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Chapter 9
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Muscle is derived from Musculus, for “Mouse”
Imagine a mouse running beneath the skin.
Functions of Muscles:
1. Body movement
2. Maintain posture
3. Produces heat
4. Propel substances
through body
5. Heartbeat
6. Breathing
Types of muscles include:
1. Smooth muscle
2. Cardiac muscle
3. Skeletal muscle
Characteristics of smooth muscles
• Involuntary control
• Tapered cells with a single, central nucleus
• Lack striations
There are two types of smooth muscles
Autonomic neuron varicosity (swelling)
• Multi-unit Smooth Muscle
• unorganized cells that contract
as individual cells
•Located within the iris of eye
and the walls of blood vessels
• Visceral (single-unit) Smooth Muscle
• Form sheets of muscle
• Cells are connected by gap junctions
• Muscle fibers contract as a group
• Rhythmic contractions
• Within walls of most hollow organs
(viscera)
Autonomic neuron varicosity (swelling)
Gap junction
• Cardiac Muscle
• Located only in the heart
• Striated cells
• Intercalated discs contain:
• gap junctions and
desmosomes
• Branching muscle fibers,
with a single central nucleus
• Muscle fibers contract
as a unit (syncytium)
• Self-exciting and rhythmic
• Skeletal Muscle
• Usually attached to bone or
other connective tissue
• Voluntary control
• Striated (light & dark bands)
• Muscle fibers form bundles
• Several peripheral nuclei
Fascia
• Dense connective tissue surrounding skeletal muscles
• Superficial fascia – beneath skin
• Deep fascia – covers muscles
• Serous fascia – surrounds serous membranes
Tendons
• Dense connective tissue that attaches muscle to bones
• Continuation of muscle fascia and bone periosteum
Aponeurosis
• Broad sheet of connective tissue attaching muscles to
bone, or to other muscles.
epicranial aponeurosis
Epimysium
• Connective tissue that covers the
entire muscle
• Lies deep to fascia
Perimysium
• Surrounds organized bundles of
muscle fibers, called fascicles
Endomysium
• Connective tissue that covers
individual muscle fibers (cells)
Fascicle
Organized bundle of muscle fibers
Muscle Fiber
Single muscle cell
Collection of myofibrils
Myofibrils
Collection of myofilaments
Myofilaments
Actin filament
Myosin filament
Muscle Fiber
Single muscle cell
Collection of myofibrils
Myofibrils
Collection of myofilaments
Myofilaments
Actin (thin) filament
Myosin (thick) filament
Sarcolemma
• Cell membrane of muscle fibers
Sarcoplasm
• Cytoplasm of muscle fibers
Sarcoplasmic Reticulum
• Modified Endoplasmic Reticulum
• Stores large deposits of Calcium
(Transverse)T-tubules:
• invaginations of sarcolemma,
extending into the sarcoplasm.
Cisternae:
• enlarged region of sarcoplasmic
reticulum, adjacent to the t-tubules
Triad
• T-tubule + adjacent cisternae
Myofibrils are bundles of
actin and myosin filaments.
• Actin – thin filament
• Myosin – thick filament
Striations appear from the
organization of actin and
myosin filaments
A sarcomere is the functional unit
of skeletal muscle
• A sarcomere is the area between
adjacent Z-lines.
•During a muscle contraction the zlines move together and the
sarcomere shortens.
Striations appear from alternate
light and dark banding patterns.
Z Line (disc) is the attachment site of
actin filaments (center of I bands)
I Bands (light band): consists of only
actin filaments
A Bands (dark band) : consists of
myosin filaments and the overlapping
portion of actin filaments
Thin filaments
composed of actin proteins
Thin filaments are
associated with troponin
and tropomyosin proteins
Thick filaments
composed of myosin proteins
During muscle contraction the
heads on myosin filaments
bind to actin filaments forming
a Cross-bridge
When a muscle is at rest, myosin heads are extended in the “cocked” position.
During a contraction, myosin heads bind to actin, forming a cross-bridge and the
myosin head pivot forward (Power Stroke) and back (Recovery stroke)
The troponin-tropomyosin complex controls the
activation of cross-bridge cycling (contraction)
Tropomyosin
• Blocks binding sites on actin when the
muscle is at rest
• Contraction (cross-bridge-cycling) begins
when tropomyosin is repositioned.
Troponin
Ca2+ binds to troponin
during a muscle contraction.
Troponin moves repositions the tropomyosin
filaments, so the myosin and actin filaments
can interact, triggers cross-bridge cycling
Synapse: Functional (not physical) junction
between an axon of a neuron and another cell
The two cells are separated by a physical
space, called the synaptic cleft.
Neurotransmitters are stored within
synaptic vesicles of the presynaptic cell
and they’re released into the synapse.
Neuromuscular Junction
Neuromuscular Junction (NMJ) refers to the
synapse between an axon and a muscle fiber.
Motor End Plate is a highly folded region
of muscle fiber at NMJ that contain
abundant mitochondria
Motor Unit
Motor neurons innervate
effectors (muscles or glands)
A motor unit includes a
motor neuron and all of the
muscle fibers it controls
1 motor unit may control between
1 and 1000 muscle fibers
motor neuron
muscle fibers
Stimulus for Contraction
Acetylcholine (ACh) is the only neurotransmitter
that initiates skeletal muscle contraction
Sequence of Actions
1. Decide to move (voluntary control)
2. An action potential (nerve impulse)
travels down axon to axon terminal.
3. Calcium channels open at the axon
terminal, Calcium diffuse into axon
4. Exocytosis of Ach from secretory
vesicles into synaptic cleft.
Stimulus for Contraction
Sequence of Actions…Continued
5. ACh binds to receptors on motor endplate,
opening Na+ channels
6. Na+ diffuses into the muscle, triggering a
muscle impulse (action potential).
7. The muscle impulse diffuses across
sarcolemma and down the t-tubules into
the cisternae of sarcoplasmic reticula.
8. The sarcoplasmic reticula release
their calcium supplies into the sarcoplasm.
Stimulus for Contraction
Sequence of Actions…Continued
7. The muscle impulse diffuses across
sarcolemma and down the t-tubules into
the cisternae of sarcoplasmic reticula.
8. The sarcoplasmic reticula release
their calcium supplies into the sarcoplasm.
9. Calcium binds to troponin and the
troponin repositions the tropomyosin,
exposing actin filaments to the myosin heads.
10. Cross-bridge cycling causes contraction of the
muscle.
Sliding Filament Theory of Contraction
During a muscle contraction
Thick (myosin) filaments and thin (actin)
filaments slide across one another
The filaments do not change lengths
Z-bands move closer together causing the
sarcomere to shorten.
I bands appear narrow
Cross Bridge Cycling
During a contraction, Calcium binds to troponin.
Tropomyosin is repositioned, exposing the myosin
binding sites on actin filaments
Cross Bridge Cycling
1.
Cross-Bridge Formation
• Myosin heads bind to actin filaments.
• The phosphate is released.
2.
Power Stroke
• Myosin heads spring forward pulling the
actin filaments.
• ADP is released from Myosin
3. Cross-Bridge Release
• New molecule of ATP binds to myosin
• Myosin head is released from actin.
4. Recovery Stroke
• ATP is hydrolyzed into ADP + P
• Energy is used to return myosin to
cocked position
Cross Bridge Cycling
Cross Bridge Cycling continues until Calcium is
removed from cytosol and tropomyosin covers
binding sites on actin filaments.
Major Events of Muscle Fiber Contraction
1. Decision to move
2. Action Potential on Motor Neuron
3. Calcium Diffuses into axon terminal
4. Exocytosis of ACh into synaptic cleft
5. ACh binds to receptors on motor end plate
6. Na+ diffuses into muscle fiber, initiating a muscle impulse
7. Muscle Impulse (action potential) reaches Sarcoplasmic
Reticulum
8. Sarcoplasmic Reticulum releases Calcium into sarcoplasm
9. Ca2+ binds to troponin
10. Troponin repositions tropomyosin
11. Cross-Bridge Cycling
• Cross-bridge Formation
• Power Stroke
End of Chapter 9, Section 3
• Cross-bridge Release
• Recovery Stroke
Relaxation
When a nerve impulse ceases, two events relax muscle fibers.
1.
Acetylcholinesterase breaks down Ach in the synapse.
• Prevents continuous stimulation of a muscle fiber.
2.
Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm and
returns it to the SR.
• Without calcium, tropomyosin covers the binding sites on actin
filaments.
Relaxation
Rigor Mortis is a partial contraction of skeletal muscles that occurs a few
hours after death.
• After death calcium leaks into sarcoplasm, triggering the muscle
contractions.
• But ATP supplies are diminished after death, so ATP is not available to
remove the cross-bridge linkages between actin and myosin.
• muscles do not relax*.
• Contraction is sustained until muscles begin to decompose.
* Notice that ATP is required for muscle relaxation!
Energy Sources for Contraction
ATP provides the energy to power the interaction between
actin & myosin filaments.
• However, ATP is quickly spent and must be replenished
New ATP molecules are synthesized by
1. Creatine Phosphate
2. Glycolysis (anaerobic respiration)
3. Aerobic Respiration
Energy Sources for Contraction
Creatine Phosphate
Creatine Phosphate can be hydrolyzed into Creatine +
Phosphate, releasing energy that is used to make new ATP.
The energy from creatine phosphate
hydrolysis cannot be used to directly
power muscles.
Instead, it’s used to produce new ATP.
Energy Sources for Contraction
Creatine Phosphate Continued
When cellular ATP is abundant, creatine
phosphate can be replenished by phosphorylating
creatine.
Creatine Phosphate provides energy for only about
10 seconds of a high intensity muscle contraction.
Energy Sources for Contraction
Glycolysis
Anaerobic respiration (glycolysis) occurs in the
cytosol of the cell and does not require oxygen.
Glucose molecules are partially broken down
producing just 2 ATP for each glucose.
If there isn’t sufficient oxygen available,
glycolysis produces lactic acid as a byproduct.
Energy Sources for Contraction
Aerobic Respiration
Aerobic respiration (uses oxygen) occurs in the
mitochondria and it includes the citric acid
cycle & electron transport chain.
Aerobic respiration is a slower reaction than
glycolysis, but it produces the most ATP.
Myoglobin
Oxygen binding protein (similar to hemoglobin)
within muscles
-Provides additional oxygen supply to muscles
Energy Sources for Contraction
During rest or moderate exercise, respiratory & cardiovascular systems
supply enough O2 to support aerobic respiration
Anaerobic (Lactic Acid) Threshold:
Shift in metabolism from aerobic to anaerobic, during strenuous muscle
activity, when the above systems cannot supply the necessary O2. Lactic
acid is produced.
Oxygen debt:
Amount of oxygen needed by liver cells to convert the accumulated lactic
acid to glucose, and to restore muscle ATP and creatine phosphate
concentrations.
Aerobic Respiration
Aerobic respiration is used primarily at rest or during
light exercise.
Muscles that rely on aerobic respiration have plenty of
mitochondria and a good blood supply.
Oxygen debt of glycolysis
Exercise and strenuous activity depends on
anaerobic respiration for ATP supplies.
During exercise anaerobic respiration causes
lactic acid to accumulate in the cells.
After exercise, when oxygen is available
the O2 is used to convert lactic acid back to
glucose in the liver.
Oxygen debt is the amount of oxygen
needed by liver cells to convert
accumulated lactic acid back to glucose.
Muscle Fatigue
• Muscle Fatigue = Inability for the muscle to contract
• Several factors can cause muscle fatigue:
• Decreased blood flow
• Ion imbalances across the sarcolemma
• Lactic acid accumulation – (greatest cause of fatigue)
• Cramp:
• A cramp is a sustained, involuntary, and painful muscle
contraction
• It’s due to electrolyte imbalance surrounding muscle
Heat Production
• Heat is produced as a by-product of cellular respiration
• Muscle cells are major source of body heat
• Blood transports heat throughout body core
Muscle Response
A muscle contraction can be observed by removing a single skeletal
muscle and connecting it to a device (myograph) that senses and
records changes in the overall length of the muscle fiber.
A threshold stimulus is the minimum
stimulus that elicits a muscle fiber contraction
all-or-none response
A threshold stimulus will cause the muscle
fiber to contract fully and completely.
A stronger stimulus does not produce a
stronger contraction!
subthreshold stimulus
The muscle fiber will not contract at all if
the stimulus is less than threshold.
Recording of a Muscle Contraction
A twitch is a single contractile response to a stimulus
A twitch can be divided into three periods.
1. Latent period
brief delay between the stimulus
and the muscle contraction
The latent period is less than 2
milliseconds in humans
2. Period of contraction
3. Period of relaxation
Summation
If the muscle is allowed to relax
completely before each stimulus than the
muscle will contract with the same force.
If the muscle is stimulated again before
it has completely relaxed, then the force
of the next contraction increases.
i.e. stimulating the muscle at a rapid
frequency increases the force of
contraction. This is called summation
Summation
Tetanic Contraction (c)
If the muscle is stimulated at a high frequency the contractions fuse
together and cannot be distinguished.
A tetanic contraction results in a maximal sustained contraction without
relaxation
Recruitment of Motor Units
all-or-none response
A muscle that is stimulated with threshold potential contracts
completely and fully.
A stronger stimulus does not produce a stronger contraction!
Instead, the strength of a muscle is increased by recruitment of
additional motor units.
Recruitment of Motor Units
Recruitment – progressive activation of motor units to
increase the force of a muscle contraction.
Recall that a motor unit is a motor neuron plus all of the fibers it controls.
• Muscles are composed of many motor units.
• As a general rule, motor units are recruited in order of their size
• Small motor units are stimulated with light activities, but additional
motor units are recruited with higher intensity activity.
As the intensity of stimulation increases,
recruitment of motor units continues until
all motor units are activated.
Sustained Contractions
The central nervous system can increase the
strength of contractions in 2 ways:
1.
Recruitment
•
Smaller motor units are recruited first, followed by larger motor units.
•
The result is a sustained contraction of increasing strength.
2.
Increased firing rate
•
A high frequency of action potentials results in summation of the muscle
contractions.
•
If the frequency is too high, however, it may produce tetanic contractions, in which
case the muscle does not relax.
Muscle tone is produced because some muscles are in a continuous
state of partial contraction in response to repeated nerve impulses from
the spinal cord.
Types of Contractions
Isotonic – muscle contracts and changes length
Concentric – shortening of muscle (a)
Eccentric – lengthening of muscle (b)
Isometric – muscle contracts but does not
change length (c)
Isometric contractions stabilizes posture
and holds the body upright
Fast twitch and slow twitch muscle fibers
Fast & Slow twitch refers to the contraction speed, and to whether muscle
fibers produce ATP oxidatively (by aerobic respiration) or glycolytically
(by glycolysis)
Slow-twitch fibers (Type I)
• Always oxidative and resistant to fatigue
• Contain myoglobin for oxygen storage “red fibers”
• Also have many mitochondria for aerobic respiration
• Good blood supply
Slow Twitch Fibers
Slow-twitch fibers (Type I)
Slow-twitch fibers are best suited for endurance
exercise over a long period with little force.
Slow-twitch fibers rely on aerobic respiration
for energy (ATP) and are resistant to fatigue.
Slow-Twitch fibers contain abundant myoglobin
for oxygen storage “red fibers” and mitochondria
to carry out aerobic respiration.
Because of their oxygen demands, slow-twitch
fibers have a good blood supply.
Geese, known for their sustained flights have
predominately red (Slow-Twitch) pectoralis muscle
fibers. The reddish appearance is from the large
amount of myoglobin.
Fast Twitch Fibers
Fast-twitch glycolytic fibers (Type IIb)
Fast-twitch glycolytic fibers contract rapidly and
with great force, but they fatigue quickly.
They are best suited for rapid
contractions over a short duration.
Fast-twitch glycolytic fibers (type IIa) contain
very little mitochondria and myoglobin and are
“white fibers”
Chickens can only fly in short bursts, so the
chicken breast is composed of primarily fasttwitch fibers for the power of liftoff. These
muscle can act more powerfully but they fatigue
quickly. They have smaller blood supply, so the
breast meat is light.
Fast Twitch Fibers
Fast-twitch Intermediate fibers (Type IIa)
Fast-twitch intermediate or fast oxidative fibers
contain intermediate amounts of myoglobin.
They contract rapidly but also have the capacity
to generate energy through aerobic respiration.
Attribution
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Muscular System Anterior View By Termininja (Pectoralis major.png Tibial anterior.png) [CC BY-SA 3.0
(http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/1/13/Muscular_system.svg
Wood Mouse Rasbak [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)],
via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/bd/Apodemus_sylvaticus_bosmuis.jpg
Flexed Arm Supinate By EncycloPetey assumed (based on copyright claims). [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/4/41/Arm_flex_supinate.jpg
Smooth Muscle Contraction By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/5/54/1028_Smooth_Muscle_Contraction.jpg
Cardiac Muscle By OpenStax CNX [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/6/6b/2017abc_Cardiac_Muscle.jpg
Skeletal, Smooth, Cardiac Muscle Fibers By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via
Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/e5/414_Skeletal_Smooth_Cardiac.jpg
Lateral Head Muscles By Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5
(http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/d/d5/Lateral_head_anatomy_detail.jpg
Skeletal Muscle Anatomy https://upload.wikimedia.org/wikipedia/commons/8/89/Illu_muscle_structure.jpg
Muscle Fibers Anatomy By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
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Muscle Fibers By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/9/9c/1022_Muscle_Fibers_%28small%29.jpg
T-tubules By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/9/94/1023_T-tubule.jpg
Attribution
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Myofibril By Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN
20018762. (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/6/6f/Blausen_0801_SkeletalMuscle.png
Sarcomere By Slashme at English Wikipedia When using this image in external works, it may be cited as follows: Richfield,
David. "Medical gallery of David Richfield 2014". Wikiversity Journal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 20018762. (http://en.wikipedia.org/wiki/Sarcomere) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0
(http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/6/6e/Sarcomere.svg
Thin and Thick Filaments By Raul654 CCBY SA3.0 https://upload.wikimedia.org/wikipedia/commons/6/66/Thin_filament.jpg
Sliding Filament By Gal gavriel (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia
Commons
https://upload.wikimedia.org/wikipedia/commons/b/b3/%D7%9E%D7%91%D7%A0%D7%94_%D7%94%D7%9E%D7%95%D7
%9C%D7%A7%D7%95%D7%9C%D7%94_-_Sliding_filament.gif
Cross-Bridge-Cycling By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/2/24/1008_Skeletal_Muscle_Contraction.jpg
Synapse By The original uploader was Nrets at English Wikipedia (Transferred from en.wikipedia to Commons.) [GFDL
(http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/b/b2/SynapseIllustration2.png
Neuromuscular Junction By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
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Contraction New with Muscle Fibers By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via
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Neuron Hand Tuned Quasar Jarosz at English Wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or
GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/b/bc/Neuron_Hand-tuned.svg
Attribution
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Motor End Plate Innervation By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/5/57/1009_Motor_End_Plate_and_Innervation.jpg
Skeletal Muscle Contraction By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/2/24/1008_Skeletal_Muscle_Contraction.jpg
Muscle Metabolism By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/f/f6/1016_Muscle_Metabolism.jpg
Mitochondrion By Mariana Ruiz Villarreal LadyofHats [Public domain], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/3/3b/Animal_mitochondrion_diagram_en.svg
Liver By Mikael Häggström (w:User:Mikael Häggström) [Public domain], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/d/d4/Liver.svg
Muscle Twitch Myogram By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/b/b5/1012_Muscle_Twitch_Myogram.jpg
Summation and Tetanus By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
Commons https://upload.wikimedia.org/wikipedia/commons/d/d4/1013_Summation_Tetanus.jpg
Types of Contractions By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia
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Canada Goose By Image taken bu Alan D. Wilson, and modified by Diliff (cropped and noise reduction applied). (Image:Canada
goose flight - natures pics.jpg) [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/e/e4/Canada_goose_flight_cropped_and_NR.jpg
Chicken By Pete Cooper [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
https://upload.wikimedia.org/wikipedia/commons/3/32/Buff_Orpington_chicken%2C_UK.jpg