Effects of stimulation and rest on the ultrastructure of the excitatory neuromuscular junctions of Locusta migratoria L.

The terminals of the fast axon on extensor tibiae muscle fibres of Locusta were examined in untreated nerve-muscle preparations and in preparations stimulated electrically at frequencies varying from 0.5 to 100 Hz. The ultrastructure of the terminals in preparations stimulated at the lower range of these frequencies, which induce twitch contractions of the muscles, is similar to that of the controls. Stimulation at the higher frequencies induced tetanic muscle responses and rapid fatigue of the muscles after which they would not respond again to high frequency stimulation for about 1 h. This loss and recovery of the responses of the muscles is correlated with changes in the ultrastructural appearance of the terminals, in particular in the number and shape of the synaptic vesicles. The ultrastructure of these "recovering" axon terminals closely resembles that of the controls.