Synaptic organization of the vertebrate retina

Abstract

Recent studies by electron microscopy have described the synaptic contacts in vertebrate retinas. In the outer plexiform layer, receptors contact bipolar dendrites and horizontal cell processes. In most species, horizontal cell synapses onto bipolar dendrites and other horizontal cell processes are observed. In the inner plexiform layer, bipolar terminals contact ganglion cell dendrites and amacrine cell processes. Amacrine processes make feedback synapses on bipolar terminals, feed-forward synapses on ganglion cell dendrites, and lateral synapses on other amacrine processes. The frequency of amacrine synapses varies between species and can be correlated with ganglion cell complexity. Intracellular recordings in mudpuppy show that distal retinal neurons respond to light with slow, sustained graded potentials. Receptors hyperpolarize in response to direct, focal illumination. Horizontal cells hyperpolarize to illumination over a wide area. Bipolars respond by depolarization or hyperpolarization to central illumination; surround illumination antagonizes this response. In the proximal retina amacrine cells depolarize transiently at on and off of illumination. Ganglion cells of two types are encountered. One type shows center-surround organization; its receptive field resembles that of the bipolars. The second type responds transiently to on and off; its activity resembles the amacrine response. The latter type of ganglion cell often shows directionally selective responses to moving spots, suggesting that the amacrine cell is involved in mediating complex ganglion cell responses such as motion and directional sensitivity.