Non monotonic morphometric changes produced at mouse neuromuscular junctions following in vivo stimulation at various frequencies.

Morphometric changes have been studied at the mouse neuromuscular junction (NMJ) as a function of the frequency of stimulation. In vivo stimulation was made at either 10, 25, 50 or 100 Hz. No significant change in the area of NMJ profiles is observed after stimulation. This indicates that stimulation does not change the volume of the nerve terminal. There is also no change of the volume density of mitochondria. However, there are significant reductions of the numerical density on area (NA, number/micron2) of the clear vesicles following stimulation at 25, 50 or 100 Hz. The NA of clear vesicles is not further reduced following stimulation at 50 or 100 Hz than after the stimulation at 25 Hz. The numerical density on area of the coated vesicles was significantly increased following the stimulation at 25 Hz and 100 Hz. It is not increased more following stimulation at 50 Hz or 100 Hz than following stimulation at 25 Hz. One possible interpretation of the clear vesicle and the coated vesicle results is that less vesicles are liberated per sec by the NMJ with stimulation at 50 and 100 Hz than at 25 Hz because the action potential may fail to invade some terminal arborizations. The total surface density (i.e. surface of membrane in micron2 per micron3 of nerve terminal) of the clear vesicles, the coated vesicles, the vacuoles and the presynaptic plasma membrane is not changed significantly following stimulation at any frequency. This indicates that there is no net loss of membrane at any of the frequencies investigated.