Postnatal development of the annulospiral endings of Ia fibers in muscle spindles of the mouse temporal muscle.

The formation of the annulospiral endings of Ia fibers in muscle spindles was investigated in the temporal muscle of developing mice. From 15 to 20 d after birth, there were only a few spindles with "++" sensory endings. With growth of mice, the primary endings of the Ia fiber began to develop its spiral endings. And by the 30th postnatal day (that is, after weaning), almost all of the Ia fibers had completely coiled endings though the formation was still continuing in some spindles at this time. Formation of the annulospiral endings of the Ia fibers of muscle spindles in the temporal muscle was completed earlier than that of fibers in the spindles in the masseter muscle.