Dorsal ocelli are small cup-like organs containing a layer of photoreceptor cells, the short axons of which synapse at the base of the cup with dendritic terminals of ocellar nerve fibers. The ocellar ERG of dragonflies, recorded from the surface of the receptor cell layer and from the long lateral ocellar nerve, contains four components. Component 1 is a depolarizing sensory generator potential which originates in the distal ends of the receptor cells and evokes component 2. Component 2 is believed to be a depolarizing response of the receptor axons. It evokes a hyperpolarizing postsynaptic potential, component 3, which originates in the dendritic terminals of the ocellar nerve fibers. Ocellar nerve fibers in dragonflies are spontaneously active, discharging afferent nerve impulses (component 4) in the dark-adapted state. Component 3 inhibits this discharge. The ERG of the cockroach ocellus is similar. The main differences are that component 3 is not as conspicuous as in the dragonflies and that in most cases ocellar nerve impulses appear only as a brief burst at "off." In one preparation a spontaneous discharge of nerve impulses was observed. As in the dragonflies, this was inhibited by illumination.

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