. Resists compression along the long axis.

• Spongy Bone (trabecular/cancellous): Porous tissue found in bone ends. Made of trabeculae that distribute stress and make the bone lightweight. Contains red marrow.

Anatomy of a Long Bone

Long bones have distinct regions that contribute to their growth and function.

• Diaphysis: The shaft; primarily compact bone. Contains the medullary cavity for marrow.

• Epiphyses: The ends of the bone; primarily spongy bone containing red marrow. Covered by articular cartilage at joints.

• Metaphysis: The narrow growth zone between diaphysis and epiphysis. Contains the epiphyseal plate (growth plate) in growing individuals, which becomes the epiphyseal line when growth stops.

• Periosteum: A membrane covering the bone’s outer surface, protecting it and providing attachment points.

Two Types of Ossification

• Intramembranous: Bone forms directly within connective tissue (e.g., skull bones).

• Endochondral: Bone replaces a hyaline cartilage model (e.g., most bones of the limbs).

Wolff’s Law

Bones adapt to stress. Use strengthens bones; inactivity weakens them.

Fractures

A break in a bone. The healing process typically takes 1-2 weeks. Fractures are classified by shape and location:

• Open (compound)

• Closed (simple)

• Transverse

• Spiral

• Displaced

• Nondisplaced

• Greenstick

Factors Regulating Bone Growth

• Hormones:

  • Growth Hormone: Elongation

  • Parathyroid Hormone (PTH): Stimulates bone breakdown

  • Calcitonin: Stimulates bone building

  • Sex steroids: Growth spurts

• Nutrition:

  • Essential minerals (calcium, phosphate)

  • Vitamins A, C, and D: Vital for bone health

Aging and Bones

With age, bone remodeling becomes imbalanced. Osteoblast activity decreases while osteoclast activity remains constant, leading to bone loss. This can result in Osteoporosis, especially in women after menopause due to reduced estrogen.

Bone Classification

Bones are classified by shape:

• Sutural bones

• Irregular bones

• Short bones

• Pneumatized bones

• Flat bones

• Long bones

Bone Markings

Bone markings are projections, depressions, and openings that serve as attachment sites or passageways.

• Projections and Processes: Bumps or extensions for muscle attachment.

  • Tubercle: Small, rounded projection

  • Tuberosity: Larger, rough projection

  • Trochanter: Very large, blunt projection (found only on femur)

• Fossa: Broad, shallow depression

• Sulcus: Narrow groove

• Foramen: Round or oval hole

• Fissure: Narrow, slit-like opening

• Meatus: Canal or passageway

Chapter 6: Axial Division of the Skeleton

Axial Skeleton

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, hyoid bone, rib cage, and auditory ossicles.

• Components: Skull, vertebral column (including sacrum and coccyx), hyoid bone, rib cage, auditory ossicles

• Functions: Provides support, protects vital organs, and serves as a site for muscle attachment

Sutures

Sutures are immovable joints of the skull.

• Coronal Suture: Separates the frontal bone from the two parietal bones

• Sagittal Suture: Separates the two parietal bones

• Lambdoid Suture: Separates the occipital bone from the two parietal bones

• Squamous Suture: Separates the temporal bone from the parietal bone

The Skull & Associated Bones

The skull is composed of the cranium (enclosing the brain) and facial bones (from the face), as well as associated bones.

Major Cranial Bones

• Frontal Bone: Forehead and upper part of the eye socket

  • Key Feature: Supra-orbital foramen/notch, ethmoid notch

• Parietal (2): Sides and roof of the skull, joined by sagittal suture

• Temporal (2): Lower sides of the skull

  • Key Features: External & internal acoustic meatus, mastoid process, styloid process, zygomatic process, squamous process, petrous portion

• Occipital: Back and base of the skull

  • Key Features: Foramen magnum, occipital condyles (articulates with C1), hypoglossal canal, external occipital protuberance

• Sphenoid: Butterfly-shaped "keystone" bone at the skull base

  • Key Features: Sella turcica, optic canal, superior orbital fissure, foramen rotundum, foramen ovale

• Ethmoid: Located inside the skull, between the nasal cavity and orbits

  • Key Features: Cribriform plate, crista galli, perpendicular plate

Major Facial Bones

• Maxillae (2): Upper jaw, forms part of the nose and orbit

  • Key Features: Infra-orbital foramen, alveolar processes, part of the palate

• Palatine (2): Posterior part of the hard palate, roof of the mouth

• Inferior Nasal Conchae (2): Lateral walls of the nasal cavity; filters, directs, and warms airflow

• Zygomatic Arch: Cheekbone prominence

  • Key Features: Temporal process, zygomatic process

• Lacrimal (2): Medial walls of the orbit; smallest facial bones

  • Key Features: Lacrimal foramen

• Vomer: Inferior nasal septum

• Mandible: Lower jaw

  • Key Features: Mental foramen, alveolar processes, mandibular condyle, coronoid process, mandibular notch, ramus, body

Associated Bones

• Hyoid Bone: U-shaped bone that does not articulate with other bones; supports tongue and larynx

• Auditory Ossicles (6 bones): Three tiny bones in each ear (malleus, incus, stapes) essential for hearing

Components of the Orbital Complex

The orbital complex is a bony cavity in the skull that holds and protects the eyeball. It is formed by the articulation of several bones:

• Frontal bone (roof)

• Zygomatic bone (lateral wall)

• Maxilla (floor)

• Palatine bone (floor)

• Sphenoid bone (posterior wall)

• Lacrimal bone (medial wall)

Passage of Cranial Nerves

Cranial nerves pass through specific foramina in the skull:

• CN I: Cribriform Plate (smell)

• CN II: Optic canal (optic nerve)

• CN III, IV, VI, V1: Superior orbital fissure (ophthalmic trigeminal)

• CN V2: Foramen rotundum (maxillary branch of trigeminal)

• CN V3: Foramen ovale (mandibular branch of trigeminal)

• CN XII: Hypoglossal canal (hypoglossal nerve)

The Vertebral Column (Spine)

Structure

The vertebral column consists of 26 bones:

• Cervical vertebrae: 7

• Thoracic vertebrae: 12

• Lumbar vertebrae: 5

• Sacrum

• Coccyx

Functions

• Protects the spinal cord

• Supports skull and trunk weight

• Transfers weight during standing/walking

• Helps maintain upright posture

Summary Table: Major Bone Markings

Key Equations

• Nernst Equation (for ion equilibrium potential): Additional info: This equation is relevant for understanding ion movement across bone cell membranes.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and context for bone markings, cranial nerves, and vertebral column functions.