Tapering of the intrafascicular endings of muscle fibers and its implications to relay of force

Authors

  • E. Eldred,

    1. Brain Research Institute and Departments of Anatomy and Cell Biology, and of Physiological Science, University of California at Los Angeles, California
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  • M. Ounjian,

    1. Brain Research Institute and Departments of Anatomy and Cell Biology, and of Physiological Science, University of California at Los Angeles, California
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  • Dr. R. R. Roy,

    Corresponding author
    1. Brain Research Institute and Departments of Anatomy and Cell Biology, and of Physiological Science, University of California at Los Angeles, California
    • Brain Research Institute, Center for the Health Sciences, University of California, Los Angeles, Los Angeles, CA 90024–1761
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  • V. R. Edgerton

    1. Brain Research Institute and Departments of Anatomy and Cell Biology, and of Physiological Science, University of California at Los Angeles, California
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Abstract

The geometric shape of the filamentous, intrafascicular type of muscle fiber ending was reconstructed as a basis for understanding the pattern in relay of the fiber's force to the muscle tendon. Single motor units (MUs) identified physiologically as being fast and slow, respectively, were isolated in cat tibialis muscles and glycogen-depleted for recognition in cross sections of the muscle frozen at its Lo. Serial measurements of cross-sectional area (CSA) using an image processing system were made along 14 intrafascicular endings of MU fibers and an additional seve, nondepleted fibers identified histochemically as slow. Comparison of coefficients of variation for the linear relation of the CSAs and of the equivalent diameters with length along the taper indicated that in both fast and slow fibers the areas bore a closer relationship, that is, the taper had the equivalent of a parabolic, rather than a conical outline. The implications of these two conformations to relay of the fiber's contractile force to surrounding structures are displayed graphically. © 1993 Wiley-Liss, Inc.

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