THE CRAYFISH VISUAL SYSTEM: INTRACELLULAR STUDIES AND MORPHOLOGIES OF IDENTIFIED INTERNEURONS
KIRK, MARK DOUGLAS
Doctor of Philosophy
A semi-intact eye-cup preparation was developed which maintains visual responses and enables intracellular recordings to be made from the optic lobes of the crayfish compound eye. Sustaining fibers (SFs) were impaled with lucifer yellow electrodes near their entrance to the second optic ganglion or Medulla (where the SFs originate). Corneal receptive fields were determined and the SFs identified based on the previous work of Wiersma and Yamaguchi (1966). Following identification of each cell, the dendritic morphology was observed with lucifer yellow iontophoresis and subsequent fluorescence microscopy. An individual SF possesses a dendritic arborization restricted to that portion of the Medulla corresponding to its corneal receptive field. Therefore, the position of the dendritic tree combined with the retinotopic organization within the distal optic neuropils determine each SF's visual receptive field. Contour sensitivity maps were obtained for SF 019. The quantitative variation in sensitivity along a given arc in visual space was correlated with 019 dendritic density along the corresponding portion of the Medulla. The SFs' responses to various stimulus conditions were recorded intracellularly in their integrating segments. No synaptic interactions between SFs were observed. Bursting in response to broad-field intense illumination results from periodic excitatory synaptic input. Other visual interneurons were impaled and dye-filled. These included, (a) Phasic light-on cells, (b) Tonic light-off cells, (c) Phasic light-on and -off cells, (d) Optic tract motion detectors, (e) Nonspiking light-on neurons, (f) Lamina monopolar cells, and (g) Lamina tangential cells (TAN2). A group of neurosecretory cells analogous to the Medulla Externa X-Organ of other crustaceans was discovered in the crayfish. These neurosecretory cells are also interneurons possessing a modality specific (visual) synaptic input. They are inhibited by step increases in illumination and the response lasts for the duration of the stimulus. Transient off-inhibition is commonly present. The IPSPs reverse near resting membrane potentia and appear to be GABA mediated and chloride-dependent. These cells project axons to the proximal edge of the Lamina and have a vertical plane of dendrites in the same layer of the Medulla as the SFs. It is proposed that these cells are involved in the neurosecretory control of the circadian rhythm of screening pigment migration and/or photoreceptor sensitivity in the retina.