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Lecture 6: Muscle Physiology

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Lecture 6: Muscle Physiology Organismal Lec X 01/25/00 Lecture 6: Muscle Physiology ASST.: READ CH. 10 I. Purpose A. In order to have N.S. produce behavior it must be able to act on effectors B. Neurons only produce electrical changes C. Need another TRANSDUCTION from electrical to mechanical or chemical energy D. Two main systems 1. Muscle - mechanical 2. Endocrine - Chemical (can lead to mechanical) II. Move down STRUCTURAL PYRAMID of Muscle A. Muscle attached via tendon to bone Bone Muscle B. Muscle is made up of MUSCLE FIBERS (CELLS) C. Muscle fiber (MULTINUCLEATED CELL) contains MYOFIBRILs D. Myofibril made up of THICK AND THIN FILAMENTS 1 Organismal Lec X 01/25/00 1. Thick - MYOSIN 2. Thin - ACTIN 3. Together complex is called ACTOMYOSIN E. Filaments made of actin and myosin molecules III. Organization of SARCOMERES (in myofibrils) A. Sarcomere Sarcomere A Band Z Line H Zone I Band B. Look at sarcomere more closely (Shows filament types associated with bands Sarcomere A Band Myosin Z Line H Zone Actin I Band 2 Organismal Lec X 01/25/00 IV. SLIDING FILAMENT THEORY A. Originally thought proteins contract (shorten) B. Huxley and Niedergerke use light microscope to look at sarcomere length 1. Showed that A - Bands maintain constant width during contraction 2. I - Bands and H - zone becomes narrower 3. Stretch → I - Bands and H - Zone get wider a) A - Band again stays constant C. Hanson and H.F. Huxley measure protein length during contraction of live muscle D. Protein length does not change E. Two Huxleys independently put forward SLIDING FILAMENT THEORY 1. Contraction of sarcomeres is due to actin sliding past myosin 2. Protein size unaffected 3. Predict changes in H - Zone and I - Band F. Length - Tension curve supports sliding filament theory 1. Stretch and hold the length of sarcomeres 2. Stimulate and measure tension 1.2 1 0.8 0.6 0.4 Tension 0.2 01.25 1.65 2.00 2.25 3.65 Sarcomere Length 3 Organismal Lec X 01/25/00 3. Start at right a) Long length → little overlap → little tension b) Less stretch → more overlap → more tension c) 20% rest lengths → max overlap → max tension d) Smaller sarcomere → overlap of actin filaments → less tension e) Eventually myosin hits Z - line and crumples → 0 tension f) Tension is a function of cross-bridges formed between actin and myosin and is thus a function of overlap V. Structure responsible for Sliding filaments A. Structure of filaments 1. Structure of filaments and cross bridge formation a) To understand how tension is related to overlap of filaments b) first look at structure of filaments 2. F - actin (fiber-like) a) Two strings of beads wound around each other 3. G - actin a) Globular b) beads in f - actin c) each bead is a G - actin molecule B. MYOSIN FILAMENT 1. Bundle of myosin molecules 2. Myosin molecule 4 Organismal Lec X 01/25/00 Heads Tail 3. Head is locus of ALL ENZYMATIC activity and ACTIN BINDING SITE 4. Myosin molecules aggregate with heads pointing out a) results in myosin filament b) Actin + Myosin → Actomyosin C. How does actomyosin complex form and Move 1. One idea a) If head can rotate it can slide actin backward → shortening of sarcomere 5 Organismal Lec X 01/25/00 2. Myosin cycle a) attach to actin b) power stroke moves head ~ 10 nm c) detachment d) start of new cycle 3. Energetics of Myosin Cycle Thin filament Cross Bridge Energized Binds to Actin C.B, Thick fil. Z Line A + M*- ADP - Pi A-M* - ADP-P Hydrolysis of Cross Bridge ATP Energizes CB ADP + Pi Moves ATP ATP binds to Myosin Causing CB to Detach A + M-ATP A-M Complex a) myosin hydrolyzes ATP in myosin complex b) energizes Myosin c) Myosin Binds to Actin d) Forms Crossbridge - ADP+Pi removed e) causes head to rock f) Myosin head rocks - slides actin 6 Organismal Lec X 01/25/00 g) ATP binds to myosin - causes head to detach D. At end of slide head is detached 1. Head can then grab on again 2. Thus, actin is passed from head to head 3. Detachment REQUIRES that ATP binds to the ATPase site on the myosin a) In absence of ATP (1) ALL heads BIND (2) RIGOR MORTIS b) ATP is then cleaved to energize myosin so it is ready for next cycle VI. What INIITIATES SLIDE A. Low levels of Ca required for muscle contraction B. Isolate actin and myosin 1. put in salt solution 2. actin and myosin will combine spontaneously to → actomyosin 3. with Ca, Mg and ATP added actomyosin will contract C. Strip away two proteins from actin filament 1. Troponin 2. Tropomyosin D. Ca no longer required E. Troponin and Tropomyosin → tonic inhibition to actin and myosin binding (Ebashi) F. Ca turns off this inhibition G. Troponin -Tropomyosin Model 1. Tropomyosin covers myosin binding site on actin 2. Troponin binds Ca and undergoes conformational change a) Actually troponin is a complex of 3 subunits (1) C (2) T (3) I b) Ca binds on C subunit 7 Organismal Lec X 01/25/00 Ca Binding Sites C Tropomyosin Filament T I Myosin Head Myosin Binding Site Actin 3. Change in troponin draws tropomyosin away from the myosin binding site 4. So myosin can reach its binding site and binds spontaneously 5. Removal of Ca returns troponin and tropomyosin to original state 6. Inhibition of myosin resumes 7. Need > 10-7M Ca conc. to → contraction H. How is Ca level controlled? 1. Depolarize near Z line get contraction on either side 2. Level of depolarization determines DEPTH of CONTRACTION 3. At Z - line is tubule which penetrates muscle fiber to deep fibrils 4. Called T - tubules 5. Adjacent to T - tubules covering all fibrils from Z - line to Z - line is sarcoplasmic reticulum 6. Next to T - tubules S.R. forms terminal cisternea (pockets) 8 Organismal Lec X 01/25/00 T Tubule Terminal Cisternea Sarc. Retic. a) terminal cisternea + T - tubule = 1 Triad 7. Ca is actively sequestered in cisternea a) as a result Free Ca conc. < 10-7M 8. Motor neuron → A.P. in muscle fiber a) travels along membrane AND DOWN T - TUBULES b) Depol. of T - tubules → release of Ca from cisternea c) Can visualize Ca++ release with Ca++ sensitive dyes d) Plunger Model for Release of Ca++ from SR (1) Depolarization of T Tubule membrane Æ comformational change in voltage sensitive Dihydropyridine receptor (2) Dihydropyridine then pulls on ryanodine receptor (feet) that act as a plunger in SR keeping Ca++ from flowing out. (3) With ryanodine feet pulled out Ca++ can flow out and bind with troponin 9 Organismal Lec X 01/25/00 Dihydropyridine Receptor ++++ ++++ ---- ---- ++++ ++++ ---- ---- ++++ ++++ ---- ---- Ryanodine Receptor ATP Ca Ca Ca Ca e) Free Ca conc. →10-6 M or higher Æ binding to troponin etc. leading to contraction f) ATP is then used to pump Ca++ back into SR against concentration gradient 9. This is EXCITATION - CONTRACTION COUPLING Ca Ca VII. Muscular Movement – MECHANICS A. Muscle is not all contractile protein 1. Stretchy connective tissue in parallel with cont. proteins 10 Organismal Lec X 01/25/00 2. This constitutes the PARALLEL ELASTIC COMPONENT 3. There is also a SERIES ELASTIC COMPONENT a) Stretchy connective tissue in series with filaments (1) tendons (2) elasticity of heads Contractile Proteins SEC Weight B. In order to move weight, muscle must first overcome SEC 1. Pull on weight via spring 2. Initially weight doesn't move (isometric) 3. When tension in spring (SEC) equals weight, movement begins (isotonic contraction) 4. Delay is longer for heavier weight C. DELAY in tension buildup due to SEC 1. Can measure movement of contractile components 2. QUICK STRETCH EXPT. a) Rapidly stretch muscle to take up slack in SEC b) Stimulate muscle c) Contractile components engage within 1-2 msec (1) This is called ACTIVE STATE (2) Tension of contractile components d) Without quick stretch whole muscle doesn't reach peak until 10-100 msec 11 Organismal Lec X 01/25/00 Tension During Quick Stretch "Active State" (without stretch) Tension Time 3. Peak tension of muscle during twitch occurs at LOW POINT of ACTIVE STATE a) Ca is quickly sequestered after spike b) This terminates active state c) SEC delays tension buildup (1) much like capacitor (2) So in one twitch muscle only gets part of max. tension capability (3) If second spike arrives before Ca is sequestered, active state remains high (a) tension can get greater - More Ca++ → More Tension (b) temporal summation High Frequency Single Twitch A.P.s Low Frequency A.P.s Tension Time (4) Series of twitches → increase in tension until cross-bridges 12 Organismal Lec X 01/25/00 start to SLIP (5) At this point tension plateaus - TETANUS 4. Some arthropods can overcome SEC to produce rapidly accelerating movements a) Result is quick and strong movement b) More than what could be accomplished with conventional contraction Muscle contraction Isotonic Movement Tension or Movement Time c) Cannot eliminate SEC but can delay movement until plateau (tetanus) is attained Delay Movement Take up SEC Rapid Accel. Tension or Movement Time d) Snapping shrimp (1) two flat disks act like wetted glass (2) hold dactyl up until max. tension is produced (a) SEC taken up (3) results in very rapidly accelerating and powerful movement VIII. Variation of tension via motor neurons control 13 Organismal Lec X 01/25/00 A. Vert. motor spike → spike in muscle (all-or-none) 1. One motor neuron innervates relatively few muscle fibers (approx.
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