Lecture 14 – Muscle Physiology II

The lengthening of muscle of muscle fibers is not an

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The lengthening of muscle of muscle fibers is not an active process produced by  contractile proteins, but a consequence of an ecternal force applied to the muscle Isometric Contraction The bound cross-bridges do exert a force on the thin filaments, but they are unable to  move the object The myosin heads just repeatedly bind to the same actin molecules Other Elements of Single Fiber Contraction Mechanical response to a single muscle fiber to one action potential is a twitch Muscle fiber AP lasts only 1-2 msec and is over beore any mechanical activity begins After AP, latent period before muscle tension increases EC coupling processes occur during this period Time period bfrom the beginning of tension unti the end of the latent period is called the  contraction time Contraction times differ across skeletal muscles 10-100 s, depending on the speed of response necessitated by the process depends in part in the time that the Ca remains elevated in the cytosol and cross  bridging continues this is dependent on the Ca-ATPase functioning the SR activity is greater is fast-twitch fivers than in slow twitch fibers Twitch duration also depends on how long it takes for cross bridges to complete their  cycle when all the Ca has been removed In isometric contraction, tension begins to rise as soon as the first cross bridge attaches Latent period due to EC coupling delay Isotonic contraction, the latent period includes both the EC coupling delay and the time  required to accumulate enough attached cross bridges to lift the load Timing of EC coupling
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AP in skeletal muscle only last s 1-2 msic and is over before any mechanical activity  begins Following a single AP, cytosolic Ca rises rapidly and then falls back to resting levels in  50 msec Once initiated, the mechanical activity my last 100 msec or more Load velocity Relationships The shortening velocity is maximal when there si no load  Velocity is ) when the load equals the maximal isometric tension At loads greater than the maximal isometric tension, the fiver will lengthen at a velocity  that increases with load The unloaded velocity is determined by the rate at which cross-bridge cycling occurs Because 1 ATP is split during each cross-bridge cycle, the rate of ATP hydrolysis  determines the shortening velocity Increasing the load slows the forward movement during the power strike, reducing the  rate of ATP hydrolysis, and decreasing the shortening velocity
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