M and L cones in macaque fovea connect to midget ganglion cells by different numbers of excitatory synapses

VISUAL acuity depends on the fine-grained neural image set by the foveal cone mosaic13. To preserve this spatial detail, cones transmit through non-divergent pathways: cone → midget bipolar cell → midget ganglion cell. Adequate gain must be established along each pathway; crosstalk and sources of variation between pathways must be minimized. These requirements raise fundamental questions regarding the synaptic connections: (1) how many synapses from bipolar to ganglion cell transmit a cone signal and with what degree of crosstalk between adjacent pathways; (2) how accurately these connections are reproduced across the mosaic; and (3) whether the midget circuits for middle (M) and long (L) wavelength sensitive cones are the same. We report here that the midget ganglion cell collects without crosstalk either 28 ±4 or 47 ± 3 midget bipolar synapses. Two cone types are defined by this difference; being about equal in number and distributing randomly in small clusters of like type, they are probably M and L.

视力取决于由中央凹视锥细胞镶嵌所设定的精细神经图像。为了保留这种空间细节，视锥细胞通过非发散通路进行传导：视锥细胞→侏儒双极细胞→侏儒神经节细胞。必须在每条通路上建立足够的增益；通路之间的串扰和变异源必须最小化。这些要求引发了关于突触连接的基本问题：（1）从双极细胞到神经节细胞有多少突触传递视锥细胞信号，以及相邻通路之间的串扰程度如何；（2）这些连接在镶嵌结构中复制得有多精确；（3）对中（M）波长敏感视锥细胞和长（L）波长敏感视锥细胞的侏儒回路是否相同。我们在此报告，侏儒神经节细胞无串扰地收集28±4个或47±3个侏儒双极突触。这种差异定义了两种视锥细胞类型；它们数量大致相等，并在同类的小簇中随机分布，它们可能是M型和L型。