1. Abortive spikes in single nerve terminals of frog muscle spindles were recorded by means of a paraffin gap method.
2. One or sometimes two kinds of abortive spikes with different amplitudes or time courses were observed in spindles of single type in the sartorius muscle, while three to four kinds of quantal potentials were detected in spindle afferents supplying two capsules which are arranged in series (tandem type) or in parallel (compound spindle) in the semitendinosus muscle.
3. The amplitude of abortive spikes (usually 0.2mV) decreased with increase of the distance between the site of the receptor and the paraffin gap.
4. Tetrodotoxin abolished both abortive and propagated spikes without any effect on the generator potential.
5. When the node (N1) next proximal to the bifurcation node (NO) along the parent axon was crushed or anaesthetized, abortive spikes and generator potentials remained unchanged, but propagated spikes disappeared.
6. The amplitude of abortive spikes decreased during increase in the amplitude of generator potentials caused by muscle stretch.
7. The frequency of abortive spikes decreased of ter transiently increased with extension of the muscle and disappeared with muscle length of+8mm, while the propagated spikes increased stepwise with this extension. Further extension of the muscle resulted in steep increase in the frequency of the propagated spikes in absence of the abortive spike.
8. Histograms of intervals between one of propagated spikes and the preceding abortive spike or the subsequent propagated and abortive spikes show that occurrence of the propagated spike was always preceded by that of an abortive one and also that of ter a propagated impulse the preparation at slack condition became silent for 50-100 msec. This duration was noted to be close to that of a positive after-potential following the propagated spike.
9. No silent period was observed after an abortive spike, and two successive abortive spikes were often superimposed with each other.
10. The site of origin of the abortive spikes and the mechanism of initiation of propagated impulses from the spindle were discussed.
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