CHAPTER 47
LECTURE
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The Musculoskeletal System
Chapter 47
Types of Skeletal Systems
• Changes in movement occur because
muscles pull against a support structure
• Zoologists recognize three types:
1. Hydrostatic skeletons
2. Exoskeletons
3. Endoskeletons
3
Hydrostatic Skeletons
• Found primarily in soft-bodied
invertebrates (terrestrial and aquatic)
• Locomotion in earthworms
– Involves a fluid-filled central cavity
(hydrostatic skeleton) and surrounding
circular and longitudinal muscles
– A wave of circular followed by longitudinal
muscle contractions move fluid down body
– Chaetae prevent slipping backward
4
Hydrostatic Skeletons
5
Exoskeletons
• Surrounds the body as a rigid hard case
• Composed of chitin in arthropods
• Provides protection for internal organs and a site
for muscle attachment
• Must be periodically shed in order for the animal
to grow
• Not as strong as a bony skeleton
• Respiratory system sets limit on body size
• Muscles cannot enlarge in size and power
6
Endoskeletons
• Rigid internal skeletons that form the
body’s framework and offer surfaces for
muscle attachment
• Echinoderms have calcite skeletons
– Made of calcium carbonate
• Vertebrate bone is made of calcium
phosphate
7
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
• Can change and remodel in response to
injury or physical stress
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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
1. Intramembranous development
• Bones form within a layer of connective tissue
2. Endochondral development
• Begin as tiny cartilaginous model
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Bone
• Intramembranous development
– Osteoblasts initiate bone development
– Some cells become trapped in the bone
matrix that they have produced
– Change into osteocytes
• Reside in tight spaces called lacunae
– The cells communicate through little canals
termed canaliculi
– Osteoclasts break down the bone matrix
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Bone
• Endochondral development
– Typically bones that are deeper in the body
– 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
– Trapped osteoblasts transform into
osteocytes
– Osteoclasts remodel bone
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Bone
• Endochondral development
– Increase in length unlike intramembranous
bone
– Limb bones have a shaft with epiphyses
– Epiphyseal growth plates separate epiphyses
from shaft
– Plates are cartilage in growing bone
– Growth pushes epiphysis away from shaft
– Cartilage becomes calcified
– Growth in length ends by late adolescence
– Growth in width does not
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16
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
– Lack osteocytes and are also called acellular
bones
17
Bone Structure
• Based on density and structure, bone falls
into three categories
1. Compact bone – outer dense layer
2. Medullary bone – lines the internal
cavity
• Contains bone marrow in vertebrates
• Bird bones are hollow
3. Spongy bone – forms the epiphyses
inside a thick shell of compact bone
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Bone Remodeling
• Bone is a dynamic tissue that can change
• Mechanical stress can remodel bone
during embryonic development and on
• Bone may thicken
• Size and shape of surface features
change in size and shape
• Large frequent forces can initiate
remodeling
• Weight-lifting is one osteoporosis
treatment
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Bone Remodeling
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Joints (articulations)
• Locations where one bone meets another
• 4 basic joint movement patterns
1. Ball-and-socket joints – permit movement in
all directions
2. Hinge joints – allow movement in only one
plane
3. Gliding joints – permit sliding of one surface
over another
4. Combination joints – movement
characteristics of two or more joint types
21
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Ball-and-Socket
a.
Hinge Joint
b.
Gliding Joint
c.
Combination Joint
d.
22
Skeletal Muscle Movement
• Skeletal muscle fibers are attached to
bones
– Directly to the periosteum
– Through a tendon attached to the periosteum
• 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
• Muscles can be antagonistic
– One counters the action of the other
23
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Flexion
Flexors
(hamstrings)
Tendon
24
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Flexion
Flexors
(hamstrings)
Tendon
Extension
Tendon
Extensors
(quadriceps)
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Muscle contraction
• 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
• Under a microscope, the myofibrils have
alternating dark and light bands – striated
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• A bands – stacked thick and thin
myofilaments
– Dark bands
– H band has interdigitating thick and thin
filaments
• 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
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Skeletal Muscle Contraction
• 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
• Thin filaments slide deeper into the A
bands, making the H and I bands narrower
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Skeletal Muscle Contraction
• Thick filament
– Composed of several myosin subunits packed
together
– Myosin consists of two polypeptide chains
wrapped around each other
– Each chain ends with a globular head
• Thin filament
– Composed of two chains of actin proteins
twisted together in a helix
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Skeletal Muscle Contraction
• Cross-bridge cycle
• Hydrolysis of ATP by myosin activates the
head for the later power stroke
• 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
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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
37
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
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Skeletal Muscle Contraction
• Muscle fiber is stimulated to contract by motor
neurons, which secrete acetylcholine at the
neuromuscular junction
• Membrane becomes depolarized
• Depolarization is conducted down the transverse
tubules (T tubules)
• Stimulate the release of Ca2+ from the
sarcoplasmic reticulum (SR)
• Excitation–contraction coupling
– Release of Ca2+ that links excitation by motor neuron
to contraction of the muscle
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Skeletal Muscle Contraction
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Skeletal Muscle Contraction
• Motor unit
– 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
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stronger contraction
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2 Types of Muscle Fibers
• A muscle stimulated with a single electric
shock quickly contracts and relaxes in a
response called a twitch
• Summation of closely spaced twitches
• Tetanus – sustained contraction with no
relaxation between twitches
• Skeletal muscles divided on the basis of
their contraction speed
– Slow-twitch or type I fibers
– Fast-twitch or type II fibers
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2 Types of Muscle Fibers
• 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 to respire anaerobically
– Adapted for rapid power generation
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2 Types of Muscle Fibers
• Skeletal muscles have different
proportions of fast-twitch and slow-twitch
fibers
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Types of Muscle Fibers
• Skeletal muscles at rest obtain most of their
energy from aerobic respiration of fatty acids
• During use, energy comes from glycogen and
glucose
• 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
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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
49
Locomotion in Water
• Water’s buoyancy reduces effect of gravity
• Primary force retarding forward movement is
frictional drag
• 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
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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
– 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
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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
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Locomotion on Land
• 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
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Locomotion on Land
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Locomotion in Air
• Flight has evolved among animals four times
• Insects, pterosaurs (extinct flying reptiles), birds,
and bats
– Convergent evolution
• All three vertebrate fliers modified the forelimb
into a wing structure, but they did so in different
ways
– Birds have wing built on a single support
– Bat wings built on multiple supports – finger bones
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Flying Vertebrates
Giraffe
Bat
Platypus
Turtle
Crocodile
Pterosaur
Dinosaurs
Hawk
Polyphyletic Group
a.
Bat
Pterosaur
Hawk
b.
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