Lack of correlation between physiological and morphological features of regenerating frog neuromuscular junctions

Morphological, ultrastructural and electrophysiological properties of endplates in the frog cutaneous pectoris muscle were monitored at various times after nerve crush. The lengths of regenerating terminals were measured in nitroblue tetrazolium-stained material. Quantal content and the frequency of spontaneous potentials were obtained from intracellular records, and they were correlated to individual terminal lengths in order to normalize their values to a standard unit of terminal length. By day 16 after nerve crush, terminal length reached a plateau near 90% of normal. In contrast, quantal content per unit terminal length quickly recovered to 67% of normal by 7-12 days but did not increase thereafter. The frequency of spontaneous potentials per unit terminal length returned much more slowly, as it remained low (near 18% of normal) throughout the entire first month before recovering to normal levels later on. Ultrastructural observations on terminal size, densities of mitochondria and synaptic vesicles, and multiple innervation could not explain these different recovery rates. It is concluded that terminal length, spontaneous transmitter release and evoked transmitter release, are not restored in parallel after reinnervation.