Abstract
We observed very different conduction velocities within the receptive field of fast type On-Off transient amacrine cells tightly coupled to each other via gap junctions. The fastest speeds were found in the dorsal area of the receptive fields, on average 5 times faster than those in the ventral area. The asymmetry was similar in the On- and Off- responses, thus being independent of the pathway, indicating the existence of a functional mechanism within the recorded cells themselves. However, the spatial decay of the graded-voltage photoresponse within the receptive field was found to be symmetrical, with the amplitude centre of the receptive field being displaced to the faster side from the minimum-latency location. Around 75% of varicosity-laden polyaxons in neurobiotin-injected cells were directed dorsally from the origin cells. Based on these results, we modelled the velocity asymmetry and the displacement of amplitude centre by adding a contribution of an asymmetric axonal feedback. Due to the asymmetry in the conduction velocity, the time delay of a light response is proposed to depend on the origin of the photostimulus movement, a potentially important mechanism underlying direction selectivity within the inner retina. [Jpn J Physiol 54 Suppl:S164 (2004)]
