Purpose A motor unit consists of an alpha motor neuron and all of the muscle fibers that it innervates. During embryonic development, many neuronal connections from motor units are formed on the same muscle fiber. Most of these synaptic connections are then eliminated leaving only one nerve terminal to synapse on a muscle fiber. Of interest in our lab are the elements that cause these neuronal axons to seek out their respective target tissues and survive the period of synapse elimination. The mouse gluteus maximus muscle is innervated primarily by a single nerve, the inferior gluteal nerve. Because the gluteus maximus muscle is thin, it approximates a 2-dimensional space. This allows us to eliminate the issue of depth from this study and focus instead on position in only the x- and y-directions. The single-nerve innervation and muscle thinness facilitate our ability to test the hypothesis that the inferior gluteal nerve forms a positionally-restricted map over the rostrocaudal axis of the gluteus maximus muscle.
Methods We examined the distribution and characteristics of single motor units of the inferior gluteal nerve supplying the gluteus maximus muscle. We used mice genetically engineered to express Yellow Fluorescent Protein (YFP) in motor neurons and their axons. The YFP protein contains a fluorophor that can be visualized by confocal fluorescent microscopy. Each axon was traced through the muscle to its respective terminal endings. We then measured the positions and areas of these nerve terminals. The number of nerve terminals was also recorded as a function of their rostrocaudal distance across the muscle.
Results Single motor units were found to occupy intervals of approximately 20–;30% of the total rostrocaudal length of the muscle. Motor units with a relatively larger number of nerve terminals were found to cover larger rostrocaudal intervals. Also it was found that the nerve terminal areas varied widely within a given motor unit.
Conclusions These results show that it is possible to map individual motor units and that variability exists in both position and nerve terminal area within a single motor unit. Ultimately our observations may be generalizable to other areas, including the CNS. This work may be useful in development of therapeutic techniques for injuries causing nervous tissue damage.
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