Site of origin of calcium spike in frog muscle spindle

In order to define the site of origin of the calcium spike along the sensory nerve terminal of the frog muscle spindle, changes in the amplitude of the spikes during long-lasting polarizations of the nerve terminal were compared with those of full-sized and abortive spikes which occurred spontaneously. The amplitude of the calcium spikes superimposed on a spindle potential was not modifiable by application of polarizing currents of 6 nA or less, as well as that of the abortive spikes; while the amplitude of the full-sized spikes depended upon the polarizations. These results suggest that the calcium spikes may initiate along the non-myelinated filaments, as in the abortive spike, in contrast with the full-sized spikes to generate at the terminal node of the myelinated axon.