Skeletal System

Anatomy & Physiology

The Skeletal System

Your skeleton comprises ~ 20% of your total body mass

There are 206 bones in your body, separated into 2 divisions:– 1. Axial skeleton: head, vertebrae and rib

cage– 2. Appendicular Skeleton: pelvis,

scapulae and limbs

Axial: pink Appendicular: green

5 Functions of Bones

1. Support: legs support the weight of body, ribs support thoracic cavity

2. Protections: protects all soft tissue organs3. Movement: muscles use bones as levers,

allowing for movement4. Storage: fat stored in internal cavities of

bones; calcium and phosphorus also stored5. Blood Cell Formation: hematopoesis; the

production of blood cells within marrow cavities

Bone Types

There are two types of bones:– 1. Compact bone: dense bone which is

smooth and solid; surrounds all bone; appears dense

– 2. Spongy bone: internal portion of bone; consists of small needle-like projections of bone called trabeculae with many open spaces filled with marrow

Bone Types

Bone Classification

Bones come in many shapes and sizes and are classified into 4 distinct groups:

1. Long Boneso Longer than wideo Built to absorb stresso Consists of a shaft and 2 heads at each endo Mostly compact but some spongy bone internallyo Examples: all bones of limbs except patella, carpals

and tarsals

Long Bone: the femur

Bone Classification

2. Short Boneso Roughly cube-likeo Contains mostly spongy boneo Thin layer of compact bone on surfaceo Examples: carpals and tarsalso Sesamoid bone: a bone embedded in a

tendon; varies in size and numbers/each individual; act to alter the pull of a tendon; i.e. patella

Short Bones: carpals of the wrist

Bone Classification

3. Flat Boneso Thin, flattened and usually curvedo 2 parallel compact surfaces with a spongy

layer betweeno examples: sternum, ribs and skull bones

Bone Classification

4. Irregular Boneso Do not fit any other classificationo Complicated shapeso Mostly spongy with thin compact layero Examples: vertebrae and hip bones

Anatomical Structure of a Long Bone Diaphysis: shaft of long bone; walls

made of compact bone Periosteum: fibrous sheath that covers

long bones– Highly vascularized– Functions in bone nourishment and

attachment sites

Anatomical Structure of a Long Bone Sharpey’s Fibers: connective tissue

fibers that secures the periosteum to underlying bone

Epiphyses: ends of long bones– Enlarged for muscle attachment – Predominately spongy bone

Anatomical Structure of a Long Bone Articular Cartilage: covers ends of

epiphyses and provides a slippery surface that decrease friction at joint surfaces

Medullary Cavity: holds marrow in center of diaphysis– Yellow marrow: fat storage in adults, found in

medullary cavity– Red marrow: found in diaphysis of infants, in flat

bones & epiphyses of adults; makes red blood cells

Anatomical Structure of a Long Bone

Endosteum: sheath covering medullary cavity

Bone Composition

Bone contains inorganic & organic components– Inorganic calcium carbonate & calcium

phosphate; provides hardness– Organic collagen: to further reinforce the

matrix

Osteoporosis: brittle bones

Normal Spongy Bone Osteoporotic Spongy Bone

Bone Cells: 3 Types

Osteoblasts: arise from embryonic cells and found on outer surfaces of adult bones; aid in matrix production

Osteocytes: mature bone cells; trapped in lacunae

Osteoclasts: secretes substances that dissolve mineral salt crystals

Bone – cell types

“Ruffled” Border

Note locations of

Osteoclasts & osteoblasts

Microscopic Anatomy of Bone

Lacunae: cavities in bones where osteocytes are found

Lamellae: a circular layer of bone

Microscopic Anatomy of Bone

Haversion Canals: a system of interconnecting canals in adults compact one; runs lengthwise through bone, carrying blood vessels and nerves to all areas of bones

Canaliculi: tiny canals that connect all the bone cells to the nutrient supply; radiate outward from Haversion Canals

Microscopic Anatomy of Bone

Volkmann’s Canals: communication system from exterior of bone to interior; runs at right angle to diaphysis

Osteon/Haversion Systems: each haversian canal with lamellae, osteocytes and caniliculi

Haversion Systems

Bone Development

Embryonic Skeleton: predominately hyaline cartilage

Fontanels: in skull at birth– Allows for growth of brain

Bone Development

Young child to late adolescence: cartilage replaced by bone– Epiphyseal Growth Plates: allows for

interstitial growth (lengthwise)– Cartilage near the epiphyses regenerates– Cartilage near the diaphysis hardens to

bone eventually they’ll meet, halting lengthwise growth

Epiphyseal Growth Plate

Epiphyseal Growth Plates

Bone Development & Growth

Ossification

The replacement of cartilage by bone Cartilage is covered by osteoblasts Cartilage is “eaten” away, leaving the

medullary cavity open within the bone

Appositional Growth

Outward growth of bone during adulthood– Bones change based on calcium levels & muscles

acting on the skeleton– Decreased blood calcium leads to bone

breakdown– Increased demand by muscles on bones causes

bone to thicken– Weight gain also increase bone diameter– Adult bone constantly remodels (breakdown &

growth) to help maintain homeostasis of blood mineral levels

Skeletal System

Axial Skeleton

Axial Skeleton

Includes 80 bones of the skull, vertebral column and bony thorax

Functions:– Supports head, neck & trunk– Protects brain, spinal cord and thoracic

organs

Skull

Composed of flat bones Function:

– Used for attachment of head muscles & protects the brain

Sutures of the Skull

Sutures: interlocking joints that unite skull bones– Coronal: where parietal bones meet frontal– Sagittal: where 2parietal bones meet

superiorly– Squamos: where parietal and temporal

bones meet on lateral aspects of skull– Lambdoidal: where parietal bones meet

occipital bones meet posteriorly

Vertebral Column aka the Spine

Location: runs from the base of the skull to the coccyx (tailbone)

Function: – Surrounds and protects the spinal cord– Provides attachment sites for ribs and back

muscles

Vertebral Column

Characteristics– 26 interconnected

irregular bones– Provides a flexible,

curved structure– Serves as axial

support of the trunk

Vertebral Column

Curvatures of the Spine:– S-shaped to prevent shock to head in motion– Allows for trunk flexibility– Increases resiliency & flexibility of the spine– Functions like a spring, not a rod

Cervical & Lumbar Curves: concave posteriorly

Thoracic & Sacral Curves: convex posteriorly

Curvatures of the Spine

Abnormal Curvatures of the Vertebral Column

Lordosis: aka sway back– An accentuated

lumbar curve

Kyphosis: aka hunchback– An exaggerated

thoracic curve

Scoliosis: the twisted disease– An abnormal lateral

curvature in the thoracic region

– Typical in girls in late childhood

Lordosis & Kyphosis

Scoliosis

Cervical Vertebrae

7 total extending from base of skull to ~ shoulder line

Numbered C1-C7 Smallest & lightest vertebrae Unique vertebrae

– Atlas or C1: no body; holds the occipital bone, allows nodding motion (“yes”)

– Axis or C2: acts as a pivot for rotation; shake head (“no”)

Thoracic Vertebrae

12 total; runs through mid-back Numbered T1-T12 Larger than cervical Longer, palpable spinous processes Ribs attach here posteriorly

Lumbar Vertebrae

5 total Numbered L1-L5 Huge bodies and

short spinous processes

Holds most of body weight & stress; very sturdy

Sacrum & Coccyx

Sacrum– Formed from 5 fused

vertebrae– Numbered S1-S5– Makes up posterior

wall of pelvis– Strengthens &

stabilizes pelvis Coccyx

– 4 fused vertebrae

Ligaments of the Spine

There are several; only 2 you need to know– Anterior Longitudinal Ligament: resists

back hyperextension– Posterior Longitudinal Ligament: resists

back flexion

Intervertebral Discs

Cushion-like pads between vertebrae Asts as shock absorbers during motion Makes up ~25% of length of column Flattens during the day

Intervertebral Discs

Ribs

Flat bones 12 total pairs Attach posteriorly to thoracic spine Function:

– Protect thoracic organs True Ribs: the superior 7 pairs

– Attach directly to sternum by costal cartilage False Ribs: the inferior 5

– 8-10: join each other by cartilage and indirectly attach to sternum

– 11& 12: the floating ribs, no anaterior attachment

Rib Cage

Pelvis

Has 2 regions: true and false pelvises False pelvis superior to true pelvis True pelvis dimensions are a concern

to child-bearing women Pelvic structure differs between men

and women

Gender Difference of Pelvis

Men– Narrow outlet– Heavier & thicker bone

structure– Ilia less flared, more

vertical– Sacrum long and curved– Ischia close together– Less rounded pubic arch

Women– Inlet circular & large– Pelvis shallow, lighter &

thinner– Ilia flare laterally– Sacrum shorter & less

curved– Ischia farther apart &

shorter– Pubic arch is more

rounded