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

Structure of the skeletal muscle 3/20 Structure of the skeletal muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-1.

Ultrastructure of the striated muscle 4/20 Ultrastructure of the striated muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-2.

Sarcomeres in cross-section 5/20 Sarcomeres in cross-section Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-3.

Structure of the thin filament 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 7/20 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

Structure of the myosin filament 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 Sliding filament theory Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-8. 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

Tubules in the muscle fiber 10/20 Tubules in the muscle fiber Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-21.

Connection of T-tubules and SR 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 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 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. 10-16. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-11.

Energetics of the contraction 14/20 Energetics of the contraction Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-29.

Types of muscle fibers tonic fibers slow-twitch (type I) fibers 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

16/20 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

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 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

Structure of the heart muscle 18/20 Structure of the heart muscle Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-50.

Smooth muscle I. not striated 19/20 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

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

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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.