Muscle. Movement with muscles movement is one of the most prominent characteristics of animal life...
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Transcript of Muscle. Movement with muscles movement is one of the most prominent characteristics of animal life...
Movement with muscles
• movement is one of the most prominent characteristics of animal life
• it can be either amoeboid, or more complicated using flagella, cilia or muscles
• Galenus (2.c. BC) – “animal spirit” is flowing from the nerves into the muscles causing swelling and shortening
• spiral shortening of proteins was the supposed mechanism until the 50’s
• new research techniques such as EM helped to elucidate the exact mechanism
• muscles can be either smooth or striated• two subtypes of striated muscles are
skeletal and heart muscle• mechanism of contraction is identical in
all muscle types
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Structure of the skeletal muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-1.
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Ultrastructure of the striated muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-2.
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Sarcomeres in cross-section
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-3.
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Structure of the thin filament
• G-actin: globular, 5.5 nm spheres• polymerized to “necklace” – two necklaces
form a helical structure – F-actin• F-actins (length about 1000 nm, width 8
nm) are anchored to z-discs (-actinin)• in the groove of the F-actin tropomyosin
(40 nm) troponin complexes are found• tropomyosin-troponin regulates actin-
myosin interaction
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-5.
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The thick filament
• the thick filament is built up of myosin molecules
• myosin molecules consist of two heavy chains (length 150 nm, width 2 nm) and 3-4 (species dependent) light chains
• heavy chains form -helices twisted around each other bearing globular heads at the end
• myosin molecules associate to form the thick filament (length 1600 nm, width 12 nm)
• head regions are arranged into “crowns” of three heads at intervals of 14.3 nm along the thick filament
• successive crowns are rotated by 40° resulting in a thick filament with 9 rows of heads along its length
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Structure of the myosin filament
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4, 6.
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Sliding filament theory
• during contraction A-band is unchanged, I-band shortens
• length of actin and myosin filaments is unchanged
• H.E. Huxley and A.F. Huxley independently described the sliding filament theory: actin and myosin are moving along each other
• best proof is the length-tension curve, longer overlap stronger contraction
• sliding is caused by the movement of cross-bridges connecting filaments
• contraction is initiated by Ca++ ions released from the SR
• excitation propagating on the sarcolemma is conducted to the SR by T-tubules invaginating at the level of z-disks
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Tubules in the muscle fiber
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-21.
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Connection of T-tubules and SR
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-25.
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Release of Ca++ ions
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4.
• AP – spreads from the sarcolemma to the T-tubule – conformational change of the voltage-dependent dihydropyridin receptor – displacement or conformational change of the ryanodin receptor – Ca++ release
• half of the ryanodin receptors are free and are opened by the Ca++ ions - trigger Ca++
• restoration by Ca++-pump
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Mechanism of sliding• released Ca++ binds to the troponin complex,
myosin binding site on actin is freed • cross-bridge cycle runs until Ca++ level is high• one cycle 10 nm displacement
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-11.
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Energetics of the contraction
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-29.
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Types of muscle fibers• tonic fibers
– postural muscles in amphibians, reptiles and birds
– muscle spindles and extraocular muscles in mammals
– no AP, motor axon forms repeated synapses– slow shortening – effective isometric contraction
• slow-twitch (type I) fibers– mammalian postural muscles– slow shortening, slow fatigue – high myoglobin
content, large number of mitochondria, rich blood supply – red muscle
• fast-twitch oxidative (type IIa) fibers– specialized for rapid, repetitive movements –
flight muscles of migratory birds– many mitochondria, relatively resistant to
fatigue
• fast-twitch glycolytic (type IIb) fibers– very fast contraction, quick fatigue– few mitochondria, relies on glycolysis– breast muscles of domestic fowl – white muscle
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Motor unit• skeletal muscles in vertebrates are
innervated by spinal or brainstem motoneurons – “final common pathway”
• one fiber is innervated by only one motoneuron
• one motoneuron might innervate several fibers (usually about 100) – motor unit
• 1:1 synaptic transmission - 1 AP, 1 twitch• regulation of tension
– AP frequency - tetanic contraction– recruitment – involvement of additional motor
units
• depending on the task, different types of fibers are activated – one motor unit always consists of fibers of the same type
• type of muscle fibers can change, it depends on the innervation and the use – swapping of axons, change in type; difference between the muscles of a heavyweight lifter and a basketball player
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Heart muscle• many differences, many similarities
compared to skeletal muscles• pacemaker properties – myogenic
generation of excitation• diffuse, modulatory innervation• individual cells with one nucleus• electrical synapses - functional syncytium• AP has plateau, long refractory period• voltage-dependent L-type Ca++-channels
on T-tubules - entering Ca++ triggers Ca++ release from SR
• Ca++ elimination: Ca++-pump (SR), Na+/Ca++ antiporter (cell membrane) - digitalis: inhibition of the Na/K pump - hypopolarization and increased Ca++ level
-adrenoceptor: IP3 - Ca++ release from SR -adrenoceptor: cAMP - Ca++ influx through
the membrane
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Structure of the heart muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-50.
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Smooth muscle I.
• not striated• actin filaments are anchored to the plasma
membrane or to the dense bodies in the plasma
• myosin filaments in parallel• single-unit smooth muscle
– myogenic contraction – electrical synapses – synchronous contraction– contracts when stretched - basal myogenic tone– innervation modulates a few cells only through
varicosities– in the wall of internal organs (gut, uterus,
bladder, etc.)
• multi-unit smooth muscle– neurogenic contraction– individual cells innervated by individual
varicosities– e.g. pupil, blood vessels
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Smooth muscle II.• activation by pacemaker cells, hormones,
mediators released from varicosities• no fast Na+-channel• AP is not necessarily generated; it might
have plateau if present• contraction is initiated by the increased level
of Ca++ ions• Ca++ influx through voltage/ligand-dependent
channels, release from the SR (less developed)
• instead of troponin-tropomyosin, caldesmon blocks the myosin binding site on actin – freed by Ca-calmodulin, or phosphorylation (PKC)
• phosphorylation of myosin light chain (LC-kinase – activated by Ca-calmodulin) also induces contraction
• light chain phosphorylation at another site by PKC - relaxation
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