Estimation of generator potentials in primary spindle endings evoked by brief tendon taps in man.

The ligamentum patellae in human subjects was tapped by a random triangular waveform, while single primary spindle afferent spikes were recorded from the femoral nerve. Cross-correlation between the primary spindle afferent spikes and the taps revealed a prominent peaked distribution of the discharge probability. The cross-correlograms showed that response time from the onsets of taps to the afferent spike was 7.5 +/- 0.6 msec in 6 units and that the width between the feet of the peaked distribution was 3.3 +/- 0.2 msec in 6 units. The width of the peak implies jitter of the response time of the primary spindle spikes which appear to correspond with the time-to-peak of generator potentials elicited by the taps in primary endings of muscle spindles. Integration of the peaked distribution showed a slow slope at the beginning and near the summit and a steep slope on the way. This curve is assumed to represent the rising phase of the generator potentials in primary spindle endings.