The Skeletal System

Introduction: Functions of the Skeletal System

• Support. The skeletal system provides structural support for the entire body.

• Leverage. Bones of the skeleton function as levers that change the magnitude and direction of the forces generated by skeletal muscles.

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Introduction: Functions of the Skeletal System

• Protection. Delicate tissues and organs are often surrounded by skeletal elements.

• Storage. The calcium salts of bone represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids.

• Blood cell production. Red blood cells and other blood elements are produced within the red marrow, which fills the internal cavities of many bones

The Structure of Bones

Macroscopic FeaturesLong bones: longer than

they are wideShort bones: just as long as

they are wideFlat bones: thin and

relatively broadIrregular bones: complex

shapes that do not fit easily into any other category

Diaphysis: a long bones central shaft

• Microscopic Features• Osteocytes: bone

cells• Lacunae: small

pockets containing bone cells

• Lamellae: narrow sheets of calcified matrix

• Canaliculi: small channels that radiate through the matrix

The Structure of Bones

Epiphyses: Expanded ends of a long bone

Compact bone: relatively solid

Spongy bone: a network of bony rods or struts separated by spaces that are normally filled with bone marrow

• Osteon: basic functional unit of compact bone

• Central canal: contains one or more blood vessels

The Structure of Bones

Periosteum: a fibrous outer layer and a cellular inner layer

Endosteum: lines the marrow cavity and other inner surfaces

• Perforating canals: provide passageways for linking the blood vessels of the central canals wit those of the periosteum of the marrow cavity

• Trabeculae: rods or plates formed by lamellae

Bone Development and Growth• Intramembranous Ossification• Intramembranous ossification:

begins when osteoblasts differentiate within fibrous connective tissue

• Ossification center: the place where ossification first occurs.

• Bone develops within sheets or membranes of connective tissue

Bone Development and Growth Continued

• Endochondral Ossification

• Endochondral ossification: bone replaces existing cartilage

• Epiphyseal plates: bone of the shaft and the bone of each epiphysis are separated by these areas of cartilage

Remodeling and Homesostatic Mechanisms

• Remodeling and Support• Regular mineral turnover gives each bone the

ability to adapt to new stresses. Heavily stressed bones become thicker, stronger, and develop more pronounced surface ridges; bones not subjected to ordinary stresses become thin and brittle. Regular exercise is thus an important stimulus that maintains normal bone structure.

Remodeling and Homeostatic Mechanism continued

• 4 Steps of the Repair Process

Remodeling and Homeostatic Mechanism continued

• Step 1: In even a small fracture, many blood vessels are broken and extensive bleeding occurs. Pooling and clotting of the blood forms a swollen area called a fracture hematona, which closes off the injured blood vessels

Remodeling and Homeostatic Mechanism continued

• Step 2: Cells of the periosteum and endosteum migrate into the fracture zone. There they form localized thickening-an external callus and internal callus. At the center of the external callus, cells differentiate into chondrocytes and build blocks of cartilage.

Remodeling and Homeostatic Mechanism continued

• Step 3: Osteoblasts replace the central cartilage of the exteral callus with spongy bone. When this process is complete, the external and internal calluses form a continuous brace of spongy bone at the fracture site.

Remodeling and Homeostatic Mechanism continued

• Step 4: The remodeling of spongy bone at the fracture site may continue from a period of 4 months to well over a year. When the remodeling is complete, the fragments of dead bone and spongy bone of the calluses will be gone, and only living compact bone will remain. The repair may be “good as new” with no sign that a fracture occurred, but the bone may be slightly thicker than normal at the fracture site.

Aging and the Skeletal System

• The bones of the skeleton become thinner and relatively weaker as a normal part of the aging process. Inadequate ossification is called osteopenia, and all people become slightly osteopenic as they age.

Aging and the Skeletal System

• The reduction in bone mass occurs because between the ages of 30 and 40, osteoblast activity begins to declining while osteoclast activity continues at normal levels. Once the reduction begins, women lose roughly 8% of their skeletal mass every decade, whereas men’s skeletons deteriorate at about 3% per decade.

Skeletal Terminology

• Each of the bones in the human skeleton not only has a distinctive shape but also has characteristic external features.

• Elevations or projections form where tendons and ligaments attach and where adjacent bones articulate.

• Depressions and openings indicate sites where blood vessels and nerves lie alongside or penetrate the bone.

• These external landmarks are called bone markings.

Skeletal Divisions

• The skeletal system consists of 206 separate bones and a number of associated cartilages.

• Axial skeleton: forms the longitudinal axis of the body

• Skull: has 22 bones plus associated bones

• Vertebral column: has 26 bones

• Thoracic cage: has 24 ribs and a sternum

The Skull

• The bones of the skull protect the brain and support delicate sense organs involved with vision, hearing, balance, smell, and taste

• The skull is made up of the 8 bones of the cranium and 14 of the face

• The cranium encloses the cranial cavity which is a fluid-filled chamber that cushions and supports the brain

The Skull

• Frontal bone: forms the forehead and superior surface of the orbits

• Orbits: the bony recesses that contain the eyes

• Parietal bone: is posterior to the frontal bone

• Sagittal structure: extends along the midline of the cranium

• Occipital bone: forms the posterior and inferior portions of the cranium

The Skull

• Foramen magnum: the opening that connects the cranial cavity with the spinal cavity

• Temporal bones: bones along the sides and base of the cranium

• Mandibular fossa: the point of articulation with the lower jaw

The Skull cont..

• Sphenoid bone: forms part of the floor of the cranium• Ethmoid bone: consists of two honeycombed masses of

bone• Maxillary bones: form the floor and medial portion of the

rim of the orbit, the walls of the nasal cavity, and anterior roof of the mouth

• Palatine bones: form the roof of the mouth• Zygomatic bones: form the lateral walls of the orbit• Zygomatic arch: the cheekbones• Nasal bones: forms the bridge of the nose• Inferior nasal conchae: form the lateral walls of the nasal

cavity• Mandible: the lower jaw

The Neck and Trunk

• Cervical region: consists of the 7 cervial vertebra of the neck

• Thoracic region: consists of the 12 thoracic vertebrae

• Lumbar region: contains the 5 lumbar vertebrae

• Sacral region: consists of the sacrum• Coccygeal region: contains the coccyx,

which consists of fused vertebrae

The Neck and Trunk cont…• Spinal curvature: the vertebrae do not form a straight and rigid structure. A

side view of the spinal column reveals four spinal cures. The thoracic and sacral curves are called primary curves because they appear late in fetal development. The cervical and lumbar curves, known as secondary curves, do not appear until months after birth.

• Vertebral Anatomy: body- more massive, weight bearing portion of the vertebrae, transverse process- projecting laterally or dorsolaterall from the pidicles serve as sites for muscle attachments, vertebral arch- form the lateral and posterior walls of the vertebral foramen, intervertebral disc- fibrocartilage

• Cervical vertebrae: The 7 cervical vertebrae extend from the head to the thorax. Atlas- hold up the head

• Thoracic Vertebrae: Distinctive features of a thoracic vertebra include: (1) a characteristic heart-shaped body that is more massive than that of a cervical vertebrae, (2) a large, slender spinous process that points inferiorly, (3) articular surfaces on the body and on the transverse processes for articulation with one or more pairs of ribs

The Neck and Trunk cont…• Lumbar Vertebrae: Distinctive features include: (1) a vertebral body

that is thicker and more oval than that of a thoracic vertebra, (2) a relatively massive, stumpy spinous process that projects posteriorly, providing surface area for the attachment of the lower back muscles, (3) bladelike transverse processes that lack articulations for ribs

• Sacrum and Coccyx: The sacrum consists of the fused elements of 5 sacral vertebrae. This structure protects the reproduction gestive, and excretory organs and attaches the axial skeleton to the appendicular skeleton by articulation with the pelvic girdle. The coccyx provides an attachment site for muscles that closes the anal opening.

• Thorax: Consists of the thoracic vertebrae, the ribs, and the sternum. The ribs and the sternum form the thoracic cage, or rib cage, and establish the contours of the thoracic cavity.

The Pectoral Girdle• The clavicle: The S-shaped clavicle bone articulates with the manubrium

component of the sternum and the acromion of the scapula. • The scapula: Its anterior face forms a broad triangle bounded by the

superior, medial, and lateral borders. The intersection of the lateral and superior borders thickens into the shallow, cup-shaped glenoid cavity. At the glenoid cavity, the scapula articulates with the proximal end of the humerus to form the should joint.

• The humerus: The prominent greater tubercle established the contour of the shoulder. The lesser tubercle lies more anteriorly, separated from the greater tubercle by a deep groove. Muscles are attached to both tubercles, and a large tendon runs along the groove. The proximal shaft of the humerus is round in section. The posterior surface of the shaft flattens and the humerus expands to either side, forming a broad triangle.

• The radius and ulna: The radius and ulna are the bones of the forearm. In the anatomical position, the radius lies along the lateral side of the forearm. The ulna forms the medial support of the forearm.

• The wrist and hand: There are 27 bones in the hand, supported the wrist, palm, and fingers. There are 4 carpal bones, 4 distal carpal bones, 5 metacarpal bones, and each hand has 14 phalangeal bones.

The Pelvic Girdle

• The coxa: The ilium is the most superior and largest coxal bone. The ilium forms broad, curved surface that provides an extensive area for the attachment of muscles, tendons, and ligaments.

• The pelvis: The pelvis consists of the coxae, the sacrum, and the coccyx. It is thus a composite structure that includes portions of both the appendicular and axial skeletons.

The Lower Limb

• The femur: The femur, or thighbone, is the longest, heaviest, and strongest bone in the body. The femur articulates with the tibia of the leg at the knee joint.

• The tibia and fibula: The condyles of the femur aticulate with the lateral and medial condyles of the tibia, or shinbone, the large medial bone of the leg. A ligament from the patella attaches to the tibial tuberosity just below the knee joint.

• The ankle and foot: The ankle includes 7 separate tarsal bones, only the talus articulates with the tibia and fibula. The basic organizational pattern of the metatarsals and phalanges of the foot resembles that of the hand.

Classification of Joints

• Immovable joints (synarthroses): the bony edges are quite close together and may even interlock

• Slightly movable joints (ampiarthroses): permits very limited movement, and the bones are usually father aprat then they are at a synarthrosis

• Freely movable joints (diarthroses): permit a wide range of motion, usually found at the ends of long bones

Articular Form and FunctionTypes of Movements

• Gliding: two opposing surfaces slide past each other• Flexion: movement in the anterior-posterior plane that reduces the angle

between the articulating elements• Abduction: movement away from the midline of the body in the frontal plane• Rotation: turning around the longitudinal axis of the body or limb• Pronation: moving the wrist and hand from palm-facing front to palm-facing

back• Supination: palm is turned forward• Inversion: twisting motion of the foot that turns the sole forward• Dorsiflexion: flexion of the ankle and elevation of the sole• Plantar flexion: extending the ankle and elevating the heel, as in standing on

tiptoes• Opposition: special movement of the thumb that enables it to grasp and hold

an object• Protraction: moving a part of the body anteriorly in the horizontal plane

A Functional Classification of Synovial Joints

• Gliding joints: have flattened or slightly curved faces, the relatively flat articular surfaces slide across one another, ex: clavicles

• Hinge joints: permit angular movement in a single plan, like the opening and closing of a door, ex: elbow

• Pivot joints: permit only rotation, ex: rotating your head• Ellipsoidal joint: an oval articular face nestles within a depression on

the opposing surface, ex: connect the fingers and toes with the metacarpals and metatarsals

• Saddle joints: have articular faces that resemble saddles, each face is concave on one axis and convex on the other, and the opposing faces nest together, permits angular motion, but prevents rotation, ex: twiddling your thumbs

• Ball and socket joints: the round head of one bone rests within a cup-shaped depression in another, ex: shoulder

Representative ArticulationIntervertebral Articulations

• The vertebrae articulate with one another in one of two ways: (1) at gliding joints between the superior and inferior articular processes, and (2) at symphyseal joints between the vertebral bodies. Articulations between the superior and inferior articular processes of adjacent vertebrae permit small movements that are associated with flexion and rotationof the vertebra column.

Representative ArticulationsArticulations of the Upper Limb

• The shoulder joints permits the greatest range of motion of any join in the body. Because it is also the most frequently dislocated joint, it provides an excellent demonstration of the principle that strength and stability must be sacrificed to obtain mobility.

• The elbow joint, which consists of two articulations: the humerus and ulna, and the humerus and radius. The elbow joint is extremely stable because (1) the bony surfaces of the humerus and ulna interlock; (2) the joint capsule is very thick; (3) the capsule is reinforced by stout ligaments.

Representative ArticulationsArticulations of the Lower Limb

• The hip joint: The articulating surface of the acetabulum has a fibro cartilage pad along its edges, a flat pad covered by snyovial membrane in its central portion, and a stout central ligament.

• The knee joint: The rounded femoral condyles roll across the top of the tibia, so the points of contact are constantly changing. The knee combines three separate joints-two between the femur and tibia, and one between the patella and fumur.

Conclusion

• The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissues that stabilize or interconnect bones. Its functions include the structural support, storage, blood cell production, protection, and leverage.