LAMINAR PATTERN OF TERMINATION OF THE IPSILATERAL CORTICAL PROJECTIONFROM SII TO SI IN CATS

The present light and electron microscopic experiments were carried ou t on the first somatic sensory area (SI) of cats to determine the lami nar distribution of axon terminals from the ipsilateral second somatic sensory area (SII) and to identify the types of synapses between thes e terminals and the neuronal elements of SI. Phaseolus vulgaris-leucoa gglutinin (PHA-L) was iontophoretically injected into multiple sites a nd at different cortical depths of the forepaw representation zone of SII. Fixed brain blocks containing the injected SII and ipsilateral SI were cut into slices and processed immunocytochemically to stain PHA- L-filled fibers and terminals. Light microscopic examination of SI rev ealed patches of anterograde labeling in the forepaw representation zo ne, concentrated mainly in supragranular layers. In these layers, thin immunolabeled fibers branched extensively and formed a dense plexus t hat was more prominent in layers II and I. Conversely, the infragranul ar layers contained fragments of vertically oriented thick fibers that rarely emitted axon collaterals. PHA-L-labeled axons had numerous swe llings along their course, interpreted as boutons en passant, and stal ked boutons. Of 19,661 labeled terminals (17,833 beads and 1,828 stalk ed boutons), 84.74% were observed in supragranular layers, with the hi ghest concentration in layer II (33.15%) and lower in layers I (26.27% ) and III (25.30%). The proportion of terminals was lower in layers IV (6.49%) and V (5.45%) and lowest in layer VI (3.32%). These counts al so showed that boutons en passant were the majority (90.70%) and stalk ed boutons, the minority (9.30%). The ratio of these two types of pres ynaptic specializations was similar (9:1) in all six layers. Electron microscopic examination of the labeled regions of SI showed that both axon swellings and stalked boutons formed synapses of the asymmetric t ype with SI neuronal elements. The majority (85.37%) of a sample of 13 0 labeled terminals synapsed on SI neurons in layers I-III. The identi fied postsynaptic profiles were dendritic spines (61.11%) or medium-si zed and small dendrites (38.89%). These results are discussed in relat ion to those of a companion study on the laminar pattern of the projec tion from SI to SII of cats (P. Barbaresi, A. Minelli, and T. Manzoni, 1994, J. Comp. Neurol. 343:582-596). Based on the anatomical organiza tion of these reciprocal connections, there seems to be no clear hiera rchicalal relationship between SI and SII in cats. (C) 1995 Wiley-Liss , Inc.