Human muscle spindles: Microfilaments in the group IA sensory nerve endings

The fine structure of the group IA sensory nerve endings from normal human muscle spindles was studied in transverse and longitudinal sections. Two arrangements of microfilaments, approximately 75 Å in diameter, were found in each of ten spindles examined. The first was a central aggregate of densely packed filaments. The aggregates were partly surrounded by mitochondria, and were oriented parallel to the longitudinal axis of the sensory ending as it encircled the intrafusal muscle fiber. Individual aggregate filaments of glycerinated endings appeared to react with heavy meromyosin. The second arrangement was a filamentous network in the periphery of the sensory ending profiles. These microfilaments approached and appeared to merge with the surface membrane. They resembled the microfilaments that others have described in growth cones of cultured neurons. Both types of microfilaments are thought to be involved in changing the shape of the sensory endings during stretch and relaxation of the spindle.