Skeletal System Functions and Structure

Skeletal System

• The skeletal system consists of bones, cartilage, tendons, and ligaments.
• Functions:
• Support
• Protection
• Movement
• Storage
• Blood cell production

Extracellular Matrix

• Composed of connective tissues
• Components:
• Collagen: tough, ropelike protein
• Proteoglycans: large molecules consisting of polysaccharides attached to core proteins
• Tendons and ligaments: large amounts of collagen fibers
• Cartilage: contains collagen and proteoglycans
• Bone: contains collagen and minerals (calcium and phosphate)

Bone Structure

• General features:
• Long bones: upper and lower limbs
• Short bones: wrist and ankle
• Irregular bones: vertebrae and facial bones
• Long bones:
• Diaphysis: central shaft
• Epiphysis: ends
• Epiphyseal plates: growth plate; where the bone grows in length
• Articular cartilage: covers the ends of the epiphyses
• Epiphyseal line: bone growth stops, and the epiphyseal plate is replaced by bone
• Medullary cavity: large cavity in the diaphysis; contains the marrow
• Layers of a bone:
• Periosteum: outermost layer; surrounds the diaphysis; contains blood vessels, nerves, and osteoblasts
• Endosteum: innermost layer; lines the medullary cavity (thinner connective tissue)

Bone Growth and Development

• Bone growth: deposition of new bone lamellae onto existing bone
• Bone elongation occurs at the epiphyseal plate; leads to increase in height (endochondral ossification)
• Chondrocytes: cartilage cells; increase in number, enlarge, and die
• Primary ossification center: where bone first begins to appear
• Osteoclasts: cells that remove calcified cartilage matrix
• Secondary ossification center: forms in the epiphyses

Bone Remodeling

• Removal of existing bone by osteoclasts
• Deposition of new bone by osteoblasts
• Responsible for change in bone shape, bone adjustment, repair, and calcium ion regulation

Bone Repair

• Clot is formed in the damaged area
• Blood vessels and cells invade the clot and form a callus (network of fibers and islets of cartilage)
• Osteoblasts enter the callus and form a spongy bone
• Bone is slowly remodeled to compact bone

Bone and Calcium Homeostasis

• Osteoclasts remove calcium, increasing blood calcium levels
• Osteoblasts deposit calcium, decreasing blood calcium levels
• Hormones that maintain calcium homeostasis:
• Parathyroid hormone (PTH)
• Occipital bone

Cranium

• Occipital bone: inferior and posterior bone of the cranium; lambdoid suture
• Foramen magnum: where the spinal cord joins the brain
• Occipital condyles: rests on the first vertebra of the vertebral column
• Foramen ovale: allows the fiber of cranial nerve 5 to pass

Sphenoid Bone

• Butterfly-shaped bone that spans the width of the skull
• Sella Turcica: saddle-shaped structure at the central region; contains the pituitary gland

Ethmoid Bone

• Irregularly shaped bone that lies anterior to the sphenoid bone

Facial Bones

• 14 bones; 13 solidly connected, 1 movable (mandible)
• Holds the facial muscles in place
• Joined together by sutures (interlocking, immovable)
• Maxillae: main bones of the face; carries the upper teeth
• Palatine bones: lies posterior to the palatine processes of the maxilla
• Zygomatic bones: cheekbones
• Lacrimal bones: bones forming part of the medial wall of each orbit
• Nasal bones: bones forming the bridge of the nose
• Inferior nasal conchae: thin curved bones projecting from the lateral wall of the nasal cavity
• Vomer bone: single bone in the median of the nasal cavity
• Mandible: lower jaw; only freely movable joint of the face

Hyoid Bone

• U-shaped bone; provides attachment to tongue muscles; elevates the larynx during speech and swallowing

Joints

• Types:
• Hinge joints: permit movement in one plane only; elbow and knee joints
• Pivot joints: restrict movement to rotation around a single axis; rotation that occurs between the axis and atlas; articulation between the ulna and radius
• Ball and socket joints: consist of a ball (head) and a socket; shoulder and hip joints
• Ellipsoid/con dyloid joints: elongated ball and socket joints; joint between the occipital condyles (skull) and the atlas (vertebral column); joints between the metacarpals and phalanges

Types of Movement

• Flexion: movement that takes place in a frontal/coronal plane
• Extension: movement that takes place in a posterior direction
• Lateral flexion: movement of the trunk in the coronal plane
• Abduction: movement away from the median plane
• Adduction: movement toward the median plane
• Pronation: rotation of the forearm so that the palm is down
• Supination: rotation of the forearm so that the palm faces up
• Eversion: opposite movement of the foot so that the sole faces in a lateral direction
• Inversion: movement of the foot so that the sole faces medially
• Rotation: movement of a part of the body around its long axis
• Circumduction: combination in sequence of the flexion, extension, abduction, and adduction

Upper Limb

• Regions:
• Arm: region between the shoulder and the elbow; contains the humerus
• Forearm: region between the elbow and the wrist; contains the radius and ulna
• Wrist: short region between the forearm and the hand; contains 8 carpal bones
• Hand: 5 metacarpal bones are attached to the carpal bones; phalanges are 3 small bones on each finger

Pelvic Girdle

• Where lower limbs attach to the body
• Coxal bones: hip bones; join each other anteriorly and the sacrum posteriorly to form a ring of bone called the pelvic girdle
• Ilium: most superior
• Ischium: inferior and posterior; sit-down bone
• Pubis: inferior and anterior
• Iliac crest: seen along the superior margin of each ilium
• Anterior superior iliac spine: important hip landmark; anterior end of the iliac crest
• Pubic symphysis: where coxal bones join anteriorly
• Sacroiliac joints: joins the sacrum posteriorly
• Acetabulum: socket of the hip joint