Phrenic motoneuron morphology during rapid diaphragm muscle growth

In the adult rat, there is a general correspondence between the sizes of mo toneurons, motor units, and muscle fibers that has particular functional im portance in motor control. During early postnatal development, after the es tablishment of singular innervation, there is rapid growth of diaphragm mus cle (Dia(m)) fibers. In the present study, the association between Dia(m) f iber growth and changes in phrenic motoneuron size (both somal and dendriti c) was evaluated from postnatal day 21 (D21) to adulthood. Phrenic motoneur ons were retrogradely labeled with fluorescent tetramethylrhodamine dextran (3,000 MW), and motoneuron somal volumes and surface areas were measured u sing three-dimensional confocal microscopy. In separate animals, phrenic mo toneurons retrogradely labeled with choleratoxin B-fragment were visualized using immunocytochemistry, and dendritic arborization was analyzed by came ra lucida. Between D21 and adulthood, Dia, fiber cross-sectional area incre ased by similar to 164% overall, with the growth of type II fibers being di sproportionate to that of type I fibers. There was also substantial growth of phrenic motoneurons (similar to 360% increase in total surface area), du ring this same period, that was primarily attributable to an expansion of d endritic surface area. Comparison of the distribution of phrenic motoneuron surface areas between D21 and adults suggests the establishment of a bimod al distribution that may have functional significance for motor unit recrui tment in the adult rat.