ADAPTATIONS OF DIAPHRAGM NEUROMUSCULAR-JUNCTION FOLLOWING INACTIVITY

We hypothesized that differences exist in the morphological adaptation s of neuromuscular junctions (NMJs) on different fiber types in respon se to prolonged inactivation, Two weeks of inactivity of both phrenic motoneurons and diaphragm muscle was induced by spinal cord hemitranse ction at C2 (spinal isolation; SI). A three-color fluorescent immunocy tochemical technique, combined with laser-scanning confocal microscopy , was used to create two- (2D) and three-dimensional (3D) images of NM Js and obtain morphological information concerning: (1) innervating ax ons and presynaptic nerve terminals; (2) motor endplates (postsynaptic apparatus consisting of acetylcholine receptors), and (3) myosin heav y chain (MHC) phenotype of muscle fibers, In both sham controls (CTL) and SI animals, planar (2D) and surface (3D) areas of motor endplates and nerve terminals on type II muscle fibers (anti-fast MHC immunoreac tive) were smaller than on type I (nonimmunoreactive to anti-fast MHC) fibers, when normalized for fiber diameter. The number of branches, t otal branch length and perimeter of both motor endplates and nerve ter minals were greater for NMJs on type II fibers than on type I fibers. The extent of overlap between nerve terminal and endplate was greater on type I fibers than on type II fibers, After SL there was a signific ant expansion of NMJs on type II fibers, Planar and surface areas of m otor endplates and nerve terminals, number of endplate and nerve termi nal branches, total branch length, and perimeter were all increased on type II fibers following SI, The extent of overlap of nerve terminal and endplate increased on type II fibers, approaching that observed in type I fiber NMJs, These results indicate that neuromuscular inactiva tion leads to a selective expansion of type II fiber NMJs through addi tion of new terminal area, and elongation of existing terminal branche s. These changes may represent a compensatory effort to improve neurom uscular transmission.