Les reponses retinomotrices: Caracteres et mecanismes

Abstract

The rods, cones and retinal epithelial pigment of teleosts, anurans, urodeles and birds undergo positional changes in response to light or darkness. The changes may take from a few minutes to several hours for completion. The phylogenetic variations observed represent an adaptive radiation of the retinomotor responses. In fishes and amphibians there is an ontogenetic parallel between the structural and functional development of the retinal elements. There is also a good correlation between retinal structure, responses, habitat and behaviour. This helps an understanding of phenomena such as phototaxis, feeding and migration. Light intensity and wavelength influence the positions of the retinal elements. Temperature and time of day do so to a lesser extent in continuous darkness and to a still lesser extent in constant light. In higher light intensities and temperatures changes from light to dark or vice versa occur faster than in lower intensities and temperatures. Retinomotor responses to wavelengths of equal energy indicate the visible spectrum of the animal. The presence of a diurnal rhythm as indicated by the position of retinal elements is not universal. Retinomotor responses occur as a result of direct photic stimulation of the eye and not of the illumination of other parts of the body. Enucleated eyes are more or less capable of retinomotor responses; this varies in animals. Unilateral illumination has a small effect on the unexposed eye indicating some neural or hormonal interactions. The action spectrum of the expansion of the retinal epithelial pigment and the contraction of cones show a direct or inverse relationship to the absorption spectrum of the scotopic visual pigment. The contraction of the cone myoids could be inhibited by Ouabain and PCMB; but neither TPA staining nor electron microscopy shows the presence of contractile elements in the myoids similar to those seen in other contractile cells like smooth muscle fibres (myosin or actin fibrils). The hypotheses of Garten, Walls and Studnitz concerning the significance of the retinomotor responses are examined in the light of our present knowledge and all three are found to be valid.