Skeletal System

1. Unit 4 Skeletal System

2. Bones • Function: • Bones protect and support body organs, serve as levers for muscles to pull on (movement), store calcium, fats, and other substances, and are the site of blood cell production.

3. Classification of Bones • Two basic types of tissue: • Compact bone = dense, looks smooth and homogenous • Spongy bone = composed of small needlelike pieces of bone and lots of open space

4. Classification of Bones • Four basic shapes: • Long = typically longer than they are wide, mostly compact • Short = generally cube shaped, mostly spongy • Flat = thin, flattened, usually curved, two layers of compact bone sandwiching layer of spongy bone • Irregular bones = do not fit one of the preceding categories (hip bone, vertebrae, etc.)

5. Structure of a Long Bone • A long bone is composed of a diaphysis (shaft) and two epiphyses (ends). • The medullary cavity of the diaphysis contains yellow marrow, the epiphyses contain spongy bone. • The epiphyseal line is the remnant of the epiphyseal plate, a flat plate of hyaline cartilage seen in young, growing bone.

6. Structure of a Long Bone • Periosteum (fibrous connective tissue membrane) covers the diaphysis. • Endosteum lines medullary cavities and the spaces of spongy bone. • Hyaline cartilage covers articular surfaces.

7. Structure of a Long Bone • In adults, the cavity of the shaft is primarily a storage area for adipose tissue, called the yellow marrow or medullary cavity. • In infants, this area forms blood cells and red marrow is found there. • In adult bones, red marrow is confined to the cavities of spongy bone in flat bone and epiphyses of some long bones.

8. Bone Markings • Two main kinds: • Projections or processes = grow out from bone surface • Depressions or cavities = indentations in the bone • Indicate sites of muscle attachment, points of articulation, and sites of blood vessels and nerve passage.

9. Microscopic Anatomy • The structural unit of compact bone is the osteon, consisting of a central Haversian canal surrounded by concentric lamellae of bone matrix. • Osteocytes (mature bone cells), embedded in lacunae (matrix), are connected to each other and the Haversian canal by canaliculi (tiny canals).

10. Microscopic Anatomy • Spongy bone has slender trabeculae containing irregularly arranged lamellae that enclose red-marrow filled cavities.

11. Bone Formation, Growth, and Remodeling • In embryos, the skeleton is primarily made of hyaline cartilage, but in the young child most of the cartilage has been replaced with bone. • Flat bones form on fibrous membranes; most other bones develop using hyaline cartilage as their “models.”

12. Bone Formation, Growth, and Remodeling • Ossification (bone formation) involves two major phases • 1. Hyaline cartilage model is completely covered with bone matrix by osteoblasts (bone-forming cells). • 2. Enclosed hyaline cartilage is digested away, opening up a medullary cavity within the newly formed bone.

13. Bone Formation, Growth, and Remodeling • By birth, most hyaline cartilage models have been converted to bone except for two regions: • Articular cartilages that cover bone ends  reduce friction • Epiphyseal plates  provide for longitudinal growth during childhood

14. Bone Formation, Growth, and Remodeling • Growing bone must widen as well as lengthen  increase in diameter is called appositional growth • Growth of long bones is controlled by growth hormone and sex hormones (during puberty) • Bone is constantly changing and remodeling due to two factors: • 1. Calcium levels in blood • 2. Pull of gravity and muscles

15. Bone Formation, Growth, and Remodeling • A drop in calcium levels can stimulate the parathyroid glands to release parathyroid hormone (PTH) into the blood. • PTH activates osteoclasts (bone destroying cells), which break down bone matrix and release calcium ions into the blood. • A spike in calcium levels can result in increased bone formation as the calcium gets deposited in bone matrix and taken out of blood. • http://www.youtube.com/watch?v=Hwj2idrQJYg

16. Bone Formation, Growth, and Remodeling • Remodeling is essential if bones are to retain normal proportions and strength during long-bone growth as the body increases in size and weight. • Bones become thicker and form large projections to increase their strength in areas where bulky muscles are attached. • Bones lose mass and begin to atrophy in bedridden or physically inactive people.

17. Bone Fractures • Bones are susceptible to fractures (breaks) all through life  during youth, fractures are due to exceptional trauma; during old age, fractures occur more often. • Closed (or simple) fracture = bone breaks cleanly but does not penetrate the skin • Open (or compound) fracture = broken bone ends penetrate skin

18. Bone Fractures

19. Bone Fractures • Treated by reduction, the realignment of the broken bone ends. • Closed reduction = bone ends are put back in normal position by physician’s hands • Open reduction = surgery is performed and bone ends are secured with pins or wires • Simple fractures heal in 6-8 weeks, longer for large bones and elderly people (poor circulation)

20. Repairing Bone Fractures • 1. A hematoma (blood-filled swelling) is formed because blood vessels are ruptured when the bone breaks. Bone cells deprived of nutrition die. • 2. The break is splinted by a fibrocartilage callus (contains cartilage matrix, bony matrix, and collagen fibers) and new capillaries form. • 3. Bony callus is formed as osteoblasts and osteoclasts migrate into the area and multiply. • 4. Over the next few months, the bony callus is remodeled in response to the mechanical stresses placed on it, forming a permanent “patch” at the fracture site.

21. The Axial Skeleton

22. Axial Skelton • Can be divided into three parts: • Skull • Vertebral column • Bony Thorax

23. Skull • Formed by two sets of bones • Cranium = encloses and protects brain, composed of eight large, flat bones • Facial bones = hold eyes in anterior position, facilitate facial muscle movement, fourteen bones total • All but one of the bones of the skull are joined together by sutures = interlocking, immovable joints. • Mandible (jawbone) is attached by freely moveable joint

24. Cranium • Frontal bone – forms forehead, bony projections under eyebrows, and superior part of each eye’s orbit • Parietal bones – paired, form most of superior and lateral walls of cranium, meet in midline of skull at sagittal suture and form coronal suture where they meet the frontal bone

25. Cranium • Temporal bones – join parietal bones at squamous sutures, bone markings: • 1. External auditory meatus = canal that leads to eardrum and middle ear • 2. Styloid process = needlelike projection that attaches to many neck muscles • 3. Zygomatic process = bridge of bone that joins with cheekbone • 4. Mastoid process = rough projection containing mastoid sinus, posterior and inferior to external auditory meatus • 5. Jugular foramen = junction of occipital and temporal bones, allows jugular vein to pass through • 6. Carotid canal = anterior to jugular foramen, allows internal carotid artery to pass through

26. Cranium • Occipital bone – most posterior bone of cranium, joins parietal bones at lambdoid suture • Foramen magnum = “large hole,” allows brain to connect with spinal cord • Occipital condyles = rest on first vertebra of spinal column

27. Cranium • Sphenoid bone – butterfly-shaped, spans width of skull, forms floor of cranial cavity • Sellaturcica = (Turk’s Saddle) small depression in midline that holds pituitary gland in place • Foramen ovale = allows fibers of cranial nerve V to pass to chewing muscles of lower jaw • Ethmoid bone – anterior to sphenoid, forms roof of nasal cavity • Cristagalli = “cock’s comb,” projects from superior surface • Cribriform plates = small holes on side, allow nerve fibers to carry impulses from olfactory receptors to brain

28. Facial Bones • Maxillae = (maxillary bones) fuse to form upper jaw, all facial bones except for mandible join the maxillae  “keystone” bones • Palantine processes form anterior part of hard palate • Paranasal sinuses = lighten bones and amplify sounds we make while speaking

29. Facial Bones • Palatine bones = lie posterior to maxillae, form posterior part of hard palate, failure to fuse = cleft palate • Zygomatic bones = cheekbones, form part of lateral walls of eye sockets • Lacrimal bones = fingernail-sized, form part of medial walls of orbits, each has a groove that is a passageway for tears

30. Facial Bones • Nasal bones = small, rectangular, form bridge of nose • Vomer bone = single bone in median line of nasal cavity, forms most of nasal septum • Inferior Conchae = thin, curved bones projecting from lateral walls of nasal cavity

31. Facial Bones • Mandible = jawbone, largest and strongest bone on face, joins temporal bones • Hyoid bone = not actually part of skull, only bone in body that does not articulate with any other bone, suspended in mid-neck region, horse-shoe shaped, serves as movable base for tongue and attachment point for neck muscles

32. Fetal Skull • Face is small compared to cranium • Skull as a whole is large compared to body length • When baby is born, skull is still unfinished • Some areas of hyaline cartilage still need to be ossified. • Fontanels = fibrous membranes connecting cranial bones of infant  “soft spots”

33. Vertebral Column (Spine) • Extends from skull, which it supports, to pelvis, where it transmits weight of body to lower limbs. • Has a central cavity containing the delicate spinal cord, which it protects. • Formed from 26 irregular bones (vertebrae) connected and reinforced by ligaments, resulting in a flexible, curved structure. • Before birth, spine consists of 33 separate vertebrae, but 9 fuse together to form the sacrum and coccyx.

34. Vertebral Column (Spine) • Superior 7 vertebrae are cervical (C1-C7) • Top two vertebrae are the atlas and axis • Next 12 vertebrae are the thoracic vertebrae (T1-T12) • Next 5 vertebrae are lumbar vertebrae (L1-L5) • Sacrum (5 fused vertebrae) and Coccyx (4 fused vertebrae) are most inferior parts of spine

35. Vertebral Column (Spine) • Single vertebrae are separated by intervertebral discs = pads of flexible fibrocartilage that cushion the vertebrae and absorb shocks • In young people, the discs have a high water content (~90%), but as people age, the water content decreases and discs become harder and less compressible. • This leaves older people more susceptible to herniated discs (slipped discs), which can press on the spinal cord or spinal nerves, resulting in severe pain or numbness.

36. Vertebral Column (Spine) • The S-shape of the spine and the vertebral discs help prevent shock to the head when we walk or run. • Primary curvatures = spinal curves in the thoracic and sacral regions, present at birth • Secondary curvatures = cervical curvature appears when babies raise their heads, lumbar curvature appears when babies start to walk

37. Abnormal Spinal Curvatures

38. Vertebrae • All vertebrae have a similar structure pattern • Centrum (body) = disclike, weight-bearing part that faces the vertebral column anteriorly • Vertebral arch = formed from the joining of all posterior extensions from the vertebral body (laminae and pedicles) • Vertebral foramen = canal through which the spinal cord passes • Transverse processes = two lateral projections from vertebral arch • Spinous process = single projection arising from the posterior aspect of vertebral arch (actually fused laminae) • Superior and inferior articular processes = paired projections lateral to vertebral foramen, allows adjacent vertebrae to form joints

39. Cervical Vertebrae • First two (atlas and axis) are different because they perform functions not shared by the other cervical vertebrae. • Atlas has no body and has large depressions on superior surface to receive occipital condyles of skull. • Axis acts as pivot for rotation of atlas and skull. It has a large, upright process (odontoid process, or dens) that acts as the pivot point. • C3-C7 are the smallest, lightest vertebrae. Their spinous processes are short and divided into two branches and their transverse processes contain openings that vertebral arteries pass through on their way to the brain

40. Thoracic Vertebrae • Larger than cervical vertebrae • Body is somewhat heart-shaped with two costal demifacets (articulating surfaces) on each side that attach to heads of ribs. • Spinous process is long and hooks sharply downward  vertebra looks like a giraffe’s head from side

41. Lumbar Vertebrae • Massive, blocklike bodies • Short, hatchet-shaped spinous processes  look like a moose head from lateral aspect • Most of the stress on the vertebral column occurs in lumbar region  sturdiest vertebrae

43. Sacrum • Formed by the fusion of 5 vertebrae • The winglikealae connect to hip bones  sacroiliac joints • Forms posterior wall of pelvis • Median sacral crest = fused spinous processes of the sacral vertebrae • Dorsal sacral foramina = holes that line each side of the median sacral crest • Sacral canal = continuation of the vertebral canal

44. Coccyx • Formed by the fusion of 3-5 tiny, irregularly shaped vertebrae • Tailbone

45. Bony Thorax • Sternum, ribs, and thoracic vertebrae • “Thoracic cage”

46. Sternum • Breastbone – attaches to 1st seven pairs of ribs • Results from the fusion of three bones – manubrium, body, and xiphoid process • Three important bony landmarks: • Jugular notch = concave upper border of manubrium • Sternal angle = where the manubrium and the body meet at a slight angle to each other so that a transverse ridge is formed at the level of the second ribs • Xiphisternal joint = point where the sternal body and xiphoid process fuse, same level as T9 vertebra

47. Ribs • Twelve pairs of ribs – males have same number as females • Attached to vertebral column posteriorly, then curve downward and around toward the anterior body surface • 1st seven pairs = “true ribs” because they attach to the sternum • 2nd five pairs = “false ribs” because they either attach indirectly to sternum or not at all, last two pairs are “floating ribs” • Intercostal spaces = spaces between ribs, filled with intercostal muscles that aid in breathing

48. Appendicular Skeleton

49. Appendicular Skeleton • Composed of 126 bones of the limbs and the pelvic and pectoral girdles, which attach the limbs to the axial skelton

50. Bones of the Shoulder Girdle • Consists of 2 bones: clavicle and scapula • Clavicle = “collarbone,” slender, doubly curved, attaches to manubrium of sternum and scapula, acts as brace to hold arm away from top of thorax and helps prevent shoulder dislocation • Scapula = “shoulder blade,” triangular, flattened body, held loosely in place by trunk muscles, doesn’t actually attach to axial skeleton, two important processes: acromion and coracoid