1 3 Muscle cell surface action potential 4 T-system transmission 3 2 5 Coupling to sarcoplasmic 3 reticulum 4 6 Ca 2+ release from sarco- plasmic reticulum 5 7 Activation of contractile proteins 8 Calcium reaccumulation by sarcoplasmic reticulum
occurs not only at the gross level, but also at reside, are very highly organized. When the muscle cell (fiber) level. First, not only examining a myofibril, one of the first obser- do muscles vary in size, they can also vary vations that can be made is that the cylindri- in the number of cells. For example, the cal organelle is made up of repeating units. muscle that is responsible for adjusting the These repeating units are known as sarco- tension of the eardrum (tensor tympani) meres. Contained in each sarcomere are all has only a few hundred muscle cells, while the structural elements needed to perform the the medial gastrocnemius (used in humans physical act of contraction at the molecular for walking) has over a million muscle cells level. Current proteomic analysis estimates (Feinstein et al. 1955). Not only does the that over 65 proteins make up the structure
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