Synaptic Transmission Between Second- and Third-Order Neurones of Cockroach Ocelli

ABSTRACT The insect ocellus contains a large number of photoreceptors which converge on a small number of large second-order neurones, called L neurones. The L neurones exit from the ocellus and project into the ocellar tract of the brain, where they make synapses with third-order neurones (reviewed by Goodman, 1981). Response characteristics of L neurones have been well documented (Goodman, 1981; Mizunami et al. 1986), but less is known about ocellar third-order neurones. In locusts, three types of ocellar third-order neurones, which descend to the thoracic ganglia, have been found (Simmons, 1980,1981; Reichert et al. 1985), and the transmission characteristics between L neurones and some of these descending neurones have been documented (Simmons, 1981). In the present paper, we examine whether the transmission characteristics reported by Simmons (1981) are specific to descending ocellar third-order neurones or are generally applicable to a variety of ocellar third-order neurones, including those terminating within the brain. Experiments were made on cockroaches, Periplaneta americana.