Matched transfer characteristics of single units in a compound slit sense organ

Summary

The transfer characteristics of the lyriform slit sense organ HS8 on the leg tibia ofCupiennius salei Keys. (Fig. 1) were investigated by electrophysiological experiments. The organ is made up of 7 slits. It was stimulated by deflecting the metatarsus toward the fixed tibia. The responses of the constituent slits were recorded individually.

• 1. The step responses of the 2nd to the 4th slits can be described bypower functions (t ^−k), where (t) is the time and (k) is a constant between 0 and 1 (Figs. 6, 8). This means that the sensilla take a position between a frequency independent system (displacement receptor) and a first order differentiator (velocity receptor). The standard deviations of the receptor responses coincide intra-individually and inter-individually (Fig. 5).

• 2. Thelinearity of the system was demonstrated using the second slit as an example:

• a) The slit responds proportionally to step amplitudes from 0.1° to 0.4°; (k) varies only slightly from 0.32 to 0.38 (Fig. 6).

• b) For stimulus frequencies of 0.01 Hz to 8 Hz the transfer functions calculated from the step response and those estimated from sinewave stimulation match well (Fig. 7).

• c) For stimulus frequencies of 0.01 Hz to 100 Hz the number of spikes/stimulus sinewave is nearly the same, no matter whether calculated from the step responses or measured by sinewave stimuli (Fig. 12).

• 3. Comparing the results for the 3 slit sensilla investigated (of a total of 7) the following conclusions are drawn:

• a) Thehigh-pass characteristic (exponentk) of the sensilla increases from the 2nd to the 4th slit, while the relative gain decreases (Fig. 8).

• b) Thebandwidth of the sensilla decreases according to decreasing sensitivity from slit 2 to slit 4, since the maximal stimulating frequency which can be encoded unequivocally (three spikes/stimulating sinewave) is less than 20–30 Hz for all the slits investigated (Fig. 12).

• c) Thefrequency-dependent threshold curves confirm the calculated transfer functions. For a stimulus frequency of 1 Hz the thresholds of the 2nd to the 4th slit are ca. 0.025°, 0.25° and 1.25° (deflection of the metatarsus), at 100 Hz 0.01°, 0.05° and 0.125° (extrapolated). From 0.01 Hz to 100 Hz no tuning of different slits to different “resonant frequencies” was seen (Figs. 11, 12).

• d) The characteristic curves (input amplitude vs. output-spike frequency) of the 2nd to 4th slit show an overlap of the amplitude ranges to which the sensilla respond; their linear parts, however, are linked up. This results in an extension of both the effective linear amplitude range and the range of precise detection of amplitude changes. The upper and lower limits of the linear parts of the characteristic curves depend on the stimulus frequency (Figs. 9, 10, 14, Eq. 6).

• 7. The receptor system can be described as consisting of a linear part with high-pass character plus a nonlinear rectifying part with a threshold. The linear part precedes the nonlinear one (Figs. 15, 16).