Neuronal mechanisms of a hydromedusan shadow reflex

Abstract

1. The photosensitive ‘O’ system neurons showed a graded hyperpolarization during various ‘instantaneous’ decreases in light intensity and graded depolarization during increases in light intensity. The ‘O’ system failed to respond to ‘instantaneous’ changes in light intensity of less than about 28% (Fig. 1a). 2. The two motor systems, the SMNs and ‘B’ system which drive swimming muscle and tentacle contractions respectively, showed graded depolarizations and spiking frequencies during various ‘instantaneous’ decreases in light intensity (Fig. 1b, c). Increases in light intensity initially cause hyperpolarizations of these two systems. The SMNs failed to respond to ‘instantaneous’ changes in light intensity of less than about 26% (Fig. 1b). 3. The ‘O’ system and SMNs respond with little habituation to repetitive changes in light intensity. Each system shows a near constant response to ‘instantaneous’ 100% changes in light intensity to all except the lowest light intensities tested (Figs. 2, 3). 4. The rate of ‘O’ system hyperpolization is directly proportional to the rate of percentage decrease in light intensity (Figs. 4, 5, 7a). 5. The SMNs and ‘B’ system show graded depolarizations and changes in spiking frequencies during ‘slow’, continuous decreases in light intensity. These two motor systems also show graded hyperpolizations during ‘slow’ increases in light intensity (Figs. 6, 7). 6. The ‘O’ system neurons show properties common to photoreceptors in general, namely graded responses to light intensity changes and electrical coupling between adjacent cells.