Calkins D J, Tsukamoto Y, Sterling P
Mahoney Institute for Neurological Sciences, University of Pennsylvania, Philadelphia 19104-6058, USA.
Vision Res. 1996 Nov;36(21):3373-81. doi: 10.1016/0042-6989(95)00333-9.
The response of a mammalian bipolar cell is generally thought to be determined by the location and morphology of synapses from the cone terminal: ON bipolar cells are believed to be depolarized strictly at invaginating contacts and OFF bipolar cells hyperpolarized at basal contacts. This hypothesis was re-investigated in the macaque fovea (1 deg nasal) using electron micrographs of serial sections. We determined the number of invaginating sites available and then identified the contacts to bipolar cells with axons in the ON level of the inner plexiform layer. A cone terminal forms about 20 active zones marked by ribbons. A few active zones house two invaginating dendrites, so there are 22 invaginating sites per cone. A midget ON bipolar cell collects 18 invaginating contacts from one cone, thus only about four invaginating sites remain for diffuse ON bipolar cells. Two diffuse ON cells were reconstructed; each collects about 25 contacts from an estimated 10 cones. Only three or four of these contacts are invaginating; the rest are basal, adjacent to the triad. This suggests that basal contacts can be depolarizing. The distance from the vesicle release site at active zones to an invaginating contact is 140 +/- 40 nm; to a basal contact adjacent to the triad is 500 +/- 160 nm, and to the next nearest basal contact is 950 +/- 370 nm.
ON双极细胞被认为仅在凹陷性接触处发生去极化，而OFF双极细胞在基底部接触处发生超极化。我们使用连续切片的电子显微镜照片，在猕猴中央凹（鼻侧1度）对这一假说进行了重新研究。我们确定了可用的凹陷性位点数量，然后在内网状层的ON层中识别与双极细胞轴突的接触。一个视锥细胞终末形成约20个以突触带为标记的活性区。少数活性区容纳两个凹陷性树突，因此每个视锥细胞有22个凹陷性位点。一个侏儒ON双极细胞从一个视锥细胞收集18个凹陷性接触，因此，对于弥散型ON双极细胞来说，仅剩下约四个凹陷性位点。重建了两个弥散型ON细胞；每个细胞从估计的10个视锥细胞收集约25个接触。这些接触中只有三到四个是凹陷性的；其余的是基底部的，与三联体相邻。这表明基底部接触可以是去极化的。从活性区的囊泡释放位点到凹陷性接触的距离为140±40nm；到与三联体相邻的基底部接触的距离为500±160nm，到下一个最近的基底部接触的距离为950±370nm。
