Threshold response phase to sinusoidal stimulation of frog cutaneous mechanoreceptor afferent units

While applying sinusoidal indentations of various frequencies and amplitudes to the receptive field of cutaneous mechanoreceptor afferent units in frogs, we measured latencies of spikes after the time point when sinusoidal stimulation crossed the neutral position from withdrawal to indentation phase. Frog type I (Ft I) and frog type II (Ft II) slowly adapting (SA) units produced spikes only at the indentation phase, and the threshold response phase (TRP), i.e.. the phase of the first spike was for ca. 45 degrees at a small amplitude of stimulation. At low frequencies of sinusoidal stimulations, the TRP of Ft II units advanced gradually with an increase in amplitude, conforming to the theoretical expectation as displacement-detectors. But the TRP of Ft I units did not show such an advance with an increase in stimulus amplitude. Rapidly adapting (RA) type I and type II units mostly produced phase-locked discharged at two different phases; the former discharged at ca. 0 degrees and 180 degrees (phase of the highest velocity) and the latter at ca. -90 degrees and 90 degrees (phase of the highest acceleration) at the smallest amplitude used. At a large amplitude, RA type II units showed advance in discharge phase like a theoretical acceleration-detector, but performance of RA type I units dit not fit that of the theoretical velocity-detectors. Both types of SA units showed frequency-phase relations which roughly agreed with the performance of the theoretical displacement-detectors in the range below 5 Hz, but RA units of either type did not follow the expectation as the theoretical velocity-detector or acceleration-detector.