LIU Chuan Yong 刘传勇 Institute of Physiology Medical School of SDU Tel 88381175 (lab)

1. LIU Chuan Yong 刘传勇 Institute of Physiology Medical School of SDU Tel 88381175 (lab) 88382098 (office) Email: liucy@sdu.edu.cn Website: www.physiology.sdu.edu.cn

2. Section 3 Muscle Contraction

3. Classification of the Muscle • According to the structure: Striated Muscle, Smooth Muscle • According to the nerve innervation: Voluntary Muscle, Involuntary Muscle • According to the Function: Skeletal Muscle, Cardiac Contraction, Smooth Muscle

4. Skeletal Muscle Cardiac Muscle Smooth Muscle

5. I Signal Transmission Through the Neuromuscular Junction

6. Skeletal Muscle Innervation

7. Illustration of the Neuromuscular Junction (NMJ)

8. New Ion Channel Players • Voltage-gated Ca2+ channel • in presynaptic nerve terminal • mediates neurotransmitter release • Nicotinic Acetylcholine Receptor Channel • in muscle neuromuscular junction (postsynaptic membrane, or end plate) • mediates electrical transmission from nerve to muscle

9. Nerve Terminal Ca2+ channels • Structurally similar to Na+ channels • Functionally similar to Na+ channels except • activation occurs at more positive potentials • activation and inactivation much slower than Na+ channels

10. Neuromuscular Transmission Myelin Axon Axon Terminal Skeletal Muscle

11. - - + + + - - + - + - + - + + + - Look here + - + - + Neuromuscular Transmission: Step by Step Depolarization of terminal opens Ca channels Nerve action potential invades axon terminal

12. Binding of ACh opens channel pore that is permeable to Na+ and K+. ACh binds to its receptor on the postsynaptic membrane ACh is released and diffuses across synaptic cleft. ACh ACh Ca2+ ACh Ca2+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ ACh Na+ Na+ Na+ Na+ Na+ Na+ Na+ Ca2+ induces fusion of vesicles with nerve terminal membrane. Nerve terminal K+ K+ K+ K+ K+ Outside Muscle membrane Inside K+ K+ K+ K+ K+ K+ K+ K+

13. EPP End Plate Potential (EPP) VNa The movement of Na+ and K+ depolarizes muscle membrane potential (EPP) 0 Muscle Membrane Voltage (mV) Threshold Presynaptic terminal -90 mV VK Time (msec) Presynaptic AP Outside Muscle membrane Inside Voltage-gated Na Channels ACh Receptor Channels Inward Rectifier K Channels

14. ACh Choline ACh ACh Choline ACh Acetate ACh Meanwhile ... ACh is hydrolyzed by AChE into Choline and acetate Choline resynthesized into ACh and repackaged into vesicle Choline is taken up into nerve terminal ACh unbinds from its receptor so the channel closes Nerve terminal Outside Muscle membrane Inside

15. Structural Reality

16. Neuromuscular Transmission • Properties of neuromuscular junction • 1:1 transmission • An unidirectional process • Time delay. 20nm/0.5-1ms • Easily affect by drugs and some factors • The NMJ is a site of considerable clinical importance

17. Clinical Chemistry Related compounds are useful in the neuroscience research Suberyldicholine is a synthetic neuromuscular agonist. Ach is the natural agonist at the neuromuscular junction. Carbachol and related compounds are used clinically for GI disorders, glaucoma, salivary gland malfunction, etc. Tubocurarine competes with ACh for binding to receptor- but does not open the pore. Tubocurarine and other, related compounds are used to paralyze muscles during surgery. So tubocurarine is a neuromuscular blocking agent. Tubocurarine is the primary paralytic ingredient in curare. Carbachol is a synthetic agonist not hydrolyzed by acetylcholinesterase.

18. Anticholinesterase Agents • Anticholinesterase (anti-ChE) agents inhibit acetylcholinesterase （乙酰胆碱酯酶） • prolong excitation at the NMJ

19. Anticholinesterase Agents 1. Normal: AChCholine + Acetate AChE 2. With anti - AchE: AChCholine + Acetate anti - AChE

20. Uses of anti-ChE agents • Clinical applications (Neostigmine, 新斯的明, Physostigmine毒扁豆碱) • Insecticides (organophosphate 有机磷酸酯) • Nerve gas (e.g. Sarin 沙林，甲氟膦酸异丙酯。一种用作神经性毒气的化学剂))

21. Sarin • comes in both liquid and gas forms, • a highly toxic and volatile nerve agent developed by Nazi scientists in Germany in the 1930s. • 500 times more toxic than cyanide （氢化物） gas.

22. NMJ Diseases • Myasthenia Gravis （重症肌无力） • Autoimmunity to ACh receptor • Fewer functional ACh receptors • Low “safety factor” for NM transmission • Lambert-Eaton syndrome（兰伯特-伊顿综合征 ，癌性肌无力综合征 ） • Autoimmunity directed against Ca2＋channels • Reduced ACh release • Low “safety factor” for NM transmission

23. II Microstructure of Skeletal Muscle

24. Skeletal Muscle • Human body contains over 400 skeletal muscles • 40-50% of total body weight • Functions of skeletal muscle • Force production for locomotion and breathing • Force production for postural support • Heat production during cold stress

25. Fascicles: bundles, CT(connective tissue) covering on each one • Muscle fibers: muscle cells

26. Structure of Skeletal Muscle:Microstructure • Sarcolemma（肌管系统） • Transverse (T) tubule • Longitudinal tubule (Sarcoplasmic reticulum, SR肌浆网) • Myofibrils（肌原纤维） • Actin肌动蛋白 (thin filament) • Troponin（肌钙蛋白） • Tropomyosin（原肌球蛋白） • Myosin肌球蛋白 (thick filament)

27. Within the sarcoplasm Triad （三联管） • Transverse tubules • Sarcoplasmic reticulum -Storage sites for calcium • Terminal cisternae - Storage sites for calcium

28. Microstructure of Skeletal Muscle (myofibril)

29. Sarcomeres • Sarcomere 肌小节: bundle of alternating thick and thin filaments • Sarcomeres join end to end to form myofibrils • Thousands per fiber, depending on length of muscle • Alternating thick and thin filaments create appearance of striations

31. Myosin 肌球蛋白 • Myosin head is hinged • Bends and straightens during contraction

32. Thick filaments (myosin) • Bundle of myosin proteins shaped like double-headed golf clubs • Myosin heads have two binding sites • Actin binding site forms cross bridge • Nucleotide binding site binds ATP (Myosin ATPase) • Hydrolysis of ATP provides energy to generate power stroke

33. Thin filaments 原肌球蛋白 肌钙蛋白 肌动蛋白

34. Thin filaments (actin) • Backbone: two strands of polymerizedglobular actin – fibrous actin • Each actin has myosin binding site • Troponin • Binds Ca2+; regulates muscle contraction • Tropomyosin • Lies in groove of actin helix • Blocks myosin binding sites in absence of Ca2+

35. Thick filament: Myosin (head and tail) • Thin filament: Actin, Tropomyosin, Troponin (calcium binding site)

36. III Molecular Mechanism of Muscular Contraction • The sliding filament model • Muscle shortening is due to movement of the actin filament over the myosin filament • Reduces the distance between Z-lines

37. The Sliding Filament Model of Muscle Contraction

38. Changes in the appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber

39. Cross-Bridge Formation in Muscle Contraction

40. Energy for Muscle Contraction • ATP is required for muscle contraction • Myosin ATPase breaks down ATP as fiber contracts

41. Nerve Activation of Individual Muscle Cells (cont.)

42. Excitation/contraction coupling • Action potential along T-tubule causes release of calcium from cisternae of TRIAD • Cross-bridge cycle

43. Begin cycle with myosin already bound to actin

44. 1. Myosin heads form cross bridges • Myosin head is tightly bound to actin in rigor state • Nothing bound to nucleotide binding site

45. 2. ATP binds to myosin • Myosin changes conformation, releases actin

46. 3. ATP hydrolysis • ATP is broken down into: • ADP + Pi(inorganic phosphate) • Both ADP and Pi remain bound to myosin

47. 4. Myosin head changes conformation • Myosin head rotates and binds to new actin molecule • Myosin is in high energy configuration

48. 5. Power stroke • Release of Pi from myosin releases head from high energy state • Head pushes on actin filament and causes sliding • Myosin head splits ATP and bends toward H zone. This is Power stroke.

49. 6. Release of ADP • Myosin head is again tightly bound to actin in rigor state • Ready to repeat cycle

50. THE CROSS-BRIDGE CYCLE Relaxed state Crossbridge energised Crossbridge attachment A + M l ADP l Pi Ca2+ present AlMlADPlPi A – M l ATP Crossbridge detachment Tension develops ADP + Pi ATP AlM A, Actin; M, Myosin