A morphological assessment of neuromuscular topography in the mouse gluteus maximus muscle

Spinal cord motor neuron pools project axons that form ordered topographic maps of connectivity onto the surface of skeletal muscles. This study looked at neuromuscular topography as it relates to the mouse gluteus maximus (GM) muscle. Experiments were designed to exploit the nature of structures found at the GM muscle neuromuscular junction. First, measuring one parameter of transmitter release, quantal content, an electrophysiological measurement of the GM topography was sought. Second, we sought a morphological correlation of this topography. Here we found that axon terminal areas differed rostrally and caudally in the GM muscle. Axon terminal areas were determined by using an activity dependent dye, FM 1-43, which was taken up only by specifically activated axons from a stimulated lumbar ventral root. Third, we performed immunocytochemistry to determine both muscle fiber type and subtype of the GM. We asked whether muscle fiber type could influence the topographic projections of the lumbar motor axons. A novel technique presented in this study shows that the axon terminal source and muscle fiber subtype could be tested in the same preparation by using activity-dependent dyes, fluorescent-labeled alpha-bungarotoxin, and myosin heavy chain subtype immunocytochemistry to identify active motor endplates, and to pair this with muscle fiber subtype. A fourth approach to analyze the mechanism for the topographic map was to use the GM muscle of mice overexpressing the ephrin-A5 gene. We asked whether the differences in axon terminal areas would remain in muscles with overexpression of ephrin-A5. Consistent with previous work showing that topography is lost in these overexpressors, we found that differences in axon terminal size were lost as well. We now have a morphological correlate of the topographic map, axon terminal size, and have found that procedures that alter this map eliminate these morphological differences. Taken together these results show that the GM is a useful model for determining specific aspects of neuromuscular topography.