1. The Musculoskeletal System
Chapter 47

2. Types of Skeletal Systems
Changes in movement occur because muscles pull against a support structure, called the skeletal system
-Zoologists recognize three types:
-Hydrostatic skeletons
-Exoskeletons
-Endoskeletons

3. Hydrostatic Skeletons
Are found primarily in soft-bodied invertebrates, both terrestrial and aquatic
Locomotion in earthworms
-Involves a fluid-filled central cavity and surrounding circular & longitudinal muscles
-A wave of circular followed by longitudinal muscle contractions move fluid down body
-Produces forward movement

4. Longitudinal muscles contracted
Hydrostatic Skeletons
Anterior
Circular muscles
Longitudinal muscles
contracted
Longitudinal muscles contracted
contract, and anterior
end moves forward
contract, and segments
catch up
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5. Hydrostatic Skeletons
Locomotion in aquatic invertebrates
-Occurs by fluid ejections or jetting
-Jellyfish produce regular pulsations in bell
-Squeezing some of water contained beneath it
-Squids fill mantle cavity with sea water
-Muscular contractions expel water forcefully through the siphon, and the animal shoots backward

6. Jellyfish propelled upward Contractile fibers Bell pulsates expelled
Water
enters
bell
Contractile fibers
expelled
from bell
Bell
pulsates
Jellyfish
propelled
upward
Water expelled
from siphon
Squid
backward b. a.
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7. Exoskeletons The exoskeleton surrounds the body as a rigid hard case
-Composed of chitin in arthropods
An exoskeleton provides protection for internal organs and a site for muscle attachment
-However, it must be periodically shed, in order for the animal to grow
-It also limits body size

8. Endoskeletons
Endoskeletons are rigid internal skeletons that form the body’s framework and offer surfaces for muscle attachment
-Echinoderms have calcite skeletons, that are made of calcium carbonate
-Bone, on the other hand, is made of calcium phosphate

9. Endoskeletons Vertebrate endoskeletons have bone and/or cartilage
-Bone is much stronger than cartilage, and much less flexible
Unlike chitin, bone and cartilage are living tissues
-They can change and remodel in response to injury or physical stress

10. Endoskeletons The vertebrate endoskeleton is divided into:
-Axial skeleton = Forms axis of the body
-Supports the body and protects internal organs
-Appendicular skeleton = Set of limb bones and their associated pectoral girdle (forelimbs) or pelvic girdle (hindlimbs)

11. Endoskeleton Chitinous outer covering Exoskeleton Sagittal section
Vertebral column
Pelvis
Femur
Tibia
Fibula
Ulna
Radius
Humerus
Skull
Scapula
Ribs
Endoskeleton a. b.
axial skeleton
appendicular
skeleton
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12. Bone
Bone is a hard but resilient connective tissue that is unique to vertebrates
Bones can be classified by the two fundamental modes of development
-Intramembranous development (simple)
-E.g.: External bones of skull
-Endochondral development (complex)
-E.g.: Bones that are deep in the body

13. Bone Intramembranous development
-Osteoblasts initiate bone development
-Some cells become trapped in the bone matrix that they have produced
-Change into osteocytes, which reside in tight spaces called lacunae
-The cells communicate through little canals termed canaliculi
-Osteoclasts break down the bone matrix

14. Undifferentiated Mesenchymal Cells
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Undifferentiated Mesenchymal Cells
Chondroblast
Fibroblast
Osteoblasts
Chondrocyte
Collagen
(fibrous tissue)
Osteocyte
Osteoclast

15. Bone Endochondral development
-Endochondral bones begin as tiny cartilaginous models
-Bone development consists of adding bone to the outside of a cartilaginous model, while replacing interior cartilage with bone
-Calcification begins with the fibrous sheath, later called the periosteum

16. Bone Endochondral development
-Cartilage that remains after the development of epiphyses serves as a pad between bone surfaces
-Bones grow by lengthening and widening
-Growth in length usually ceases in humans by late adolescence
-Growth in width continues by bone addition just beneath the periosteum

17. Red marrow in spongy bone Capillary in Haversian canal Canaliculi
Lamellae
Compact
bone
Growth plate
transition to
medullary
Haversian
system
Outer layers
Sharpey’s
fibers
Medullary cavity
Medullary
cavity
Periosteum
(osteoblasts
found here)
Lacunae containing
osteocytes
Shaft
Epiphysis
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18. Bone Structure
In most mammals, bones retain internal blood vessels and are called vascular bones
-These typically have osteocytes and are also called cellular bones
-Vascular bone has a special internal organization termed the Haversian system
In birds and fishes, bones are avascular
-These typically lack osteocytes and are also called acellular bones

19. Bone Structure
Based on density and structure, bone falls into three categories
-Compact bone = Outer dense layer
-Medullary bone = Lines the internal cavity
-Contains bone marrow in vertebrates
-Spongy bone = Honeycomb structure
-Forms the epiphyses inside a thick shell of compact bone

20. Bone Remodeling The phenomenon of remodeling is known for all bones
-Small forces may not have a great effect
-But larger forces – if frequent enough – can initiate remodeling by osteoblasts
It is possible that mechanical stress in bones deforms the hydroxyapatite crystal, thus producing a piezoelectric effect

21. Bone Remodeling Force Reaction force Medullary cavity a. b. c. d.
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22. Joints Joints are the locations where one bone meets another
-1. Immovable joints = Join bones
-2. Slightly movable joints = Involve fibrous connective tissue or cartilage
-3. Freely movable joints = Also called synovial joints
-Contain a lubricating fluid

23. Slightly Movable Joints
Body of
vertebra
Fibrous
joints
Fibrous Joints
Intervertebral
disk
Articular
cartilage
connective
tissue
Bone
Suture
Immovable Joint a.
Slightly Movable Joints
Cartilaginous Joints b.
Synovial membrane
Synovial fluid
Fibrous capsule
Articular cartilage
Freely Movable Joint c.
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24. Joints Movable joints can be divided into four types
-Ball-and-socket joints = Permit movement in all directions
-Hinge joints = Allow movement in only one plane
-Gliding joints = Permit sliding of one surface over another
-Combination joints = Allow rotation and side-to-side sliding

25. Ball-and-Socket Combination Joint Hinge Joint Gliding Joint a. d. b.
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26. Skeletal Muscle Movement
Skeletal muscle fibers are attached to the periosteum of bones in one of two ways
-Directly; or through a strong, fibrous cord called the tendon
One attachment of the muscle, the origin, remains stationary during contraction
-The other end, the insertion, is attached to a bone that moves when muscle contracts

27. Skeletal Muscle Movement
Skeletal muscles occur in antagonistic pairs
-Agonist = Muscle group causing an action
-Antagonist = Muscle group that counters movement
Isotonic contraction – The force of contraction remains relatively constant as the muscle shorten in length
Isometric contraction – The length of the muscle does not change as force is exerted

28. Flexion Extension Endoskeleton Flexor Joint Exoskeleton Extensor
Flexors
(hamstrings)
Tendon
Exoskeleton
Endoskeleton
Joint
muscles
contract
Extensor
Extensors
(quadriceps)
Extension
Flexion a. b.
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29. Skeletal Muscle Structure
Each skeletal muscle contains numerous muscle fibers
-Each muscle fiber encloses a bundle of 4 to 20 elongated structures called myofibrils
-Each myofibril in turn is composed of thick and thin myofilaments

30. Tendon Muscle fascicle (with many muscle fibers) Plasma membrane
Skeletal
muscle
Muscle fascicle
(with many
muscle fibers)
Muscle fiber (cell)
Myofilaments
Myofibril
Plasma
membrane
Nuclei
Striations
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31. Skeletal Muscle Structure
A bands = Stacked thick & thin myofilaments
-Dark bands
H bands = Center of the A band, consisting of thick bands only
I bands = Consist only of thin myofilaments
-Light bands
-Divided into two halves by a disc of protein called the Z line
Sarcomere = Distance between two Z lines
-Smallest subunit of muscle contraction

32. Skeletal Muscle Contraction
A muscle contracts and shortens because the myofibrils contract and shorten
-Myofilaments themselves do not shorten
-Instead, the thick and thin filaments slide relative to each other
-Sliding filament mechanism
-Z lines move closer together, as the I and H bands become shorter
-A band does not change in size

33. Thin filaments (actin) Thick filaments (myosin)
Relaxed Muscle
Contracted Muscle
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H Band
A band
I band
Z line
Thin filaments (actin)
Thick filaments (myosin)

34. Skeletal Muscle Contraction
A thick filament is composed of several myosin subunits packed together
-Myosin consists of two polypeptide chains wrapped around each other
-Each chain ends with a globular head
A thin filament is composed of two chains of actin proteins twisted together in a helix

36. Skeletal Muscle Contraction
Muscle contraction involves a series of events called the cross-bridge cycle
-Hydrolysis of ATP by myosin, activates the head for the later power stroke
-The ADP and Pi remain bound to the head, which binds to actin forming a cross-bridge
-During the power stroke, myosin returns to its original shape, releasing ADP and Pi
-ATP binds to the head which releases actin

37. Cross-bridge formation
Myosin head
ATP hydrolysis
Cross-bridge
formation
Power stroke
ATP binding,
actin release
Actin
ADP Pi a. b. c.
ATP d.
Myosin
binding site
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38. Thin filaments (actin) Thick filament (myosin) Cross-bridges
Sarcomere
H band
A band
I band a. b.
Z line
Thin filaments (actin)
Thick filament (myosin)
Cross-bridges
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39. Skeletal Muscle Contraction
When a muscle is relaxed, its myosin heads cannot bind to actin because the attachment sites are blocked by tropomyosin
-In order for muscle to contract, tropomyosin must be removed by troponin
-This process is regulated by Ca2+ levels in the muscle fiber cytoplasm

40. Skeletal Muscle Contraction
-In low Ca2+ levels, tropomyosin inhibits cross-bridge formation
-In high Ca2+ levels, Ca2+ binds to troponin
-Tropomyosin is displaced, allowing the formation of actin-myosin cross-bridges
Ca2+ a. b.
Actin
Myosin
head
Troponin
Tropomyosin
Binding sites for
cross-bridges blocked
Binding sites for cross-bridges exposed
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41. Skeletal Muscle Contraction
A muscle fiber is stimulated to contract by motor neurons, which secrete acetylcholine at the neuromuscular junction
-The membrane becomes depolarized
-Depolarization is conducted down the transverse tubules (T tubules)
-Stimulate the release of Ca2+ from the sarcoplasmic reticulum (SR)

42. Skeletal Muscle Contraction
Myofibril
Na+
Sarcolemma
Neuromuscular
junction
Motor neuron
Nerve
impulse
Neurotransmitter
Sarcoplasmic
reticulum
Transverse
tubule
(T tubule)
Release
of Ca2+
Ca2+
Muscle depolarization
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43. Skeletal Muscle Contraction
A motor unit consists of a motor neuron and all of the muscle fibers it innervates
-All fibers contract together when the motor neuron produces impulses
Muscles that require precise control have smaller motor units
Muscles that require less precise control but exert more force, have larger motor units
Recruitment is the cumulative increase in motor unit number and size leading to a stronger contraction

44. Fewer Motor Units Activated More Motor
Muscle
fiber
Fewer Motor
Units Activated
More Motor
Motor
unit
Tapping Toe
Running a. b.
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45. Types of Muscle Fibers
A muscle stimulated with a single electric shock quickly contracts and relaxes in a response called a twitch
Summation is a cumulative response when a second twitch “piggy-backs” on the first
Tetanus occurs when there is no relaxation between twitches
-A sustained contraction is produced

46. ' ' ' ' ' ' Complete tetanus Incomplete
Amplitude of Muscle Contractions
Complete
tetanus
Twitches
Incomplete
Time
Summation
Stimuli
' ' ' ' ' '
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47. Types of Muscle Fibers
Skeletal muscle fibers can be divided based on their contraction speed
-Slow-twitch, or Type I, fibers
-Rich in capillaries, mitochondria and myoglobin (red fibers)
-Sustain action for long periods of time
-Fast-twitch, or Type II, fibers
-Poor in capillaries, mitochondria and myoglobin (white fibers)
-Adapted for rapid power generation

48. Types of Muscle Fibers
Skeletal muscles have different proportions of fast-twitch and slow-twitch fibers
Contraction Strength
Time (msec)
eye muscle (lateral rectus)
calf muscle (gastrocnemius)
deep muscle of leg (soleus)
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49. Types of Muscle Fibers
Skeletal muscles at rest obtain most of their energy from aerobic respiration of fatty acids
-During muscle use, energy comes from glycogen and glucose
The maximum rate of oxygen consumption in the body is called the aerobic capacity
Muscle fatigue is the use-dependent decrease in the ability to generate force
-Usually correlated with the production of lactic acid by the exercising muscle

50. Modes of Animal Locomotion
Locomotion in large animals involves:
-Appendicular locomotion
-Produced by appendages that oscillate
-Axial locomotion
-Produced by bodies that undulate, pulse or undergo peristaltic waves
The physical constraints to movement – gravity and frictional drag – occur in every environment, differing only in degree

51. Locomotion in Water Water’s buoyancy reduces effect of gravity
Some marine invertebrates move about using hydraulic propulsion
All aquatic invertebrates swim
-Swimming involves using the body or its appendages to push against the water
-An eel uses its whole body
-A trout uses only its posterior half

52. Eel Trout thrust reactive force lateral force push 90o a. b.
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53. Locomotion in Water
Many terrestrial tetrapod vertebrates are able to swim, usually through limb movement
-Most birds that swim propel themselves by pushing against water with their hind legs
-These typically have webbed feet
-Animals that swim with their forelegs usually have these modified as flippers and pull themselves through the water
-Sea turtles and penguins

54. Locomotion on Land Terrestrial locomotion deals mainly with gravity
Mollusks glide along a path of mucus
Vertebrates and arthropods have a raised body, and move forward by pushing against the ground with jointed appendages – legs
-Vertebrates are tetrapods; all arthropods have at least six limbs
-Having extra legs increases stability, but reduces the maximum speed

55. Locomotion on Land
The basic walking pattern of quadrupeds generates a diagonal pattern of foot falls
-Left hind leg, right foreleg, right hind leg, left foreleg
-Allows running by a series of leaps
Some vertebrates are also effective leapers
-Kangaroos, rabbits and frogs have powerful leg muscles

56. Locomotion on Land

57. Locomotion in Air Flight has evolved among animals four times
-Insects, pterosaurs (extinct flying reptiles), birds, and bats
-Propulsion is achieved by pushing down against the air with wings
In birds and most insects, wing raising and lowering is achieved by alternate contraction of extensor muscles (elevators) and flexor muscles (depressors)

58. Locomotion in Air
These different vertebrates all have lightened bones and forelimbs transformed into wings