Three morphological types of axons have been recognized in previous studies of the serotoninergic (5-HT) innervation of the cat cerebral cortex (Mulligan and Törk, 1988; DeFelipe at al., 1991): thick, nonvaricose axons, fine axons with small varicosities, and beaded axons with large, spherical varicosities. In the present study, the laminar density and distribution of the three 5-HT fiber types in area 17 are characterized. In both coronal and sagittal immunostained sections, 5-HT axons exhibit an overall gradient of decreasing density from layer I to the white matter. The three 5-HT axon types exhibit distinctive innervation patterns. (1) Fine axons with small varicosities comprise the greatest number of fibers in each layer (56–98%). They usually have oblique or radial trajectories, but some horizontally oriented fibers run through layers I, III, VI, and the white matter. (2) Non-varicose axons, the preterminal portions of the beaded axons with the large varicosities (Mulligan and Törk, 1988), comprise only about 7% of the total 5-HT fiber population in area 17, and are found mainly in layer I and in the white matter where they form a horizontally oriented plexus. (3) Large varicose axons (“beaded” axons) are rare (about 3% of the total 5-HT population) and are restricted to layers I and V. Although large varicose axons often form elaborate pericellular basketlike arbors around the soma and dendrites of neurons in other parts of the cat cerebral cortex (Mulligan and Törk, 1987, 1988), such arrays are rare in area 17. When observed in area 17, pericellular arrays are typically simple structures with a few large varicosities apparently contacting only the somata of a small population of layer I neurons. Comparison of these results with reports of 5-HT innervation of area 17 in other species suggests that the 5-HT innervation of the cortex is highly species specific.

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