Scanning electron microscopic observation of the sensory region in the frog muscle spindle.

The equatorial sensory region of muscle spindles in the fourth toe extensor digitorum longus muscle of the adult frog was examined by scanning electron microscopy. Segments of this thin and long muscle after fixation were longitudinally cut with a razor blade and then treated with an HCl-hydrolysis method to remove connective tissues. Cells of the inner capsule extended thin and flattened cytoplasmic processes, showing a sieve-like appearance. Some specimens after a partial disruption of the inner capsule reevaluated at the fine structural level that numerous sensory terminals with varicose swellings longitudinally arranged along each intrafusal muscle fiber.