Effects on Muscle Spindle Development of Multiple and Single Exposure to X-Irradiation

The effect on muscle spindle development of multiple exposure to x-irradiation was studied in the soleus and plantaris muscles of young rats. Growth in muscle volume and extrafusal fiber cross-sectional area was strongly reduced following exposures of 2400 to 4100 rad given within two weeks after birth. Spindles also demonstrated reduction in size of the nuclear bag and chain intrafusal fibers in the irradiated as compared to the contralateral muscle, but the difference was relatively less than that of the extrafusal fibers. The intracapsular space was reduced in some heavily irradiated spindles to the point where virtually no space was seen beneath the capsule wall. The number of clearly recognizable spindles was less in the heavily irradiated muscles. Essentially similar results were found in one rat given a single exposure to 4000 rad. The effects on the fibers are interpreted to indicate relative resistance to irradiation in development of the intrafusal musculature as compared to extrafusal fibers.