An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.
Article| February 01 1972
FINE STRUCTURE OF RAT INTRAFUSAL MUSCLE FIBERS : The Equatorial Region
William K. Ovalle, Jr.
From the Department of Anatomy, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
Dr. Ovalle's present address is the Neurophysiology Laboratory, Department of Surgery, The University of Alberta, Edmonton 7, Alberta, Canada.
Received: August 11 1971
Revision Received: October 12 1971
Online Issn: 1540-8140
Print Issn: 0021-9525
Copyright © 1972 by The Rockefeller University Press
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William K. Ovalle; FINE STRUCTURE OF RAT INTRAFUSAL MUSCLE FIBERS : The Equatorial Region . J Cell Biol 1 February 1972; 52 (2): 382–396. doi: https://doi.org/10.1083/jcb.52.2.382
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