Muscle spindles are not always distributed more in postural muscles with many slow-twitch-oxidative (SO) myofibers than in locomotory muscles with few SO myofibers. The purpose of present study was to examine the distribution, density, and structure of muscle spindles in the vastus intermedius muscle: an antigravity muscle and the peroneus longus muscle: a locomotory muscle in the sheep. Muscle spindles were reconstructed from serial sections at 300 μm intervals throughout the muscles. Myofiber types were classified into SO, fast-twitch-oxidative-glycolytic, and fast-twitch-glycolytic myofibers by differences in histochemical reactivity. No significant difference in the density of muscle spindles (DMS) existed between the vastus intermedius (DMS: 5.3) and peroneus longus (DMS: 5.7) muscles. The muscle spindles were distributed more in the distal portion than in the proximal portion of the vastus intermedius muscle. The muscle spindles were distributed in the proximal and middle portion but hardly in the distal portion of the peroneus longus muscle. Muscle spindles were classified into simple, tandem, and compound muscle spindles. Most of the muscle spindles were the simple type. The differences in size of the muscle spindle and numbers of the intrafusal myofibers were not significant between the two muscles. The results show that the density and structure of the muscle spindles do not differ between the postural and locomotory muscles in the sheep.
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