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The differentiation of two types of intrafusal fibres in rabbit muscle spindles

An electron microscopic study

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Summary

  1. 1.

    The ultrastructure of muscle spindles in deep lumbrical muscles of adult rabbit has been investigated. Contrary to findings obtained by previous authors at the light microscope, indicating morphologic uniformity of intrafusal fibre type in rabbit spindles, two populations of intrafusal muscle fibres have been detected.

  2. 2.

    The first type of fibres is made by myofibrils which have a thick Z line and lack a distinct M line. These fibres are also characterized by the apparent fusion of fibrillar fields due to the paucity of the sarcoplasm, which contains very little glycogen. The mitochondria are scattered and of small size. The sarcoplasmic reticulum is poorly developed and the triads are quite rare. These fibres resemble the nuclear bag fibres of cat muscle spindles.

  3. 3.

    The second type of fibres is made by myofibrils which have a thin Z line and a prominent M line. There is a great separation of the fibrillar fields as a result of abundant sarcoplasm which contains large accumulation of glycogen and ribosomes. The mitochondria are densely distributed and of large size. The sarcoplasmic reticulum is well developed and the triads are rather numerous. These fibres resemble the nuclear chain fibres of cat muscle spindles.

  4. 4.

    The demonstration of two types of intrafusal muscle fibres in rabbit spindles may account for the duality of physiological responses obtained from these receptor organs in this animal species.

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Authors and Affiliations

  1. Istituto di Fisiologia Umana, Cattedra II, Università di Pisa, Via S. Zeno, 31, Pisa, Italia

    N. Corvaja & O. Pompeiano

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  1. N. Corvaja
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  2. O. Pompeiano
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Additional information

This investigation was supported by PHS research grant NB 07685-02, from the National Institute of Neurological Diseases and Blindness, N.I.H., Public Health Service, U.S.A. and by research grant n. 69.01076-115.2857 from the Consiglio Nazionale delle Ricerche, Italy.

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Corvaja, N., Pompeiano, O. The differentiation of two types of intrafusal fibres in rabbit muscle spindles. Pflugers Arch. 317, 187–197 (1970). https://doi.org/10.1007/BF00586503

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  • DOI: https://doi.org/10.1007/BF00586503

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