Ultrastructural changes of the sensory nerve terminals in frog muscle spindle during dynamic stretch

The sensory ending of the frog muscle spindle consists of bulbous swellings interconnected by thin, tube-like axonal branches. This study was made to determine if the bulb or thin tube regions are deformed to the same degree during dynamic stretch, by comparing spindles prepared in the relaxed and stretched states. Isolated muscle spindles were rapidly frozen, either in a completely relaxed state or at the end of dynamic stretch to 130% of the in situ length. Longitudinal sections for ultra-high-voltage electron microscopy revealed that dynamic stretch caused a decrease of 61.7% in bulb diameter, an increase of 88% in bulb length and an increase of 34.6% in bulb volume. The cross-sectional areas of the nonmyelinated terminals in the reticular and compact zones measured in electron micrographs decreased by 79.2% in the bulb region and 66.7% in the tube region during dynamic stretch.