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Essentials of Human Anatomy & Physiology - Chapter 5: The Skeletal System

• The skeletal system is comprised of bones, joints, cartilages, and ligaments
• The skeleton is divided into two subdivisions: axial and appendicular
• The axial skeleton forms the longitudinal axis of the body and includes the skull, vertebral column, and bony thorax.
• The appendicular skeleton is composed of the limbs (appendages), pectoral girdle, and pelvic girdle

Functions of the Bones

• Support the body
• Protect soft organs (skull and vertebrae protect brain and spinal cord, rib cage protects thoracic cavity organs)
• Allow movement via attached muscles
• Store minerals and fats (calcium and phosphorus, fat in the internal marrow cavity)
• Blood cell formation (hematopoiesis)

Classification of Bones

• The adult skeleton has 206 bones
• Two basic types of osseous tissue: compact bone and spongy bone
  • Compact bone is dense, smooth, and homogeneous
  • Spongy bone has small needlelike pieces of bone with many open spaces
• Bones are classified by shape into four groups: long, flat, short, and irregular
  • Long bones are typically longer than they are wide, with a shaft and enlarged ends. Examples include the femur and humerus.
  • Flat bones are thin, flattened, and usually curved, with two thin layers of compact bone sandwiching a layer of spongy bone. Examples include most bones of the skull, ribs, and sternum.
  • Short bones are generally cube-shaped, containing mostly spongy bone with an outer layer of compact bone. Examples include carpals (wrist bones) and tarsals (ankle bones). Sesamoid bones, like the patella, are a type of short bone.
  • Irregular bones have irregular shapes and don't fit into the other categories. Examples include vertebrae and hip bones.

Structure of Bone

• Long Bone Anatomy:
  • Diaphysis (shaft): Composed of compact bone, makes up most of the bone’s length.
  • Periosteum: Fibrous connective tissue membrane that covers the outside of the diaphysis; Perforating (Sharpey's) fibers secure the periosteum to the underlying bone.
  • Epiphysis (ends): Composed mostly of spongy bone, enclosed by a thin layer of compact bone; Articular cartilage covers the external surface of the epiphyses and decreases friction at joint surfaces.
  • Epiphyseal line: Remnant of the epiphyseal plate, seen in adult bones.
  • Epiphyseal plate: Flat plate of hyaline cartilage that causes lengthwise growth of a long bone in young, growing bone.
  • Medullary cavity: Cavity inside the shaft; Contains yellow marrow (mostly fat) in adults, and red marrow for blood cell formation until age 6 or 7.
  • Endosteum: Lines the inner surface of the shaft, made of connective tissue.
• Bone Markings:
  • Sites of attachments for muscles, tendons, and ligaments
  • Passages for nerves and blood vessels
  • Projections (processes) and depressions (cavities)
    • Projections generally begin with "T" (e.g., tuberosity, trochanter).
    • Depressions/cavities generally begin with "F" (e.g., fossa).
• Microscopic Anatomy of Spongy Bone:
  • Composed of small, needlelike pieces of bone called trabeculae
  • Open spaces are filled by marrow, blood vessels, and nerves.
• Microscopic Anatomy of Compact Bone:
  • Osteocytes: Mature bone cells situated in the bone matrix.
  • Lacunae: Cavities in the bone matrix that house osteocytes.
  • Lamellae: Concentric circles of lacunae situated around the central (Haversian) canal.
  • Central (Haversian) canal: Opening in an osteon containing blood vessels and nerves.
  • Canalculi: Tiny canals that radiate from the central canal to lacunae, forming a transport system connecting all bone cells to a nutrient supply.
• Bone is relatively lightweight and resists tension and other forces; organic parts (collagen fibers) provide flexibility with great tensile strength; calcium salts make bone hard and resistant to compression.

Bone Formation, Growth, and Remodeling

• Ossification: The process of bone formation that occurs on hyaline cartilage or fibrous membranes.
• Two major phases in long bone ossification:
• In embryos, osteoblasts cover a hyaline cartilage model with bone matrix.
• In fetuses, the cartilage is replaced by bone, creating a medullary cavity.
• Appositional growth: Bones grow in width by osteoblasts in the periosteum adding bone matrix on the outside and osteoclasts in the endosteum removing bone from the inner surface.
• Bone remodeling throughout life in response to two factors: calcium ion levels in the blood and the pull of gravity and muscles acting on the skeleton.
  • Parathyroid hormone (PTH): Released when calcium levels are low; Activates osteoclasts (bone-destroying cells), which break down bone and release calcium ions into the blood.
  • Hypercalcemia (high blood calcium): Prompts calcium storage in bones by osteoblasts.

Bone Fractures

• Fracture: a break in a bone

  • Types: closed (simple) and open (compound)

• Treatment: reduction (manually coaxed into place or secured with pins/wires), immobilization.

• Healing time: 6–8 weeks

• Repair involves four major events: hematoma formation, fibrocartilage callus formation, bony callus formation and bone remodeling.

Axial Skeleton

• Forms the longitudinal axis of the body and includes the skull, vertebral column, and bony thorax.

Skull

• Two sets of bones (cranial and facial) form the skull.
• Cranial bones enclose the brain, hold the eyes in anterior position (allow facial muscles to express feelings).
• Bones joined by immovable joints (sutures) except for mandible.
• 8 cranial bones: Frontal, Occipital, Ethmoid, Sphenoid, Parietal (pair), Temporal (pair)
• 14 facial bones: Maxillae(pair), Palatine(pair), Lacrimal(pair), Zygomatic(pair), Nasal(pair), Vomer, Inferior nasal conchae(pair), Mandible.
• Paranasal sinuses are hollow portions of bones surrounding the nasal cavity that lighten the skull and amplify sounds.
• Hyoid bone is a U-shaped bone that is closely related to mandibular and temporal bones; it's the only bone that doesn't articulate with another bone, as it serves as a movable base for the tongue; important for swallowing and speech.

Vertebral Column

• Vertebral column provides axial support; extends from the skull to the pelvis.
• 26 vertebrae separated by intervertebral discs.
  • 7 Cervical vertebrae
  • 12 Thoracic vertebrae
  • 5 Lumbar vertebrae
  • Sacrum (5 fused vertebrae)
  • Coccyx (3-5 fused vertebrae)
• Primary and secondary curvatures of the vertebral column.
• Common features of vertebrae: Body (centrum), vertebral arch, pedicle, lamina, vertebral foramen, transverse processes, spinous process, superior and inferior articular processes.

Thoracic Cage

• Bony thorax (or thoracic cage) protects thoracic cavity organs
• Consists of three parts:
  • Sternum
  • Ribs (true, false, floating)
  • Thoracic vertebrae

Appendicular Skeleton

• Composed of 126 bones, divided into limbs (appendages), pectoral girdle, and pelvic girdle.

Bones of the Pectoral Girdle

• Consist of two bones: Clavicle and Scapula.
• Light, poorly reinforced structure, but allows for exceptional flexibility of the upper limb.

Bones of the Upper Limbs

• Humerus (arm): head articulates with glenoid cavity of the scapula; distal end articulates with the bones of the forearm.
• Radius and ulna (forearm).
• Carpals (wrist), Metacarpals (palm), Phalanges (fingers and thumb)

Bones of the Pelvic Girdle

• Pelvic girdle and pelvis are made up of the Ilium, Ischium, and Pubis fused together.
• Protects several organs (reproductive, urinary bladder, part of large intestine).
• Female anatomy has a wider and proportionally shallower pelvic opening than male anatomy.

Bones of the Lower Limbs

• Femur (thigh): Head articulates with the acetabulum of the coxal bone; distal end articulates with the tibia.
• Tibia and fibula (lower leg).
• Tarsals (ankle), metatarsals (foot sole), phalanges (toes).
• Arched foot structure for support and balance.

Joints

• Joints are articulations where two or more bones meet.
• Functions: hold bones together securely, allow for mobility.
• Classification (two types):
  • Functionally: Synarthroses (immovable), Amphiarthroses (slightly movable), Diarthroses (freely movable).
  • Structurally: Fibrous (generally immovable; sutures, syndesmoses, gomphoses), Cartilaginous (immovable or slightly movable; synchondrosis, symphysis), Synovial (freely movable: plane, hinge, pivot, condylar, saddle, ball-and-socket).
• Bursae and tendon sheaths are flattened fibrous sacs or elongated bursae that wrap around tendons; lined with synovial membranes; filled with synovial fluid, not part of the joint.

Developmental Aspects of the Skeleton

• Birth to adulthood: bones develop from hyaline cartilage models or fibrous membranes by ossification; most cartilage converted to bone, except for articular cartilage and epiphyseal plates.
• Fontanels: fibrous membranes connecting cranial bones in the fetal skull, act as "soft spots" that allow brain growth. Usually ossify by two year age.
• Growth after birth is related to the brain; the facial skeleton enlarges (due to tooth development and respiratory passages).
• Size of cranium in relation to body; body proportions change during puberty (pelvis broadens for females). Epiphyseal plates completely ossify by the end of adolescence.
• Osteoporosis (age-related disease): bone-thinning; vertebral collapse can occur