Scanning electron microscopic study of muscle spindles in the tenuissimus muscle of the Chinese hamster: with reference to the outer capsule, inner capsule and fusimotor endings.

摘要

The outer capsule, inner capsule and fusimotor endings of muscle spindles in the tenuissimus muscle of mature Chinese hamsters were examined by scanning electron microscopy (SEM). The thin and long tenuissimus muscle was transversely cut into several segments, and the segments were longitudinally or obliquely cut with a razor blade to mechanically remove the spindle sheath (outer capsule). These specimens were treated with 8 N HCl at 60 degrees C to isolate the muscle spindles and clearly expose the internal structures such as the inner capsule and fusimotor endings innervating intrafusal muscle fibers. The spindle sheath was about 1 mm long and at the equator about 50 microm in diameter. Within the outer capsule, cells of the inner capsule in the equatorial region were polygonal in shape, and continuously surrounded the axial bundle like a sleeve. This continuous sheath became sieve-like in the juxta-equatorial region, depending on the twining of thin and flattened cytoplasmic extensions and/or processes of the cells. Owing to the continuous inner capsular sheath in the equatorial region, the arrangement of sensory endings were not visualized under SEM. Fusimotor endings observed in the juxta-equatorial and polar regions consisted of poorly-developed subneural apparatuses with predominantly pit-like and short slit-like junctional folds.