L16_NPB_101 - Lecture 16 SmartSite: Lecture 16 Notes...

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Unformatted text preview: Lecture 16 SmartSite: Lecture 16 Notes Midterm 1: Reading (Recommended): Relevant por:ons of Chapters 7 & 8 Announcements: 30 MC ques:ons Covers lectures through Tuesday (Star:ng Wednesday, lectures on Midterm 2) Requires students to bring and use SCANTRON 2000 (blue) M-W 5-6 Review Muscle Skeletal muscle Skeletal muscle CAF Office Hrs (197 Briggs) KR Office Hrs (2041A SLB) Tuesday 9-10 Wednesday 3-4 Thursday 11:30-12:30 (normal) Canceling Thursday 3-4 1 REV: Muscle Muscle as a :ssue similari:es, differences & func:on Skeletal muscle morphology muscle to myofiber to myofibril to sarcomere Four proteins myosin, ac:n, tropomyosin, troponin Contrac:on sliding filament mechanism 2 REV: Cross Bridge (Power Stroke) Ac:vated cross bridge bends toward center of thick filament, "rowing" in thin filament to which it is aaached Ca2+ released into sarcoplasm (cytoplasm of muscle cell) Myosin heads bind to ac:n Myosin heads swivel toward center of sarcomere (power stroke) ATP binds to myosin head and detaches it from ac:n Hydrolysis of ATP transfers energy to myosin head and reorients it Contrac:on con:nues if ATP is available and Ca2+ level in sarcoplasm is high 3 Cross-Bridge Cycle Fig. 8-12; pg. 267 4 Excita:on-Contrac:on Coupling 5 The Neuromuscular Junction Fig. 7-4; pg. 249; Fig. 7-5a; pg. 250 6 Transverse Tubules T-tubules Run perpendicularly from surface of muscle cell membrane into central por:ons of the muscle fiber Since membrane is con:nuous with surface membrane ac:on poten:al on surface membrane also spreads down into T-tubule Spread of ac:on poten:al down a T-tubule triggers release of Ca2+ from sarcoplasmic re:culum into cytosol Fig. 8-9; pg. 264 7 Sarcoplasmic Re:culum Modified endoplasmic re:culum Consists of fine network of interconnected compartments that surround each myofibril Not con:nuous but encircles myofibril throughout its length Segments are wrapped around each A band and each I band Ends of segments expand to form saclike regions lateral sacs (terminal cisternae) Fig. 8-9; pg. 264 8 Rela:onship Between T-Tubule and Sarcoplasmic Re:culum Fig. 8-10; pg. 265 9 Ca2+ in Excita:on-Contrac:on Coupling Fig. 8-11; pg. 266 10 Relaxa:on Reuptake of Ca2+ into sarcoplasmic re:culum (SR) Ac:ve transport with Ca2+ pump. Acetylcholinesterase breaks down ACh at neuromuscular junc:on Muscle fiber ac:on poten:al stops When local ac:on poten:al is no longer present, Ca2+ moves back into SR 11 The Mechanics of Muscle Contrac:on 12 Speed of Contrac:on is Related To the Load on the Muscle Fig. 8-20; pg. 274 13 Length-Tension Rela:onship Maximum tension is generated at the op:mal length (L0) Fig. 8-19; pg. 273 14 Length-Tension Rela:onship Less than op:mal length Bunching thick filaments Thin filament overlap Ca2+ influx reduced Too Stretched Less than op:mal overlap Fig. 8-19; pg. 273 15 Muscle Contrac:ons Contrac:ons of whole muscle can be of varying strength Twitch Brief, weak contrac:on Produced from single ac:on poten:al Too short and too weak to be useful Normally does not take place in body Two primary factors which can be adjusted to accomplish grada:on of whole-muscle tension Number of muscle fibers contrac:ng within a muscle Tension developed by each contrac:ng fiber 16 The Motor Unit Motor unit One motor neuron and the muscle fibers it innervates Number of muscle fibers varies among different motor units Number of muscle fibers per motor unit and number of motor units per muscle vary widely Muscles that produce precise, delicate movements contain fewer fibers per motor unit Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor unit 17 Fig. 8-16; pg. 270 Contrac:on Strength of Small Versus Large Motor Units Fig. 8-17; pg. 270 18 Motor Unit Recruitment Asynchronous recruitment of motor units helps delay or prevent fa:gue Factors influencing extent to which tension can be developed Frequency of s:mula:on Length of fiber at onset of contrac:on Extent of fa:gue Thickness of fiber 19 Twitch Summa:on and Tetanus Twitch summa:on Results from sustained eleva:on of cytosolic calcium Tetanus Occurs if muscle fiber is s:mulated so rapidly that it does not have a chance to relax between s:muli Contrac:on is usually three to four :mes stronger than a single twitch 20 Summa:on and Tetanus Fig. 8-18; pg. 271 21 Major Types of Skeletal Muscle Fibers Classified based on differences in ATP hydrolysis and synthesis Three major types Slow-oxida:ve (type I) fibers Fast-oxida:ve (type IIa) fibers Fast-glycoly:c (type IIx) fibers 22 Characteris:cs of Skeletal Muscle Fibers Table 8-1; pg. 280 23 ...
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