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8/13/2019 Sliding theory
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Muscle Cross Section
Bundles of fibres /
fibers Fasciculi
connective tissue
Perimysium. Each Fasciculicontains
10 to 100 muscle
fibres
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Sliding Theory of Contraction
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Muscle Fibers
Muscle fibers can rangefrom 10 to 80micrometers indiameter and may beup to 35cm long.
A large strong muscle,such as thoses formingyour Quadriceps wouldhave a large number offibers within eachbundle.
A smaller muscle usedfor precisionmovement, such asthose in the hand wouldcontain far fewer fibresper Fasciculi.
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Sarcolemmafibres cell membrane
Sarcoplasmcells cytoplasm
Myofibrils: cylindrical organelles
Sarcoplasmic Reticulum: network of tubules,through which nerve impulses travel,Calcium storage,
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Myofibrils
Each muscle fiber contains hundreds to
thousands of Myofibrils.
These are bundles of Actin and Myosin
proteins which run the length of the muscle
fiber and are important in muscle contraction.
Each Myofibril can be broken down into
functional repeating segments called
Sarcomeres.
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Sliding Theory
At a very basic level each muscle fibre is made
up of smaller fibres called myofibrils.
These contain even smaller structures called
actin and myosin filaments.
These filaments slide in and outbetween each
other to form a muscle contraction, hence
called the sliding filament theory.
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Structures Invoved
Myofibril:A cylindrical organelle running the length of themuscle fibre, containing Actin and Myosin filaments.
Sarcomere: The functional unit of the Myofibril, dividedinto I, A and H bands.
Actin: A thin, contractile protein filament, containing'active' or 'binding' sites.
Myosin: A thick, contractile protein filament, withprotusions known as Myosin Heads.
Tropomyosin:An actin-binding protein which regulates
muscle contraction. Troponin:A complex of three proteins, attached to
Tropomyosin.
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Sarcomere
Part of myofibril
This is the smallest unit of skeletal muscle that
can contract.
Sarcomeres repeat themselves over and over
along the length of the myofibril.
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The process of a muscle contracting can be
divided into 5 sections:
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Step 1
1. A nervous impulse arrives at theneuromuscular junction, which causes arelease of a chemical called Acetylcholine.
The presence of Acetylcholine causes thedepolarisation of the motor end plate whichtravels throughout the muscle by thetransverse tubules, causing Calcium (Ca+) tobe released from the sarcoplasmic reticulum.
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2. In the presence of high concentrations of Ca+,
the Ca+ binds to Troponin, changing its shape
and so moving Tropomyosin from the active
site of the Actin. The Myosin filaments can
now attach to the Actin, forming a cross-
bridge.
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3 The breakdown of ATP releases energy which
enables the Myosin to pull the Actin
filaments inwards and so shortening the
muscle.
This occurs along the entire length of every
myofibril in the muscle cell.
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4 The Myosin detaches from the Actin and thecross-bridge is broken when an ATP moleculebinds to the Myosin head.
When the ATP is then broken down the Myosinhead can again attach to an Actin binding sitefurther along the Actin filament and repeat the'power stroke'.
This repeated pulling of the Actin over themyosin is often known as the ratchetmechanism.
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5 This process of muscular contraction can last
for as long as there is adequate ATP and Ca+
stores.
Once the impulse stops the Ca+ is pumped
back to the Sarcoplasmic Reticulum and the
Actin returns to its resting position causing
the muscle to lengthen and relax.
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Stretched Muscle
the I - bands and the H -zone is elongated dueto reduced overlappingof the myosin and actin
filaments.
There would bereduced musclestrength because fewcross bridges can formbetween actin andmyosin.
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Partially Contracted Muscle
more overlapping of the
myosin and actin with
lots of potential for
cross bridges to form. The I - bands and H -
zone are shortened.
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Fully Contracted Muscle
Because the thin actin
filaments have
overlapped there is a
reduced potential forcross bridges to form
again.
Therefore there will be
low force productionfrom the muscle.