The Z disk ultrastructure of white, intermediate, and red fibers from mammalian muscle is examined. Three models are proposed that explain the differences between the three types of Z disk. The three models are all based on the same concept, i.e., looping filaments from both sides of the Z disk. The differences between the models are in terms of the spatial relationships of adjacent loops within the Z disk. In the white fiber Z disk model all the loops from one side of the Z disk are on the same plane. In intermediate fibers there are two planes of loops from both sides of the Z disk, whereas in red fibers there are three planes of loops from both sides. The implications of these three structures are discussed in relation to known physiological differences between the fiber types.

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