Characteristics of interneurons in the ocellar system of the honeybee.

Intracellular recording and staining has revealed new aspects of the anatomy and physiology of ocellar interneurons in the honeybee (Apis mellifera). Homologous large first-order interneurons (L-neurons) from different individuals vary in structure as well as in their response to light. Even the response patterns of one type of L-neuron may present a continuum from graded responses to spiking responses. The response depends on the state of the L-neuron and cannot be correlated with particular morphological types. Graded responses are unimodal and can be elicited exclusively by stationary illumination of the ocelli, whereas spiking L-neurons can also respond to stimuli of other sensory modalities. Recordings from the same type of multimodal spiking L-neuron in different individuals demonstrate considerable variation in responses to different modalities. The existence of two response patterns in the same cell that can encode different information may be of general importance with respect to the processing of information in the brain. There appears to be a variety of different pathways enabling interaction of ocellar and compound eye signals either by direct contact between ocellar and optic lobe neurons or by convergence of the two pathways to a common descending or local interneuron. Two examples are discussed: an ocellar small first-order interneuron (S-neuron) that project directly to the optic lobes and a local interneuron that is situated in the posterior midbrain close to situated in the posterior midbrain close to ocellar terminals and optic lobe outputs and that responds to stimulation of both visual systems.(ABSTRACT TRUNCATED AT 250 WORDS)