Muscle Movement with muscles 2/20 • movement is one of the most prominent characteristics of animal life • it can be either amoeboid, or more complicated.
Download ReportTranscript Muscle Movement with muscles 2/20 • movement is one of the most prominent characteristics of animal life • it can be either amoeboid, or more complicated.
Muscle Movement with muscles 2/20 • 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 3/20 Structure of the skeletal muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-1. 4/20 Ultrastructure of the striated muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-2. 5/20 Sarcomeres in cross-section Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-3. 6/20 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. 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 7/20 8/20 Structure of the myosin filament Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4, 6. Sliding filament theory 9/20 • during contraction A-band is unchanged, Iband 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 crossbridges 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 10/20 Tubules in the muscle fiber Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-21. 11/20 Connection of T-tubules and SR Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-25. Release of Ca++ ions 12/20 • AP – spreads from the sarcolemma to the Ttubule – conformational change of the voltagedependent 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 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4. Mechanism of sliding 13/20 • 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. 14/20 Energetics of the contraction Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-29. 15/20 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 Motor unit 16/20 • 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 Heart muscle 17/20 • 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 Ttubules - 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 18/20 Structure of the heart muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-50. Smooth muscle I. 19/20 • 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 Smooth muscle II. 20/20 • 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 End of text Length-tension relation Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-8. Role of the troponin complex Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-16.