A morphological technique for exploring neuromuscular topography expressed in the mouse gluteus maximus muscle

Motor neuron pools innervate muscle fibers forming an ordered topographic map. In the gluteus maximus (GM) muscle, as well as additional muscles, we and others have demonstrated electrophysiologically that there exists a rostrocaudal distribution of axon terminals on the muscle surface. The role of muscle fiber type in determining this topography is unknown. A morphological approach was designed to investigate this question directly. We combined three different methods in the same muscle preparation: (1) the uptake of activity-dependent dyes into selected axon terminals to define the spinal segmental origin of a peripheral nerve terminal; (2) the fluorescent labeling of nicotinic acetylcholine receptors to determine motor endplate size; (3) the immunocytochemical staining of skeletal muscle to determine fiber subtype. We applied these methods to the mouse GM muscle to determine the relationship between muscle fiber type and the topographic map of the inferior gluteal nerve (IGN). Results from this unique combination of techniques in the same preparation showed that axon terminals from more rostral spinal nerve segments of origin are larger on rostral muscle fibers expressing myosin heavy chain (MyHC) IIB epitope than caudal type IIB fibers. Because type IIB fibers dominate the GM, this suggests that for these rostral axons terminal size is independent of fiber type. How this axon terminal size is related to the topographic map is the next question to be answered.