8 muscle i sr2002 2013 al
TRANSCRIPT
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Skeletal Muscle I
SR2002
Dr. Arimantas Lionikas
October 24, 2013
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Muscle IPlan
• Muscles and connective tissue• Structure and components of
muscle fibre• Muscle contraction
Reading list:1. Enoka R. Neuromechanics of human movement. 2008. Publishers: Human Kinetics, p. 205-213; 2. MacIntosh, B.R., Gardiner, P.F. McComas, A.J. Skeletal muscle, 2nd edition. 2006. Publishers: Human Kinetics, p. 151-160.
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Muscles in numbers• Humans have ~660 skeletal muscles• Average muscle contains ~100,000
fibres• Diameter of muscle fibres is 10 -100 m• Muscle fibre length is 1-400 mm • Note:
Ratio of fibre length / muscle length is 0.2 – 0.6(Muscle fibres do not go from end to end of muscles)
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Muscles and connective tissue• Skeletal muscles are connected
to bones by tendons• Muscles are enclosed by
epimysium (connective tissue)• Groups of muscle fibres form
muscle fascicles surrounded by perimisium
• Muscle fibres are enlosed by endomysium
• Connective tissue maintains integrity of skeletal muscles and is important in transmission of force. Connective tissue makes around 6% of total dry muscle weight.
• How are muscle fibres connected to endomysium? Via focal adhesions, consisting of a number cytoskeletal anchor proteins.
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Muscle shape and arrangement of fibres
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Contractile machinery of the muscle• Muscle fibres (10-100
μm) are covered by sarcolemma (≈7.5 nm thick)
• T-tubules are invaginations of sarcolemma
• Sarcoplasmic reticulum is a network of tubes surrounding myofibrils
• Sarcoplasm is fluid enclosed within the fibre
• Myofibrils ( ~1 μm) are packed bundles of myofilaments running along the fibre
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Essentials of muscle contraction• Myofibril contains two
myofilaments, known as think and thin filaments. Each filament is composed of several proteins.– Thick filaments consist of
myosin and myosin-binding proteins: C protein, H protein, M protein, myomesin
– Thin filament is composed of actin, tropomyosin and troponin complex (TN-T, TN-I, TN-C
• Myofilaments within myofibril are arranged in a series of repeating units, the sarcomere, which is basic contractile unit of muscle.
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Sarcomere• One end of thin filaments projects into the sarcomere while another connects
the Z line• Thin filaments contain troponin and tropomyosin proteins that participate in
blocking and unblocking of thin filaments. Because of this function troponin and tropomyosin are referred to as regulatory proteins.
• Thick filaments (myosin) are in the centre of sarcomere and overlap thin filaments from both sides
Myosin head
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A Closer Look: Myosin
Myosin light chains
ATPase
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Cytoskeletal proteins of the sarcomere
Prado et al. J. Gen. Physiol. 2008:126:461-480
Titin acts as a molecular spring permitting return of stretched sarcomeres to “optimal” length.
What holds actin and myosin in place?
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• Myosin heads drag thin filaments from both ends towards each other
• The distance between Z lines shortens (sarcomere shortens)
• Shortening of sarcomeres in series add up.
• Will variation in the number of sarcomeres affect contraction speed of a fibre?
Muscle contraction
≈ 3.0
≈ 2.0
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Force-length relationship (sarcomere)
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Cross-bridge cycle• Cross bridges circulate between different states:
1) No binding; 2) Weak binding; 3) Power stroke; 4) Strong binding; 5) Rigor
• Velocity of muscle shortening is determined by the rate of transition between these states
1
2
3
4
5
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Myosin
Ca2+ATPase
Sarcolema
Ca2+SR
T-tubule
DHP
RYR
Relaxation
Myosin
Myosin binding site
AP
Contraction
ECC
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Muscle contraction: in vivo & in vitroElectrodes
Dynamometerstrap
Force
5 mm
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Force–velocity relationshipMuscle fibre
• Max speed of shortening increases with a decrease in external load (force)
• Muscle force is greatest in lengthening (eccentric) contractions
• Muscle cross-bridges are stretched in eccentric contractions
Ecc. Con.
• Ecc is eccentric (muscle lengthens)• Con is concentric (muscle shortens)
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Muscle ISummary
• Muscles contain significant amounts of connective tissue
• Muscle fibres are muscle cells which contain myofibrils with contractile elements
• Sarcomere is the basic contractile unit. Myosin filaments in the middle of the sarcomere pull actin filaments from both sides
• Muscle contraction is based on conformational changes in the shape an orientation of myosin heads due to ATP hydrolysis
• Speed of contraction decreases with an increase in resistance
• Cytoskeletal proteins provide a frame for actin and myosin filaments