Terminal sprouting of mouse motor nerves when the post-synaptic membrane degenerates

Injection of echothiophate, 4-aminopyridine and caffeine into the mouse calf produced necrosis in the endplate region of approximately 40% of soleus muscle fibres. Within two days terminal sprouting, as seen by zinc iodide/osmium tetroxide staining, had occurred at nearly a quarter of such endplates, but not at neighbouring intact ones. Almost half of these sprouts were greater than 50 microns in length. Terminal sprouting at degenerating endplates was also seen in identically treated silver-stained gluteus maximus muscles. Muscle degeneration caused by mechanical damage produced similar effects. In transverse sections of the gluteus maximus preparation, the terminals could be found within the necrotic muscle fibres, having penetrated the synaptic basal lamina. It is concluded that motor nerve terminals have an intrinsic tendency to grow, and are normally prevented from doing so by their formation of synapses with muscle fibres. Destruction of this relationship alone can cause terminal sprouting.