No abstract

The effects of denervation and direct stimulation in fast and slow latissimus dorsii muscles were investigated in chicken. In slow ALD muscle, denervation resulted in an incompleteness of the relaxation, a decrease in MDH and CPK activities and an increase in fast myosin light chains (MLC) accumulation. Direct stimulation at either fast or slow rhythm prevented the effects of denervation on relaxation and CPK activity but was ineffective on MDH activity and fast MLC accumulation. Moreover, direct stimulation of denervated ALD caused rhythm-dependent change in tetanic contraction. In fast PLD muscle, the main changes in muscle properties following denervation were a slowing down of the time course of the twitch and an incompleteness of the relaxation, a decrease in LDH and CPK activities and in LC3F accumulation. Stimulation at a high frequency partly prevented the effects of denervation and resulted in a large accumulation of LC3F, while a low frequency stimulation did not restore the twitch time to peak, increased MDH activity and induced synthesis of slow MLC. This study emphasizes the role of muscle activity and its pattern in some properties of slow and fast chicken muscles following denervation.

The present study investigated the effects of spinal cord stimulation, neuromuscular blockade, or a combination of the two on neuromuscular development both during and after the period of naturally occurring motoneuron death in the chick embryo. Electrical stimulation of the spinal cord was without effect on motoneuron survival, synaptogenesis, or muscle properties. By contrast, activity blockade rescued motoneurons from cell death and altered synaptogenesis. A combination of spinal cord stimulation and activity blockade resulted in a marked increase in motoneuron death, and also altered synaptogenesis similar to that seen with activity blockade alone. Perturbation of normal nerve-muscle interactions by activity blockade may increase the vulnerability of developing motoneurons to excessive excitatory afferent input (spinal cord stimulation) resulting in excitotoxic-induced cell death.

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.