Projection patterns of individual X‐ and Y‐cell axons from the lateral geniculate nucleus to cortical area 17 in the cat

Abstract Horseradish peroxidase was injected intracellularly into single, physiologically‐identified X‐ and Y‐cell geniculocortical axons projecting to area 17 of the cat. This injection anterogradely labeled the axon terminal fields in cortex and retrogradely labeled the somata of these same axons in laminae A and A1 of the lateral geniculate nucleus (LGN). The laminar projections of 21 X‐ and 15 Y‐cell axons were analyzed. For these, the laminar terminations of ten X‐ and seven Y‐cell axons were also related to their cells' positions in the A‐laminae. The terminal fields of X‐ and Y‐cell axons overlapped substantially in layers IV and VI of area 17. Some X‐cells terminated mainly in IVb, others mainly in IVa, and still others throughout IVa and IVb. The latter two groups also projected up to 100 μm into lower layer III. Y‐cells terminated primarily in layer IVa and projected up to 200 μm into lower layer III. Some also arborized throughout the depth of layer IVb. Both X‐ and Y‐cell axons terminated throughout the depth of layer VI, although more so in the upper half. We found no relationship between the diameter of the parent axon and its sublaminar projection within layer IV. Within layer IV, X‐cell axons generally terminated within a single, continuous clump and had surface areas of 0.6 to 0.9 mm 2 . Axons of Y‐cells often terminated in two to three separate clumps, separated by terminal free gaps 400 to 600 μm wide. Their total surface areas, including gaps, were 1.0 to 1.8 mm 2 , roughly 1.6 times the surface areas of X‐cell axons. Despite considerable overlap, Y‐cell arbors contained significantly more boutons than did X‐cell arbors. The sublaminar projections of the X‐ and Y‐cell axons within layer IV reflected the locations of the cells' somata within the depth of the A‐laminae. X‐cells located in the dorsal or ventral thirds of the depths of the laminae projected mainly to layer IVa or throughout layer IV in cortex. Those located in the central thirds projected mainly to layer IVb. Y‐cells showed a similar positional relationship, but they appeared to follow different rules. Y‐cells in the outer thirds of the A‐laminae projected mainly to layer IVa; those in the central thirds, in addition, expanded their projections to include layer IVb. In general, larger sized somata in the LGN gave rise to more widely spreading terminal arbors and greater numbers of boutons in cortex than did smaller somata. However, we found no significant relationship between soma size and terminal arbor extent or total boutons within each cell class (X or Y), and thus the correlation noted may result from Y‐cells having larger somata and terminal arbor extents than do X‐cells. Our results demonstrate considerable heterogeneity in the laminar projections of X‐ and Y‐cell axons within area 17. This heterogeneity reflects an underlying sublaminar organization of the parent somata within the depths of the LGN A‐laminae. The functional significance of this organization, both in the LGN and cortex, is unknown. It is clear, however, that the result of the geniculocortical projection upon layer IV is not to segregate X‐ and Y‐ afferents into lower and upper tiers. Rather, it may be to re‐establish a positional organization existing within the depths of the LGN laminae.