Bipolar cells in the “grouped retina” of the elephantnose fish (Gnathonemus petersii)
To elucidate the specific properties of retinae with grouped photoreceptors the neural pathways in the outer and inner plexiform layer were studied. Photoreceptor bundles in this species consist of more than 100 rods and up to 50 cones, and are usually regarded as functional units. Golgi impregnation in thick and thin sections and light microscopy were used to identify bipolar cell types linking photoreceptors to amacrine and/or ganglion cells. Nine different types were distinguished based on their dendritic morphology and the position of the axon terminal in the inner plexiform layer. Small cells have dendritic fields smaller than the diameter of a photoreceptor bundle and are contacted mostly by cones. The dendritic field size of bushy cells matches that of a photoreceptor bundle; they are contacted mainly by rods. Flat cells receive about equal input from rods and cones; their dendritic field size exceeds the bundle diameter. Within the three major classes there are subtypes addressing three sublaminae of the inner plexiform layer, the proximal On-centre region (sl b), the distal Off-centre region (sl a) and a central sublayer (sl c) probably with transient activity. These observations suggest that cone vision has a spatial acuity better than the “bundle grain”. In rod dominated vision the resolution matches that of the bundles; for this pathway, the hypothesis of the bundle as a functional unit is confirmed. The mesopic flat cell pathway has a resolution inferior to the “bundle grain”; it may therefore be dedicated to movement detection.(Received November 30 2006)
(Accepted April 11 2007)
Key Words: Outer plexiform layer; Inner plexiform layer; Rod-dominant pathway; Cone-dominant pathway.
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