VISUAL SYSTEM: STATE OF THE ART 



ROD 



59 



CONE 



ROD 



CONE 



25 fJrn 



Figure 12 Receptor/horizontal cell organization in the 

 retina of Mustelus canis. HI represents the external 

 layer of horizontal cells which appears to contact rods 

 alone. H2, the intermediate layer, also contacts only 

 rods. The internal layer of horizontal cells (H3) appears 

 to contact cones alone. ELM = external limiting 

 "membrane"; ENL = external nuclear layer; EPL = 

 external plexiform layer; INL = internal nuclear layer. 

 (Redrawn from Stell and Witkovsky (19736). Repro- 

 duced by kind permission of the authors and the 

 Journal of Comparative Neurology © 1973 Wistar 

 Press.) 



mobranchs were also reported by Stell (1966, 1972a) and by Hama in a 

 personal communication to Stell (1972a). The importance of gap junctions, 

 which may be defined as a special type of close membrane apposition with 

 space between opposing cells reduced to 20-40 A, is that they form sites of 

 low resistance to electrotonic spread. This implies that horizontal cells are 

 electrically coupled, which Kaneko elegantly demonstrated by recording the 

 S-potential (discussed in detail later) of adjacent horizontal cells. He found 

 that injection of current into one cell polarized neighbors separated by up to 

 five cells. Such electrotonic spread was not found between different horizon- 

 tal cell layers or between horizontal and bipolar cells. Coupling was con- 

 firmed by injection of Procion (fluorescent) dye into horizontal cells which 

 diffused into neighboring cells. Procion did not enter bipolar cells or cells of 

 other horizontal cell layers. Thus evidence was presented favoring Yamada 

 and Ishikawa's (1965) suggestion that horizontal cell layers form an elec- 

 trical syncitium, which also explained the extraordinarily large receptive 

 fields of S-potentials that originate in horizontal cells. 



In a preliminary study Kaneko et al. (1976) investigated completely 

 isolated horizontal cells of several elasmobranchs. The method of isolation 

 involved digesting the retina in trypsin. They produced excellent photo- 

 micrographs of individual horizontal cells under Nomarsky optics. Kaneko et 

 al. hope to improve their techniques so that healthy retinal neurons can be 

 isolated and their physiological and biochemical properties characterized. 



Bipolar cells— Bipolar cells, as their name suggests, are interneurons 

 that send processes from the internal nuclear layer toward photoreceptors as 

 well as ganglion cells. They thus vertically connect the input and output 



