Elimination of muscle afferent boutons from the cuneate nucleus of the rat medulla during development.

There is developmental refinement of the proprioreceptive muscle afferent input to the rat ventral horn. This study explored the extent to which this occurs in the medulla. Muscle afferents were transganglionically labeled from the extensor digitorum communis forelimb muscle with cholera toxin B subunit. Tracer amounts and transport times were adjusted for animal size. Immunohistochemistry revealed tracer localization in the medulla and dorsal root ganglia. Labeled muscle afferent boutons were counted in the cuneate nucleus between postnatal days 7 and 42, during which time a large decrease in the density of labeled boutons was observed qualitatively. Localization of input to dorsolateral parts of the nucleus remained broadly the same at different ages, although disappearance of a marked innervation of ventromedial regions in more caudal sections was observed. Bouton counts were corrected for growth of the medulla with age, and any spread of tracer to adjacent muscles indicated by counts of labeled dorsal root ganglion neurons. There was a statistically significant, approximately 40% reduction in the number of muscle afferent boutons in the cuneate nucleus during this developmental period. Previous studies suggest that perturbations to the corticospinal input during a developmental critical period influence the eventual size of the muscle afferent input to the ventral horn. Corticocuneate fibers invade the nucleus during the same period and may influence reorganization of its muscle afferent input, making it another potential site for aberrant reflex development in cerebral palsy.