Unit 4 - Skeletal System

Functions of the skeletal system

• 💪🏻support (bear the weight of the body)

• 🛡️protection (encase essential organs)

• 🏃🏻‍♀️movement (from joints)

• 📦storage (store minerals to be released into the bloodstream)

• 🏭manufacturing (production of red and white blood cells from red bone marrow)

!!! Bones = solid matrix of living cells and fibers surrounded by calcium deposits, classified by shape

Bone is created/dissolved to maintain a certain level of calcium in the blood at all times

Bones are like storage tanks for calcium 

Broken bone repair process:

1. Hematoma forms (blood enters wound, cells begin to die, phagocytes ingest dead bone cells/debris)

2. Callus forms (blood vessels grow, cartilage forms to hold bone tgt)

3. Callus ossifies (spongy bone forms to replace the cartilage)

4. Compact bone forms (osteoclasts form a larger medullary cavity, spongy bone is converted to compact bone)

Bone shapes

• flat 

• irregular

• short

• long

• sesamoid

Features of a long bone

• Epiphyses: ends of bone

  • Articular cartilage: external layer of cartilage on the ends of epiphyses, provides smooth movement of joints and cushioning from shock

• Diaphysis: shaft, long part of bone

  • Medullary cavity: hollow, filled w/ yellow bone marrow (when young starts out red) where new blood cells are produced (hematopoiesis)

• Epiphyseal plate: thin layer of internal cartilage

Layers of bone

• Periosteum 

  • outer layer made of tough connective tissue

  • location of muscle attachment and bone repair

• Compact bone

  • thick layer

  • arranged in cylinders called osteons, which are arranged in concentric circles called lamellae, which surround a central (Haversian) canal that has blood vessels and nerves

  • central canals are connected by perforating (Volkmann’s) canals running perpendicularly

• Spongy bone

  • at the ends of long bones

  • lattice of trabeculae that are found along lines of stress (perfect to resist compression) - btwn trabeculae are spaces filled w/ marrow or blood vessels

Specialized bone cells and their functions

• Osteocytes 

  • mature bone cells that are a majority of bone structure

  • connected w/ tentacle-like canaliculi

• Osteoclasts

  • break down bone _{^{(think crack)}}

  • secrete acid to enlarge medullary cavity as bone grows so that marrow is available for all cells

• Osteoblasts

  • build/produce bone

_{^{^Osteoclasts/osteoblasts line Haversian canals and the surfaces of the compact and spongy bone}}

Canaliculi

connect all bone cells, allowing them to receive nutrients and remove wastes

How bone is formed

Ossification - process of incorporating calcium and minerals into cartilage to calcify _^(harden) and become bone

1. embryo’s skeleton made of cartilage

2. osteoblasts secrete mineral deposits to replace the cartilage

3. osteoblasts mature into osteocytes

How bones grow as a child ages

Tall columns of chondrocytes (cartilage cells) at the epiphyseal plate divide then deteriorate as the matrix around them calcifies → osteoblasts, forming spongy bone 

Bone composition

• Osteoid (organic)

  • 35% (made of ground substance & collagen)

  • provides flexibility/tensile strength so bones aren’t constantly breaking _{^{(lack of collagen causes ‘Brittle Bone Disease’)}}

• Mineral salts (inorganic)

  • 65%

  • crystalline salts made of hydroxyapatites (Ca₁₀(PO₄)(OH)₂) ^{_{(lack of hydroxyapatites causes ‘Rickets’ - soft/weak bones)}}

  • provides strength/hardness

2 hormones that trigger bone remodeling 

• Calcitonin (deposits extra calcium from blood into bones, when Ca in blood is too high; from Thyroid gland)

• Parathyroid hormone (PTH) (stimulates osteoclasts to break down bone, when Ca in blood is too low) 

Major sections of the skeleton

• Axial

  • central axis of the body

  • skull, ribs, sternum, vertebrae; 80 bones

• Appendicular

  • pectoral and pelvic girdles

  • bones of the arms, legs, pelvis, shoulders; 126 bones

Axial skeleton bones

• Cranium 

  • frontal

  • parietal

  • temporal

  • occipital

  • sphenoid

  • ethmoid

  • +nasal

  • +lacrimal

  • +zygomatic

  • +maxilla

• Mandible 

• manubrium

• sternum

• ribs

• Cervical vertebrae

• thoracic vertebrae

• lumbar vertebrae

• sacrum

• coccyx

Vertebral column structure

• from skull to pelvis 

• provides support & protects the spinal cord

• vertebrae _{^{(that aren’t fused)}} separated by intervertebral discs that provide cushioning and absorb shock 

• primary (convex) and secondary (concave) curvatures of the spine allow for better balance and distribution of weight

_{^{*convexly curved at birth, later 2 portions in the cervical and lumbar vertebrae develop concave curves; cervical 7, thoracic 12, lumbar 5, sacrum 5 fused, coccyx 4 fused}}

Appendicular skeleton bones

• Clavicle

• Scapula

• humerus

• ulna

• radius

• carpals

• metacarpals

• phalanges (hands)

• pelvis

• femur

• patella

• tibia

• fibula

• tarsals

• metatarsals

• phalanges (feet)

Types of joints

• Fibrous

  • immovable or slightly movable

  • held tgt by fibrous connective tissue (ex: skull)

• Cartilaginous

  • immovable or slightly movable

  • held tgt by cartilage (ex: ribs)

• Synovial _{^{(see pic)}}

  • highly movable

  • joint capsule at ends of bones contains synovial fluid for frictionless movement (ex: knee)

  • synovial membrane and articular cartilage line the joint cavity

Ligaments vs. tendons 

connect bones to bones 

vs. 

connect bones to muscle

Types of synovial joints (ADD PIC)

• Pivot

• saddle

• hinge

• condylar

• gliding/plane

• ball & socket