Serotoninergic innervation of the cat cerebral cortex

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 μm) and bear fusiform varicosities that are generally less than 1 μm 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 μm), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 μm or more in diameter) borne on fine‐caliber fibers. These axons often form basketlike arbors around the somata of single neurons. In the simplest basketlike 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.