Response properties of dipteran giant visual interneurones involved in control of optomotor behaviour

Abstract

GIANT homolateral interneurones in the third optic ganglion of flies respond to visual stimulation with graded potentials only, lacking spike generation for signal transmission in physiological conditions¹. (The term ‘homolateral’ is used here in its anatomical connotation of neurones which are anatomically restricted to one side of the brain⁵.) Graded potentials were thought to be the means of signal transmission of all three classes of homolateral giant elements of this neuropile region, that is, the ‘vertical cells’ (V cells), the ‘centrifugal horizontal cells’ (CH cells) and the ‘horizontal cells’ (H cells): both CH and H cells are mainly sensitive to horizontal pattern movement, whereas the V cells respond mainly to vertically moving patterns^1,2. (The CH cells have an additional sensitivity to vertical, some V cells to horizontal pattern motion.) However, there is experimental proof that these findings might be valid only for the CH cells in which exclusively graded potentials could be recorded showing discrete excitatory and inhibitory postsynaptic potentials^1,2. For the V cells, however, it has been shown that spiking can be induced by application of hyperpolarising currents². In the absence of hyperpolarising currents, graded potentials with superimposed ‘local’ action potentials of small amplitude were observed^1,2. The H cells, on the other hand were reported to generate only ‘local’ action potentials superimposed on to postsynaptic slow potentials¹: these findings¹ are not compatible with the previous³ results or with the results presented here which demonstrate regular action potential activity in H cells in physiological conditions.