NEURONAL MECHANISMS OF A HYDROMEDUSAN SHADOW REFLEX .1. IDENTIFIED REFLEX COMPONENTS AND SEQUENCE OF EVENTS

1. Intracellular recordings from three neuronal systems (swimming motor neurons - SMNs, burster - ''B'' system, oscillator - ''O'' system) and two effector systems (swimming and tentacle myoepithelium) are used to demonstate the mechanism of the stereotypic shadow reflex of the hydromeduan Polyorchis penicillatus. 2. The two motor systems, SMNs and ''B'' system which drive swimming muscle and tentacle muscle contractions respectively, spike spontaneously in lighted conditions. At light OFF, these systems depolarize and give a burst of action potentials (Figs. 2a, 3a, b). This response is lost if the ocelli are removed (Figs. 2b, 3c). Neither system showed any bursting activity in response to decreasing light intensity when chemical synapses are blocked by bathing the preparation in excess Mg++ (Figs. 2c, 3d, 4e). 3. The non-spiking ''O'' system shows regular membrane potential oscillations in lighted conditions. At light OFF, the ''O'' system rapidly hyperpolarizes and the oscillations cease (Fig. 4a, b). There is a similar response to shadows after the ocelli are removed (Fig. 4c). Even when chemical synaposes are blocked by excess Mg++, the ''O'' system shows a rapid hyperpolization at light OFF and depolarization at light ON (Fig. 4e). 4. Typical ''O'' system recordings can be made from deep within the ocellar cup (Fig. 5). 5. ''B'' system action potentials show a 1:1 correlation with large EPSPs which can be recorded from the tentacle myoepithelium and epithelium covering the outer nerve-ring (ONR) (Figs. 6-9). Thsee 4-6 mV EPSPs are delayed by 8-10 ms from the peak of the "B'' system action potentials and are correlated with tentacle contractions. 6. The behavior, sequence of the shadow reflex consist of initial rapid swimming muscle contraction, followed by several rapid tentacle contractions and is completed by 2-4 additional swimming muscle contractions (Fig. 11).