Antal M, Kraftsik R, Székely G, van der Loos H
Department of Anatomy, University Medical School, Debrecen, Hungary.
J Neurocytol. 1992 Jan;21(1):34-49. doi: 10.1007/BF01206896.
Cobalt-labelled motoneuron dendrites of the frog spinal cord at the level of the second spinal nerve were photographed in the electron microscope from long series of ultrathin sections. Three-dimensional computer reconstructions of 120 dendrite segments were analysed. The samples were taken from two locations: proximal to cell body and distal, as defined in a transverse plane of the spinal cord. The dendrites showed highly irregular outlines with many 1-2 microns-long 'thorns' (on average 8.5 thorns per 100 microns 2 of dendritic area). Taken together, the reconstructed dendrite segments from the proximal sites had a total length of about 250 microns; those from the distal locations, 180 microns. On all segments together there were 699 synapses. Nine percent of the synapses were on thorns, and many more close to their base on the dendritic shaft. The synapses were classified in four groups. One third of the synapses were asymmetric with spherical vesicles; one half were symmetric with spherical vesicles; and one tenth were symmetric with flattened vesicles. A fourth, small class of asymmetric synapses had dense-core vesicles. The area of the active zones was large for the asymmetric synapses (median value 0.20 microns 2), and small for the symmetric ones (median value 0.10 microns 2), and the difference was significant. On average, the areas of the active zones of the synapses on thin dendrites were larger than those of synapses on large calibre dendrites. About every 4 microns 2 of dendritic area received one contact. There was a significant difference between the areas of the active zones of the synapses at the two locations. Moreover, the number per unit dendritic length was correlated with dendrite calibre. On average, the active zones covered more than 4% of the dendritic area; this value for thin dendrites was about twice as large as that of large calibre dendrites. We suggest that the larger active zones and the larger synaptic coverage of the thin dendrites compensate for the longer electrotonic distance of these synapses from the soma.
在电子显微镜下，从一系列长的超薄切片中拍摄了位于第二脊髓神经水平的青蛙脊髓中钴标记的运动神经元树突。对120个树突节段进行了三维计算机重建分析。样本取自两个位置：在脊髓横切面上定义的细胞体近端和远端。树突呈现出高度不规则的轮廓，有许多1 - 2微米长的“刺”（平均每100平方微米树突面积有8.5个刺）。总体而言，来自近端部位的重建树突节段总长度约为250微米；来自远端部位的为180微米。所有节段上共有699个突触。9%的突触位于刺上，更多的位于树突干基部附近。突触分为四类。三分之一的突触是具有球形囊泡的不对称突触；一半是具有球形囊泡的对称突触；十分之一是具有扁平囊泡的对称突触。第四类小的不对称突触具有致密核心囊泡。不对称突触的活性区面积大（中值为0.20平方微米），对称突触的活性区面积小（中值为0.10平方微米），差异显著。平均而言，细树突上突触的活性区面积大于大口径树突上突触的活性区面积。大约每4平方微米的树突面积有一个接触点。两个位置突触的活性区面积存在显著差异。此外，每单位树突长度的数量与树突口径相关。平均而言，活性区覆盖超过4%的树突面积；细树突的这一数值约为大口径树突的两倍。我们认为，细树突较大的活性区和较大的突触覆盖面积弥补了这些突触与胞体之间较长的电紧张距离。
