The skeletal system consists of bones, cartilage, joints and ligaments that work together to support the body, protect organs and allow for movement. There are two types of ossification that can form bone - intramembranous, where bone develops in connective tissue, and endochondral, where bone replaces cartilage. Bones are constantly remodeled through the actions of osteoblasts which build bone and osteoclasts which break it down. Factors like nutrition, hormones, exercise and aging can impact bone remodeling and strength over time.

1. The Skeletal System

2. Parts of the skeletal system
 Bones (skeleton)
 Joints
 Cartilages
 Ligaments (bone to bone) (tendon=bone to muscle)
Divided into two divisions
 Axial skeleton- skull, spinal column
 Appendicular skeleton – limbs and girdle

3. Cartilage Tendons Ligaments
Tough Attaches bone to
muscle
Attaches bone to
bone
Flexible Sturdy Elastic
At end of bone Non elastic Stabilise
Cushions Size changes
depending on
muscle
Made of many
fibres
Anchor Strong

4. Functions of Bones
 Support of the body
 Protection of soft organs
 Movement due to attached skeletal
muscles
 Storage of minerals and fats
 Blood cell formation

5. Bones of the Human Body
 The skeleton has 206 bones
 Two basic types of bone tissue
 Compact bone
 Homogeneous
 Spongy bone
 Small needle-like
pieces of bone
 Many open spaces

6. Classification of bones (Shapes)
1. Long- bones are longer than they are wide
(arms, legs)
2. Short- usually square in shape, cube like
(wrist, ankle)
3. Flat- flat , curved (skull, Sternum)
4. Irregular- odd shapes (vertebrae, pelvis)

7. Lets put the bones into the four categories
Long Bones Short Bones Flat Bones Irregular Bones
Femur Tarsals Patella Atlas
Humerus Carpals Cranium Axis
Tibia Pelvis (Llium) Cervical
Radius Scapula Thoracic
Ulna Sternum Lumbar
Fibula Ribs Sacrum
Phalanges Coccyx
Meta Tarsals
Meta Carpals
Clavicle

9. Types of Bone Cells
 Osteocytes
 Mature bone cells
 Osteoblasts
 Bone-forming cells
 Osteoclasts
 Bone-destroying cells
 Break down bone matrix for remodeling and release of calcium
 Bone remodeling is a process by both osteoblasts and
osteoclasts

10. Changes in the Human Skeleton
 In embryos, the skeleton is primarily hyaline
cartilage
 During development, much of this cartilage is
replaced by bone
 Cartilage remains in isolated areas
 Bridge of the nose
 Parts of ribs
 Joints

11. Bone Fractures
 A break in a bone
 Types of bone fractures
 Closed (simple) fracture – break that does not penetrate
the skin
 Open (compound) fracture – broken bone penetrates
through the skin
 Greenstick- frays, hard to repair, breaks like a green twig
 Bone fractures are treated by reduction and immobilization
 Realignment of the bone

12. Axial skeleton
• Axial skeleton supports and protects organs
of head, neck and trunk
• skull (cranium and facial bones)
• Hyoid bone (anchors tongue and muscles
associated with swallowing)
• vertebral column (vertebrae and disks)
• Bony thorax (ribs and sternum)

13. Appendicular skeleton
• Appendicular skeleton includes bones of limbs
and Bones that anchor them to the axial
skeleton
• Pectoral girdle (clavicle, scapula)
• Upper limbs (arms)
• Pelvic girdle (sacrum, coccyx)
• Lower limbs (legs)
• Articulation- where joints meet, connect, and are
formed.

15. The Skull
• 8 sutured bones in cranium
• Facial bones: 13 sutured bones 1 mandible
Cranium
• Encases brain
• Attachments for muscles
• Sinuses

17. Bones of the Skull
Figure 5.11

18. Paranasal Sinuses
 Hollow portions of bones surrounding
the nasal cavity
Figure 5.10

19. The Hyoid Bone
 The only bone that
does not articulate
with another bone
 Serves as a
moveable base for
the tongue, and
other muscle
attachments

20. The Vertebral Column
 Vertebrae separated
by intervertebral discs
made of cartilage
 The spine has a
normal S curvature
 Each vertebrae is
given a name
according to its
location

22. Thoracic cage
ribs
thoracic Vertebrae
sternum
costal cartilages
•True ribs are directly attached to the
sternum
(first seven pairs)
•Three false ribs are joined to the 7th
rib
•Two pairs of floating ribs

24. Internal Structure of Bone
• Bone consists of two different types of
tissue—compact bone and spongy bone.
• Another type of tissue called marrow fills the
spaces in bones
• There are two types of marrow—red and
yellow.

25. Compact Bone
Compact bone makes up the
outer layer of all bones. Although
it looks dense and solid, It is full
of holes for nerves and blood vessels.
Spongy Bone
Spongy bone contains flat
and needlelike structures
that resist stress. Red bone
marrow may fill the open
spaces in some bones.
Central Cavity
Central cavities in long
bones usually contain
yellow bone marrow (fat).
Outer Membrane
An outer membrane
covers most of a long bone.
The inner portion of a membrane
contains cells that build up and
breakdown bone.

26. What are the types and functions of
bone cells?

27. Bone (Osseous) Tissue
• Dense, supportive connective tissue
• Contains specialized cells
• Produces solid matrix of calcium salt deposits
around collagen fibers

28. Characteristics of Bone Tissue
• Dense matrix, containing:
– deposits of calcium salts
– bone cells (osteocytes) within lacunae organized
around blood vessels
• Canaliculi:
– form pathways for blood vessels
– exchange nutrients and wastes

29. Characteristics of Bone Tissue
• Periosteum:
– covers outer surfaces
of bones
– consist of:
• outer fibrous layer
• inner cellular layer

30. Matrix Minerals
• Two-thirds of the bone matrix is calcium
phosphate, Ca3(PO4)2:
– Reacts with calcium hydroxide, Ca(OH)2 to form
crystals of hydroxyapatite, Ca10(PO4)6(OH)2
– which incorporates other calcium salts and ions
Ca3(PO4)2 + Ca(OH)2 Ca10(PO4)6(OH)2

31. Bone Cells
• Make up only 2% of bone
mass:
– osteocytes
– osteoblasts
– osteoprogenitor cells
– osteoclasts
Matrix Proteins
• One-third of the bone
matrix is protein fibers
(collagen)

32. Osteocytes
• Live in lacunae
• Are between layers (lamellae) of matrix
• Connect by cytoplasmic extensions through
canaliculi in lamellae
• Do not divide
Functions
• To maintain protein and mineral content of
matrix
• To help repair damaged bone

33. Osteoblasts & Osteoid
• Immature bone cells that secrete matrix
compounds (osteogenesis)
• Matrix produced by osteoblasts, but not yet
calcified to form bone
• Osteoblasts surrounded by bone become
osteocytes

34. Osteoprogenitor Cells
• Mesenchymal stem cells that divide to
produce osteoblasts
• Are located in the inner, cellular layer of
periosteum (endosteum)
• Assist in fracture repair

35. Osteoclasts
• Secrete acids and protein-digesting enzymes
• Giant, multinucleate cells
• Dissolve bone matrix and release stored
minerals (osteolysis)
• Are derived from stem cells that produce
macrophages

36. What is the difference between
compact bone and spongy bone?

37. Compact Bone

38. Osteon
• The basic unit of mature compact bone
• Osteocytes are arranged in concentric lamellae
• Around a central canal (Haversian canal) containing
blood vessels
Perforating Canals
• Perpendicular to the central canal
• Carry blood vessels into bone and marrow
Circumferential Lamellae
• Lamellae wrapped around the long bone
• Binds osteons together

39. Spongy Bone
• Does not have osteons
• The matrix forms an open network of
trabeculae
– Trabeculae have no blood vessels

40. Red Marrow
• The space between
trabeculae is filled with red
bone marrow:
– has blood vessels
– forms red blood cells
– supplies nutrients to
osteocytes
Yellow Marrow
• In some bones, spongy
bone holds yellow bone
marrow:
– is yellow because it stores fat

41. Periosteum and Endosteum
• Compact bone is covered with membrane:
– periosteum on the outside
– endosteum on the inside

42. Periosteum
• Covers all bones:
– except parts enclosed in
joint capsules
• It is made up of:
– an outer, fibrous layer
– and an inner, cellular
layer

43. Perforating Fibers
• Collagen fibers of the periosteum:
– connect with:
• collagen fibers in bone
• fibers of joint capsules
• attached tendons
• ligaments

44. Functions of Periosteum
1. Isolate bone from surrounding tissues
2. Provide a route for circulatory and nervous
supply
3. Participate in bone growth and repair

45. Endosteum
• An incomplete cellular layer:
– lines the marrow cavity
– covers trabeculae of spongy bone
– lines central canals
• Contains:
– osteoblasts
– osteoprogenitor cells
– osteoclasts
• Is active in bone growth and repair

46. What is the difference between
intramembranous ossification
and endochondral ossification?

47. Bone Development
• Human bones grow until about age 25
• Osteogenesis:
bone formation
• Ossification:
the process of replacing other tissues with bone
• Calcification:
The process of depositing calcium salts
Occurs during bone ossification and in other
tissues

48. Ossification
• The 2 main forms of ossification are:
– intramembranous ossification
– endochondral ossification

49. Intramembranous Ossification
• Also called dermal ossification:
– occurs in the dermis
– produces dermal bones such as the mandible and
clavicle

50. Endochondral Ossification
• Ossifies bones that originate as hyaline
cartilage
• Most bones originate as hyaline cartilage

51. How does bone form and grow?

52. Blood Supply of
Mature Bones
• 3 major sets of blood
vessels develop

53. Blood Vessels of Mature Bones
• Nutrient artery and vein:
– a single pair of large blood
vessels
– enter the diaphysis through
the nutrient foramen
– femur has more than 1 pair
• Metaphyseal vessels:
– supply the epiphyseal
cartilage
– where bone growth occurs
• Periosteal vessels provide blood
to:
– superficial osteons
– secondary ossification centers
• The periosteum also contains:
– networks of
• lymphatic vessels
• sensory nerves

54. How does the skeletal system
remodel and maintain homeostasis,
and what are the effects of
nutrition, hormones, exercise, and
aging on bone?

55. Remodeling
• The adult skeleton:
– maintains itself
– replaces mineral reserves
• Remodeling:
– Recycles and renews bone matrix
– involves osteocytes, osteoblasts, and osteoclasts

56. KEY CONCEPTS
• Bone continually remodels, recycles, and
replaces
• Turnover rate varies
• If deposition is greater than removal, bones
get stronger
• If removal is faster than replacement, bones
get weaker

57. Bone Degeneration
• Bone degenerates quickly
• Up to 1/3 of bone mass can be lost in a few
weeks of inactivity

58. KEY CONCEPTS
• What you don’t use, you lose
• Stresses applied to bones during physical
activity are essential to maintain bone
strength and mass

59. Effects of Hormones
and Nutrition on Bone
• Minerals: A dietary source of calcium and
phosphate salts plus small amounts of
magnesium, fluoride, iron, and manganese
• Calcitriol: The hormone calcitriol, is made in
the kidneys and helps absorb calcium and
phosphorus from digestive tract its synthesis
requires vitamin D3 (cholecalciferol)

60. • Vitamins:
• Vitamin C is required for collagen synthesis,
and stimulates osteoblast differentiation
• Vitamin A stimulates osteoblast activity
• Vitamins K and B12 help synthesize bone
proteins

61. Other Hormones
• Growth hormone and thyroxine stimulate
bone growth
• Estrogens and androgens stimulate
osteoblasts
• Calcitonin and parathyroid hormone regulate
calcium and phosphate levels

62. Hormones for Bone Growth
and Maintenance

63. Chemical Composition of Bone

64. Calcium Regulation
• Calcium ions in body fluids:
– must be closely regulated
• Homeostasis is maintained:
– by calcitonin and parathyroid hormone
– which control storage, absorption, and excretion

65. Calcitonin and Parathyroid Hormone
Control
• Bones:
– where calcium is stored
• Digestive tract:
– where calcium is absorbed
• Kidneys:
– where calcium is excreted

66. Parathyroid Hormone (PTH)

67. Parathyroid Hormone (PTH)
• Produced by parathyroid glands in neck
• Increases calcium ion levels by:
– stimulating osteoclasts
– increasing intestinal absorption of calcium
– decreases calcium excretion at kidneys

68. Calcitonin

69. Calcitonin
• Secreted by C cells (parafollicular cells) in
thyroid
• Decreases calcium ion levels by:
– inhibiting osteoclast activity
– increasing calcium excretion at kidneys

70. Joints
A joint, or articulation, is the place where two
bones come together.
• Fibrous- Immovable: connect bones, no movement.
(skull and pelvis).
• Cartilaginous- slightly movable, bones are attached
by cartilage, a little movement (spine or ribs).
• Synovial- freely movable, much more movement
than cartilaginous joints. Cavities between bones are
filled with synovial fluid. This fluid helps lubricate and
protect the bones.

71. The Synovial Joint

72. Types of Synovial Joints Based
on Shape

73. Types of Synovial Joints Based
on Shape

74. Ball-and-Socket Joint
A ball-and-socket joint allows
movement
in all directions. Your shoulders and
hips
are ball-and-socket joints.
Hinge Joint
Hinge joints allow
bending and straightening
movements.Your knees
and elbows are
hinge joints.
Gliding Joint
Gliding joints allow
movement in many
directions as the bones
slide along each other.
Your wrists and ankles contain
gliding joints.
Pivot Joint
A pivot joint connects your
head to the first vertebra in
your backbone. It allows you
to turn your head from
side to side.

75. General Structure of Synovial Joints
1. Articular cartilage
 Hyaline
 Spongy cushions absorb
compression
 Protects ends of bones
from being crushed
2. Joint (synovial) cavity
 Potential space
 Small amount of synovial
fluid

76. 3. Articular (or joint) capsule
 Two layered
 Outer*: fibrous capsule
of dense irregular
connective tissue
continuous with
periosteum
 Inner*: synovial
membrane of loose
connective tissue (makes
synovial fluid)
 Lines all internal joint
surfaces not covered by
cartilage*
*
*
*

77. 4. Synovial fluid
 Filtrate of blood
 Contains special glycoproteins
 Nourishes cartilage and functions
as slippery lubricant
 “Weeping” lubricatioin
5. Reinforcing ligaments (some
joints)
 Capsular (most) – thickened parts
of capsule
 Extracapsular
 Intracapsular

78. 6. Nerves
 Detect pain
 Monitor stretch (one of the
ways of sensing posture and
body movements)
7. Blood vessels
 Rich blood supply
 Extensive capillary beds in
synovial membrane (produce
the blood filtrate)

79. Types of Joints
Hinge- A hinge joint allows extension and
retraction of an appendage. (Elbow, Knee)

80. Ball and Socket- A ball and socket joint
allows for radial movement in almost
any direction. They are found in the hips
and shoulders. (Hip, Shoulder)

81. Gliding- In a gliding or plane joint bones
slide past each other. Mid-carpal and mid-
tarsal joints are gliding joints. (Hands,
Feet)

82. Saddle- This type of joint occurs when the
touching surfaces of two bones have both
concave and convex regions with the shapes
of the two bones complementing one other
and allowing a wide range of movement.
(Thumb)

83. Joints (Types of Movements at Synovial Joints)
 Gliding
 Simple movement back-and-forth and from side-to-side
 There is no significant alteration of the angle between the bones
 Limited in range
 Intercarpal joints
 Angular Movements
 Increase or a decrease in the angle between articulating bones
 Angular movements include
Flexion
Extension
Lateral flexion
Hyperextension
Abduction
Adduction
Circumduction

84. • Flexion
– Decrease in the angle between articulating bones
– Bending the trunk forward
• Extension
– Increase in the angle between articulating bones
– Flexion and extension are opposite movements
• Lateral flexion
– Movement of the trunk sideways to the right or left at the waist
• Hyperextension
– Continuation of extension beyond the normal extension
– Bending the trunk backward
• Abduction
– Movement of a bone away from the midline
– Moving the humerus laterally at the shoulder joint
• Adduction
– Movement of a bone toward the midline
– Movement that returns body parts to normal position from abduction

85. • Circumduction
– Movement of a body part in a circle
– Moving the humerus in a circle at the shoulder joint
• Rotation
– A bone revolves around its own longitudinal axis
– Turning the head from side to side as when you shake your head “no”

87. • Special Movements
– Elevation
– Depression
– Protraction
– Retraction
– Inversion
– Eversion
– Dorsiflexion
– Plantar flexion
– Supination
– Pronation
– Opposition

88. • Elevation
– Upward movement of a part of the body
– Closing the mouth
– Its opposing movement is depression
• Depression
– Downward movement of a part of the body
– Opening the mouth
• Protraction
– Movement of a part of the body anteriorly
– Thrusting the mandible outward
– Its opposing movement is retraction
• Retraction
– Movement of a protracted part of the body back to normal

89.  Inversion
 Movement of the foot medially
 Its opposing movement is eversion
 Eversion
 Movement of the sole laterally
 Dorsiflexion
 Bending of the foot at the ankle in an upward direction
 Its opposing movement is plantar flexion
 Plantar flexion
 Bending of the foot at the ankle in a downward direction
 Supination
 Movement of the forearm so that the palm is turned upward
 Its opposing movement is pronation
 Pronation
 Movement of the forearm so that the palm is turned downward
 Opposition
 Movement of the thumb in which the thumb moves across the palm to touch
the tips of the fingers on the same hand

90. Muscle Structure
• Each muscle fibre has its own motor nerve ending;
the neuromuscular junction is where the motor
neuron terminates on the muscle fibre
• The axon terminal is the enlarged tip of the motor
neuron; it contains sacs of the neurotransmitter
acetylcholine (ACh).
• The membrane of the muscle fibre is the
sarcolemma, which contains receptor sites for
acetylcholine, and an in-activator called
cholinesterase.

91. • Within the muscle fibre are thousands of individual
contracting units called sarcomeres, which are
arranged end to end in cylinders called myofibrils
• The Z lines are the end boundaries of a sarcomere.
• Filaments of the protein myosin are in the centre of
the sarcomere, and filaments of the protein actin are
at the ends, attached to the Z lines. Myosin filaments
are anchored to the Z lines by the protein titin.
• Myosin and actin are the contractile proteins of a
muscle fibre. Their interactions produce muscle
contraction.

92. • the myosin and actin filaments partially
interdigitate and thus cause the myofibrils to
have alternate light and dark bands
• The light bands contain only actin filaments
and are called I bands because they are
isotropic to polarized light.
• The dark bands contain myosin filaments, as
well as the ends of the actin filaments where
they overlap the myosin, and are called A
bands because they are anisotropic to
polarized light.

93. • Also present are two inhibitory proteins, Troponin
and Tropomyosin, which are part of the actin
filaments and prevent the sliding of actin and myosin
when the muscle fibre is relaxed.
• Surrounding the sarcomeres is the sarcoplasmi
reticulum, the endoplasmic reticulum of muscle
cells.
• The sarcoplasmic reticulum is a reservoir for calcium
ions (Ca2), which are essential for the contraction
process.

94. • All of these parts of a muscle fibre are involved in the
contraction process.
• Contraction begins when a nerve impulse arrives at
the axon terminal and stimulates the release of
acetylcholine.
• Acetylcholine generates electrical changes (the
movement of ions) at the sarcolemma of the muscle
fibre.
• These electrical changes initiate a sequence of
events within the muscle fibre that is called the
sliding filament mechanism of muscle contraction.

97. The Sliding Filament Theory/
Physiology of Muscle Contraction
• Nerve impulse causes depolarization of a
muscle fibre, and this electrical change
enables the myosin filaments to pull the actin
filaments toward the centre of the sarcomere,
making the sarcomere shorter.
• All of the sarcomeres shorten and the muscle
fibre contracts.