An equivalent circuit for the quantitative description of inter-receptor coupling in the retina of the desert antCataglyphis bicolor

Summary

• 1. An ionic equivalent circuit model is developed for the purpose of quantifying the strengths of electrical coupling existing between certain receptor cells in the compound eye ofCataglyphis bicolor which show wavelength dependence in the slopes of their ‘V logI’ relations and orientations of maximum PS (Mote and Wehner, 1980). It combines the pure electrical equivalent of the plasma membrane, as proposed by Finkelstein and Mauro (1963), and a closed series circuit such that Kirchhoff's laws are applicable.

• 2. The model is applied to hypothetical pairs of cells with different sensitivities and different strengths of interaction. It predicts that cells are most strongly coupled when at rest and most weakly coupled when most active. When one is active and the second is not then “apparent rectification” can occur since the coupling strength is not symmetrical. This means that both “resting” and “dynamic” coupling coefficients must be considered. Transition between these modes causes irregularities in the ‘V logI’ relation of a weakly excited cell which is coupled to a strongly excited cell.

• 3. The model is then applied to data obtained from photoreceptors in the compound eye ofCataglyphis. It adequately simulates the wavelength dependence of both the ‘V logI’ relation (Fig. 6) and the orientation of maximum PS (Fig. 7) measured in these cells. In its simplest form the model permits an estimate of the coupling coefficients which are greater than 0.6 for strongly coupled cells and less than 0.4 for weakly coupled cells.

• 4. A hypothetical treatment of pairs of coupled cells under conditions approaching the natural situation in the animal's environment suggests that super-numerary analyzers of polarized light in the u.v. could arise through receptor coupling in certain ommatidial types found in the eye ofCataglyphis.