Sensory Innervation of Atypical Spindles After Nerve Crush in Newborn Rats

Abstract

Denervation experiments (Zelená and Hník, 1963a; Zelená, 1976) and ultrastructural studies indicate (Landon, 1972; Milburn, 1973; 1984; Kozeka and Ontell, 1981) that sensory axons trigger the development of muscle spindles by inducing the formation of nuclear bags in selected primary myotubes and by initiating the encapsulation of nascent spindles. Sensory axons are also mainly responsible for further differentiation and maintenance of the muscle spindle structure (Zelená and Soukup, 1973; 1974; Kucera, 1987), whereas fusimotor axons contribute to a full maturation of myosin profiles of intrafusal fiber types (teKronnie et al., 1982). If skeletal muscles are denervated in newborn (Zelená and Hník, 1960; 1963b) or 4-day-old rats (Schiaffino and Pierobon Bormioli, 1976), most muscle spindles disintegrate during the transient denervation period due to the loss of innervation; only occasional spindles survive with a reduced number of intrafusal fibers and can be reinnervated again when the muscle nerves regenerate.