Scanning electron microscopic observation on the equatorial region of the rat intrafusal muscle fibres, especially on the subsarcolemmal sarcoplasmic reticulum

We examined intracellular structures in the equatorial region of the muscle spindles of rat soleus muscles by scanning electron microscopy, paying particular attention to the ultrastructure of the sarcoplasmic reticulum (SR) beneath the sarcolemma. The subsarcolemmal SR was more developed in nuclear chain fibres than in nuclear bag fibres as was reported in the sleeve and extracapsular regions. In addition, the subsarcolemmal SR of the chain fibre formed a fenestrated sheet, whereas that of the bag fibre organized a layer of fenestrated bands beneath the sarcolemma where the sensory nerve endings are associated. No T-tubules were discerned in the subsarcolemmal SR of both fibres, which may be concerned with the little contraction of the equatorial region.