DIFFERENTIAL-EFFECTS OF NEONATAL DENERVATION ON INTRAFUSAL MUSCLE-FIBERS IN THE RAT

The response of developing muscle spindles to denervation was studied by sectioning the nerve to the medial gastrocnemius muscle of rats at birth. The denervated spindles were examined daily throughout the firs t postnatal week for changes in ultrastructure and expression of sever al isoforms of myosin heavy chain (MHC). Each of the three different t ypes of intrafusal muscle fiber exhibited a different response to dene rvation. Within 5 days after the nerve section nuclear bag, fibers deg enerated completely; nuclear bag1 fibers persisted, but ceased to expr ess the 'spindle-specific' slow-tonic MHC isoform and thereby could no t be differentiated from extrafusal fibers; nuclear chain fibers did n ot form. The capsules of spindles disassembled, hence spindles or thei r remnants could no longer be identified 1 week after denervation. Neo natal deefferentation has little effect on these features of developin g spindles, so removal of afferent innervation is presumably the facto r that induces the loss of spindles in denervated muscles. Degeneratio n of the bag2 fiber, but not bag1 or extrafusal fibers, reflects a gre ater dependence of the bag2 fiber than the bag1 fiber on afferent inne rvation for maintenance of its structural integrity. This difference i n response of the two types of immature bag fiber to denervation might reflect an origin of the bag2 fibers from a lineage of myogenic cells distinct from that giving rise to bag1 or extrafusal fibers, or a dif ference in the length of contact with afferents between the two types of bag fiber prior to nerve section.