Lecture 16 – Muscle Physiology IV

At moderate levels of contraction most of the atp

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At moderate levels of contraction, most of the ATP used forcontraction comes from  oxidative phosphorylation During the first 5-10 minute, the breakdown nof muscle glycogen to glucose provides the  major fuel for oxidative phosphorylation For the next 30 minutes, glucose and fatty acids diffusing into the cell from the blood  provide most of the fuel with blood glucose and fatty acids contributing about equally
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After that, fatty acids are dominant source of fue Phosphorylation od ADP in the glycolytic pathway Does not produce nearly as much Very fast and can keep rapid production as long as there is a lot of substrate Glycolotic Pathway Important because it is rapid if the intensity is over 70% Contributes an increasing amount If there is enough substrate, it can produce a lot in a short amount of time Without oxygen you get lactic acid buildup and this affects the rate at which muscles  fatigue Clasified According to a few criteria Basis of mazimal velocity of shortening Fast or slow Major pathway for ATP production Oxidative or glycolytic Fast v slow Called type II fibers Difference in structure of myosin Determines the maximal time for crose bridge formation  Fast fibers have high rates of ATPase activity Need high supply of ATP
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Rate of cross bridge formation is 4x faster in fast fibers HOWEVER, the firce generated by both is about the same  Fast twitch fibers pump Ca2+ back into the R much faster Last only about .7ms Slow twitch fibers can take 10x as long for that to happen Ca is pumped into SR more slowly Actin binding site coverage is slower Oxidative v Glycolitic CP strts off all pathways Oxidative fibers have lots of mitocondira In O fibers, most of the ATP produced by OP requires a lot of blood flow because it  needs oxygen and fatty acid Surrounded by a lot of capillaries and small blood vessel networks Oxidative fibers also have high lvels of myoglobulin which is an oxygen binding protein Increases the rate of oxygen diffusion thourought the muscle  High concentrations of myoglobulin leads to red colored muscles Glycolitic fibers do not have as many mitochondria or blood flow Have very little myoglobulin, fairly light colored Slow oxidative  Low myoglobulin ATPase activity + high oxidative capacity Fast oxidative glycolytic fibers Combine high mglobulin ATPase with high oxidative capacity and intermediate glycolytic  capacity
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Fast glycolytic fivers Combine high myosin ATPase with high glycolytic capacity Differences Glycolytic fiber hve  greater diamemeters than oxidative fibers
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