Sensory terminal responses of frog muscle spindle recorded across vaseline gap onto intrafusal muscle fibre.

When electrical activities of the sensory terminal in the frog muscle spindle were recorded across a vaseline gap which was oriented transversely to the intrafusal muscle bundle (IFM-gap), large and small spikes occurred in coincidence with propagated and abortive spikes respectively recorded across the axon-gap. The IFM-gap placed on the receptor region displayed triphasic spikes in which the first small positive deflection occurred approximately 0.2 ms prior to the propagated and abortive spikes. No such deflection was observed when the IFM-gap was placed more than 50 mum away from the receptor region. Increasing the distance from the receptor region to the IFM-gap resulted in an exponential decay in the amplitudes of both the large and small spikes. Regardless of the position of the IFM-gap relative to the receptor region, bimodal distributions in the amplitude histograms of the large spikes, and large variations in the shapes of the small spikes were observed. These results indicate that the IFM-gap responses may represent electrical activity of the non-myelinated branches, along which propagated impulses may arise from at lest two separate origins, while abortive spikes arise from many more sites.