Strettoi E, Dacheux R F, Raviola E
Istituto di Neurofisiologia del C.N.R., Pisa, Italy.
J Comp Neurol. 1994 Sep 1;347(1):139-49. doi: 10.1002/cne.903470111.
In the mammalian retina, rod signals are transmitted by rod bipolars to the narrow-field, bistratified (AII) amacrine cell. This neuron, in turn, makes gap junctions with the axonal arborization of cone bipolar cells that reside in the vitreal half (sublamina b) of the inner plexiform layer (IPL). After examining rod bipolars and AII amacrines in the rabbit retina, we have now reconstructed from electron micrographs of continuous series of thin sections the synaptic connections of the axonal arborizations of cone bipolar cells that make the highest number of gap junctions with AII amacrines. These axonal arborizations were narrowly confined to stratum 4 (S4) of the IPL and made ribbon synapses to dyads of postsynaptic dendrites that belonged to either ganglion or amacrine cells. In the population of postsynaptic processes, 30% were ganglion cell dendrites. These dendrites were probably originating, at least in part, from on-center ganglion cells because their course was confined to sublamina b of the IPL. Of the remaining postsynaptic processes, 51.7% belonged to amacrine cells and 18.3% were not identified. Among the postsynaptic amacrine cell processes, 33.3% returned a reciprocal synapse onto the cone bipolar endings. These reciprocal synapses represented 21.3% of the total input onto the axonal arborizations, the remaining fraction (78.7%) arising from a heterogeneous population of amacrine dendrites that were purely presynaptic to the cone bipolars endings. Pre- and postsynaptic amacrines were part of several distinct microcircuits which suggest complex local processing of both rod and cone signals. Thus, the cone bipolars that make gap junctions with AII amacrines in sublamina b of the rabbit IPL exhibit a substantial output onto ganglion cells. This fact, in conjunction with our previous observations that in this sublamina ganglion cells receive negligible input from rod bipolars and AII amacrines, demonstrates that in the rabbit cone bipolars represent a necessary link in the pathway followed by rod signals to enter on-center ganglion cells. Thus, rod and cone signals ultimately share the same integrating mechanisms and converge onto the same set of ganglion cells.
在哺乳动物视网膜中，视杆信号由视杆双极细胞传递至窄场双分层（AII）无长突细胞。反过来，该神经元与位于内网状层（IPL）玻璃体半层（b亚层）的视锥双极细胞的轴突分支形成缝隙连接。在检查了兔视网膜中的视杆双极细胞和AII无长突细胞后，我们现在从连续系列薄切片的电子显微照片中重建了与AII无长突细胞形成最多缝隙连接的视锥双极细胞轴突分支的突触连接。这些轴突分支狭窄地局限于IPL的第4层（S4），并与属于神经节细胞或无长突细胞的突触后树突的双联形成带状突触。在突触后过程群体中，30%是神经节细胞树突。这些树突可能至少部分起源于中心型神经节细胞，因为它们的行程局限于IPL的b亚层。在其余的突触后过程中，51.7%属于无长突细胞，18.3%未被识别。在突触后无长突细胞过程中，33.3%在视锥双极终末上返回一个反向突触。这些反向突触占轴突分支总输入的21.3%，其余部分（78.7%）来自对视锥双极终末仅为突触前的异质无长突树突群体。突触前和突触后无长突细胞是几个不同微回路的一部分，这表明视杆和视锥信号存在复杂的局部处理。因此，在兔IPL的b亚层中与AII无长突细胞形成缝隙连接的视锥双极细胞对神经节细胞有大量输出。这一事实，结合我们之前的观察结果，即在该亚层中神经节细胞从视杆双极细胞和AII无长突细胞接收的输入可忽略不计，表明在兔中视锥双极细胞是视杆信号进入中心型神经节细胞所遵循途径中的必要环节。因此，视杆和视锥信号最终共享相同的整合机制并汇聚到同一组神经节细胞上。
