Journal of Neuroscience, Vol 11, 3840-3850, Copyright © 1991 by Society for Neuroscience
Influence of spinal cord stimulation on the innervation pattern of muscle fibers in vivo
C Fournier Le Ray and J Fontaine-Perus
Universite de Nantes, Faculte des Sciences et des Techniques, Centre National de la Recherche Scientifique, URA 1340, France.
In chick embryo, chronic stimulation of the brachial spinal cord at a fast
rhythm from days 7 to 18 of development induced an increase in AChE
activity sites and ACh receptor (AChR) clusters in slow anterior latissimus
dorsi (ALD) muscle. Most AChR clusters and AChE spots were contacted by
nerve endings. A previous study showed that such spinal cord stimulation
causes changes in ALD muscle properties, especially the appearance of a
high proportion of fast type II fibers (Fournier Le Ray et al., 1989).
Analysis of the synaptic pattern in different fiber types of experimental
ALD muscle indicated a decrease in the distance between successive AChE
spots in slow type III fibers compared to controls, whereas the
intersynaptic distance in fast type II fibers was very similar to that in
the rare fast fibers developing in control ALD. Fast fibers of experimental
muscles exhibited less AChR than did slow fibers. The increased number of
neuromuscular junctions in ALD muscle after spinal cord stimulation
appeared to be preferentially located in slow fibers. Electron microscopy
showed no change in the number of axons in ALD nerve after spinal cord
stimulation. The activity imposed on brachial motoneurons apparently caused
terminal sprouting of ALD nerve in target muscle, thus accounting for the
increase in neuromuscular contacts in ALD muscle fibers. Differences in the
distribution of nerve contacts indicate that the type of muscle fiber
innervated may play a critical role in the synaptic pattern during chick
embryogenesis.