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Journal of Neuroscience, Vol 8, 623-642, Copyright © 1988 by Society for Neuroscience
Architecture of rod and cone circuits to the on-beta ganglion cell
P Sterling, MA Freed and RG Smith
Department of Anatomy, University of Pennsylvania, Philadelphia 19104.
Photoreceptors connect to the on-beta ganglion cell through parallel
circuits involving rod bipolar (RB) and cone bipolar (CB) neurons. We
estimated for a small patch in the area centralis of one retina the 3-
dimensional architecture of both circuits. This was accomplished by
reconstructing neurons and synapses from electron micrographs of 189 serial
sections. There were (per mm2) 27,000 cones, 450,000 rods, 6500 CBb1,
30,300 RB, 4100 All amacrines, and 2000 on-beta ganglion cells. The
tangential spread of processes was determined for each cell type, and, with
the densities, this allowed us to calculate the potential convergence and
divergence of each array upon the next. The actual numbers of cells
converging and diverging were estimated from serial sections, as were the
approximate numbers of chemical synapses involved. The cone bipolar circuit
showed narrow convergence and divergence: 16 cones----4 CBb1----1 on-beta 1
cone----1 CBb1----1.2 on- beta This circuit is thought to contribute
significantly to the on-beta cell's photopic receptive field because the
CBb1 has a center-surround receptive field whose center diameter is greater
than the spacing between adjacent CBb1s. Consequently, the receptive fields
of the CBb1s converging on a beta cell are probably largely concentric and
thus mutually reinforcing in their contributions to the on-beta. The rod
bipolar circuit showed a wider convergence and divergence: 1500 rods----
100 RB----5 AII----4 CBb1----1 on-beta 1 rod----2 RB----5 AII----8 CBb1-
---2----2 on-beta The 1500 rods converging via this circuit account for the
spatial extent of the beta cell's dark-adapted receptive field. This
convergence also accounts for the ganglion cell's maintained discharge,
which is thought to arise from about 6 quantal "dark events" per second.
This many dark events would appear in the ganglion cell if each rod in the
circuit contributed 0.004 dark events per second, and this is close to what
has been measured in monkey rods (Baylor et al., 1984). Divergence in this
circuit serves to expand the number of copies of the quantal signal (1
rod----8 CBb1) and so to engage large numbers of chemical synapses that
provide amplification. Reconvergence at the last stage (8 CBb1----2
on-beta) may reduce (by signal averaging) the synaptic noise that would
otherwise accumulate along the pathway.
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