Electrophysiology of the Insect Dorsal Ocellus : II. Mechanisms of generation and inhibition of impulses in the ocellar nerve of dragonflies

Large nerve fibers in the ocellar nerves of dragonflies are spontaneously active. In the absence of inhibitory influence the spontaneous activity is rhythmic. Inhibition occurs in the dark-adapted state and during illumination. Miniature inhibitory postsynaptic potentials occur in the dark-adapted state. These modulate by temporary suppression the otherwise rhythmic discharge of ocellar nerve impulses. The presence of random spontaneous receptor cell excitations is inferred from the presence of the miniature i.p.s.p.'s. Light stimulates many or all the receptor cells simultaneously, masking the random spontaneous activity of individual receptor cells. The result is a sustained hyperpolarizing i.p.s.p. and sustained inhibition of the nerve discharge. Preceding resumption of the spontaneous activity at "off" the i.p.s.p. may oscillate, overshoot the baseline as a negative after-potential, or do both. These phases of the off-effect may generate nerve impulses in an off-burst.