Mulligan K A, Törk I
School of Anatomy, University of New South Wales, Sydney, Australia.
J Comp Neurol. 1988 Apr 1;270(1):86-110. doi: 10.1002/cne.902700108.
Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.
用一种针对血清素（5-羟色胺，5-HT）的单克隆抗体，通过免疫组织化学方法在猫脑皮质中显示了血清素能轴突。在光学显微镜水平上，根据5-HT轴突形态的差异可区分出三种类型。小的曲张轴突很细（小于0.5微米），带有梭形曲张体，其直径通常小于1微米。这些轴突贯穿皮质的整个宽度并频繁分支，产生广泛分布的侧支。无曲张轴突很光滑，具有相对较大且恒定的直径（约1微米），沿直线、水平轨迹走行，且很少分支。大的曲张轴突的特点是在细纤维上带有大的圆形或椭圆形曲张体（直径1微米或更大）。这些轴突常常在单个神经元的胞体周围形成篮状分支。在最简单的篮状分支中，少数轴突上的几个大的圆形曲张体与胞体接触。在复杂的篮状结构中，相互交织的侧支与胞体接触，并明显沿着细胞的主要树突攀爬并勾勒出其轮廓。攀爬轴突所显示的模式表明，多种非锥体细胞类型选择性地接受密集的5-HT神经支配。对皮质内5-HT轴突的连续重建显示，大的曲张轴突是无曲张轴突偶尔发出的侧支。单个无曲张的母轴突会产生几个大的曲张轴突侧支，这些侧支可能参与不同的篮状分支。相反，单个篮状分支可能由来自多个无曲张母轴突的大的曲张轴突侧支形成。小的曲张轴突在皮质内似乎与无曲张或大的曲张轴突类型均无关联。这些结果支持这样一种假说，即投射到皮质的5-HT分为两个子系统，其中一个可能通过高度发散的分支在皮质中发挥广泛影响，而另一个分布较局限，作用于特定类型的皮质神经元。
