AbstractThis study was conducted to determine the effects of chronic phrenicotomy on spinal hemisection-induced morphological plasticity occurring in the phrenic nucleus. Young adult rats were divided into a hemisection-alone and two hemisection-plus-phrenicotomy (HPP) groups. HPP animals received a left phrenicotomy two or four weeks prior to sacrificing; whereas hemisection-alone animals did not. All animals received a left C2 spinal hemisection 24 hours prior to death. Quantitative morphometric analysis of the phrenic nucleus showed significant reductions in phrenic dendritic size and the number of dendrodendritic appositions in HPP (two week) animals and in the length of dendrodendritic appositions in HPP (four week) animals. Significant increases in microglial area fraction in HPP (two week) animals and in astroglia area fraction in HPP (four week) animals were also detected. The results suggest that the alterations in the spinal hemisection-induced dendrodendritic apposition formation is most likely influenced by the different stages of the glial reactions induced by the chronic phrenicotomy/spinal hemisection. (J Spinal Cord Med: 19:58–70)