assign_3_solns - Handed out 2 March 06 Due 9 March 06...

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Handed out: 2 March 06 Due: 9 March 06 16.423J/HST515J/ESD65 Space Biomedical Engineering and Life Support Systems Muscle Homework Assignment Illustrations are highly desired in answers! 1. (1 points) From the readings and lecture, please describe the sliding filament mechanism. SOLUTION: Muscle filaments are composed of actin and myosin proteins. The muscle myofibril contraction results from the relative sliding of actin and myosin. The contraction results from the myosin heads forming crossbridges on active sites of the actin. The isometric tension output is directly proportional to the number of crossbridges formed (T-L curve). The myosin heads go through a cyclic motion (attach on actin / pull / detach / attach…), allowing to create the pulling force while contracting Muscle shortening should be based on the relative motion of sliding filaments because: A. A-band width stays constant during stretch and shortening B. A-band disappears when myosin is dissolved away C. Actin filaments begin at Z-line, run through I-bands into A-band, but stop before reaching H-zone (muscle at rest length) D. I-band is entirely actin filaments E. H-zone is myosin filaments In shortening, the relationship between the Force and Velocity is (Hill's Curve, Hyperbolic Form): (T+a)(v+b) = (T 0 +a)b where T 0 = isometric tension (velocity = 0) o Muscles shorten more rapidly against light loads than they do against heavy ones. o Muscles which are actively shortening can produce less force than those which contract isometrically. There is a discontinuity in the slope of F-V curve at zero velocity. Active muscle yields when the load exceeds about 1.8 T 0 (T 0 – Tetanus tension). Hill's observations of the Fenn effect give a linear relation between total rate of energy liberation and tension. Fenn = muscle produces a certain extra heat when it shortens a given distance, whether shortening velocity is fast or slow. Muscular energy liberation should be based on the splitting of a high-energy phosphate as actomyosin attachments separate.
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2. (1 points) Briefly discuss muscle coactivation. SOLUTION: Typically referred to as the α−γ (alpha-gamma) coactivation principle, it can be used to better understand movement control. More specifically, α−γ (alpha-gamma) coactivation can be described as muscle commands being signaled simultaneously with increases in both α and γ activity by higher order motor centers. This alleviates the problem of contraction of the extrafusal muscle mass and a slackening of the spindles if only α motor activity changes (say increased). The stretch reflex would then come into play as the Ia
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assign_3_solns - Handed out 2 March 06 Due 9 March 06...

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