Influence of spinal cord stimulation on the innervation pattern of muscle fibers in vivo.

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.