Muscle tissue is specialized for movement in humans and animals. The compound adenosine triphosphate (ATP) contains high-energy phosphate bonds, and these bonds can be broken to convert chemical energy into work by the myofibrils. Muscle contraction occurs when a nerve signal causes the depolarization of the muscle cell membrane and the release of calcium from the sarcoplasmic reticulum to activate the myofibril contractile proteins. Adenosine triphosphate is required to power the contraction as well as to pump the calcium back into the sarcoplasmic reticulum and restore the sodium and potassium at the cell membrane. A diagram showing the pathways for ATP production and utilization is shown in Fig. 1. In the living animal, the most efficient pathways of ATP production involve conversion of pyruvate into carbon dioxide in the mitochondria. However, after the animal dies, substrates such as glucose, fatty acids, and oxygen from the bloodstream are no longer available. Creatine phosphate (CP) can regenerate a small amount of ATP, but only the glycolysis pathway remains active. In postmortem muscle, the glycogen is converted to lactic acid and the latter accumulates. The pH also declines to below 6.0 in most cases, and the final or ultimate pH depends on species and muscle type. A typical pattern for the postmortem changes in several chemical and physical factors is shown in Fig. 2. Although this pattern is for normal pig muscle, other species would display similar patterns except for differences in the time axis.
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